Allowable pressure Sparton double rifle

[bobmn]

Sundles: I would be cautious with your recommendation of allowable pressure in the 45/70 double rifle based upon extrapolation from the 30/06 version. The factor you are not considering is case head thrust. Because the area of the case head of a 45/70 is much larger than the area of a 30/06 case head, the maximum allowable pressure is lower for the larger sized case. This is the reason you can safely fire a .223 T/C contender at 50,000 psi but if you tried a .308 Contender you will stretch the frame. This subject is covered very thoroughly by J. D. Jones in "Case Head Thrust" Handloader's Digest 9th edition page 59.

[Sundles]

I know about case head thrust. I considered it. Shoot all of our 40,000CUP 45-70 ammo you like out of the Spartan.

All five of our 45-70 loads run right at 40,000CUP.

[James H.]

Gents

From the viewpoint of fundamental physics, case head thrust, as a function of case head area and maximum working pressure, just doesn't exist. This is like saying that it hurts more for a 200lb person to step on your toes with size 12 workboots on than it does for the same person wearing spike heals to step on your foot. It just doesn't work that way.

What does exist is Force which is the product of Mass and Acceleration. Maximum pressure is important when trying to figure out if a barrel of a certain diameter and material strength will deform or burst.

The Force generated by a .30-06 round can go as high as 10,000 Newtons (Federal High Energy 180gr @ 2,880 in a 23" barrel). The Force generated by Tim's .45-70 rounds will go as high as 10,600 Newtons (the 300gr @ 2,350 in a 22" barrel). If the wall thickness of the barrel can handle the pressure generated, there is no reason why this ammo can't be used.

If anyone is interested, I have Excel spreadsheets that calculate both Force generated by a given load and bursting strength of various wall thicknesses (this is a predictive model only!). Please PM me an e-mail address and I will send them along.

[bobmn]

John: I might buy your argument about F=MA in a bolt action rifle where the lock up of the action is so much stronger, but the Spartan rifle is a break action rifle and I believe it to be subject to the same sort of limitations imposed on the T/C Contender action. That is the reason T/C added the thickend ribs at the top of the G2 action and Winchester thickened the top of the Big Bore Model 94 action. Have you read J. D. Jones' article?

[Jeff]

James,

I'm interested in understanding the assumptions you made to create your model. Do you assume that bullet acceleration is constant or a linear function of position in the barrel or a non-linear function? I’ve never been able to make these kind of calculations work out right since I don’t have access to the resources to do the numerical integration of the pressure vs time curve to get the acceleration correct here at home. Are you using strain gage data that you gathered, or have you found a public accessible source from the manufacturers or a lab? Does your "bursting strength" model predict the difference in safety factor that a handloader could gain by optimizing powder choice?

[James H.]

Physics is physics. The calculations aren't really all that complicated.

Force (F) = Mass (m) x Acceleration (a)

Force is measured in Newtons.
Mass is measured in Kilograms.
Acceleration is measured in Meters Per Second Squared.

(Sorry, it is easier to convert to metric values and then do the calculation!)

An example: 180gr bullet @ 2,700fps from a 22" barrel with a 55gr charge of powder.

Mass= 180gr + 55gr = 235gr/437.5 = .537 ounces * 28.4 = 15.25g = .01525kg.

Velocity(final)= 2,700fps/3.3 = 818 m/s

Velocity (average) = (0+818)/2 = 409 m/s

Barrel Length = 22 inches x 2.54 = 55.88 cm = .5588 m

Time (in barrel) = Distance Travelled/Average Velocity = .5588 m /409 m/s = .00136 seconds

(Note this is only an approximation because the length of the cartridge case, the depth of seating and even the depth of crown all have minute impacts on the actual distance the bullet travels during its acceleration)

Acceleration = Difference in Velocity/Time = 818m/s - 0 m/s / .00136 = 601,470 m/s2

Force = Mass x Acceleration = .01525 kg x 601,170 m/s2 = 9172 kg m/s2 = 9172 Newtons

Note that getting 2,700 fps from a 18 inch barrel requires much greater Force than getting the same 2,700 fps from a 24 inch barrel because the acceleration has to be much greater.

Because we know that for every Force there is an equal and opposite reaction (recoil in the case of firearms) we can calculate the recoil of any given weight of firearm. Recoil (and the force exerted on the bolt or breachface) is a function of the force exerted on the projectile to accelerate it to the muzzle velocity. It has nothing to do with the size of the cartridge base or the peak or average pressure of the cartridge. Yes, it takes higher pressures (or more properly, the area under the Pressure/Time curve has to be greater) to achieve higher accelerations to get good velocities out of short barrels but it is the acceleration and mass of the projectile (including propellant) that determines both recoil and the force exerted on the action. What determines whether the action can handle the Force are its materials and how the Force is applied. From here we get into engineering, not physics and that's above my paygrade. I recommend checking out Varmint Al and Dan Ljilla's websites for more technical examinations of things like the effects of Force and pressure on bolt action rifles.

Regarding Mr. Jones article, I haven't read it yet. I'd be very interested if someone could e-mail me/fax me a copy. I'm a big fan of SSK products having two JDJ chamberings on my Contender carbine.

The reason the Contender was re-designed, in part, was to stand up to the greater stresses created by cartridges like the JDJ's and the .45-70 loaded above factory specs. The forces generated to achieve some of the accelerations required to get the kind of velocities these loadings produce must be pretty impressive for such a small action.

[don]

I perfer a simpler formula as thrust = area x pressure
a 30-06 is .486 diameter at case head
a 47/70 is .502 diameter measured at the base of the case. not rim
.486 /2 = .243 squared = .0590 X Pi 3.141 = .1855 Sq inch area.
.502/2 = .2510 squared = .0630 X Pi
3.141 = .1979 Sq inch area.
so the 45/70 has a headcase thrust a factor of 1.0669 more, compared to a 30-06
don

[bobmn]

James H.: Your calculations predict the pressure limits resulting in the structural limit of the barrel. In a break open action such as the Contender, Spartan or bolts with rear locking lugs such as the Winch. 94 or Remington 788, long before the pressure which would result in deformation of the barrel is reached, the frame will stretch. I believe the reason case head thrust is a factor is because the larger area of the bigger cases more efficiently transfers a greater percentage of the "Newtons" you have calculated to the receiver and stretches the frame at a lower chamber pressure than the smaller cases do. Since you have a Contender, why did T/C chamber barrels in .223 but not in .308 Win.? They both have about the same SAAMI pressure limits and your spreedsheet will show that neither will burst the T/C barrel. The .308 will stretch the frame while the .223 will not.
Concerning the Spartan, Remington states that only 28,000 cup loads should be used. Sundles has stated that his 40,000 cup loads are safe in the Spartan. The Spartan is made by Baikal in Russia and is probably proofed at that 28,000 cup plus whatever the safety factor is required by the gov't. proof house (if there still is one considering the chaos in Russia). In the US, we have no gov't. proof house but rely on SAAMI for standardization. As an interesing aside, in 1977 I was in the Italian proof house in Gardonne ValTrampia and saw stacks of Winchester 94 rifles. These were imported for sale in Italy (remember the speghetti westerns?)and had to be proofed prior to sale in Italy because they were not proofed in the US and had no proof marks on them. So we know that the Spartan rifle can handle 28,000 cup 45/70 loads and 50,000 cup 30/06 loads. The question is, will a steady diet of 40,000 cup 45/70 loads stretch the frame of a Spartan? Sundles says no, although I do not believe he has fired his rifle yet. I certainly have not, nor have I proofed or ran a durability test by firing a large number Sundles' loads in the Spartan and than comparing the headspace before and after. I am confident that if I were to fire nothing but Sundles' loads in a Spartan and they stretched the frame, Remington would say that I violated the terms of their warranty and may or may not replace the rifle. If they would not, I am sure Sundles would because he is a stand-up guy. Knowing the Russian proclivity for over-engineering, Sundles is probably right but I do not know his criteria for publicly stating his approval for 40,000 cup loads in contrast with the importer's warranty stipulation of 28,000 cup loads.

[Jeff]

That's what I was afraid of. Force exists at an instant in time. Your calculations are only telling you the amount of force necessary to produce your acceleration number on a mass equal to the mass of the bullet in a frictionless, adiabatic world at an instant in time. Because the acceleration function of the bullet in the barrel is so far from a constant, and there are factors like friction, inertia, bullet pull, etc that you're neglecting from the calculation, your model badly underestimates the peak force on the locking lugs.

Just a couple example to illustrate what I'm trying to say:

if you have two loads that produce identical velocities in the same firearm, one using a slow burning rate powder and one using a fast burning rate powder do they both stress the firearm equally? The fast powder load will give you sticky extraction etc before the slow powder load, yes?

if you have a cast bullet load and a jacketed bullet load with equal weight bullets and equal velocities do both put the same amount of stress on the firearm? Won't the cast bullet puts less stress on the firearm for a given velocity because of the better lubricity and lower friction?

[James H.]

bobmn

Let's run the numbers on a number of different cartridges through the T/C Contender and see what we get. All ballistics will be based on data provided in the Hornady Handbook of Cartridge Reloading 4th Edition.

.223 Remington, 14" Contender, 50gr @ 3,000 fps with 25.1gr of powder. Total Force generated is 5,655 N.

.7-30 Waters, 14" Contender, 120gr @ 2,500 fps with 38.5gr of powder. Total Force generated is 8,288 N.

.30 Herrett, 10" Contender, 110gr @ 2,400 fps with 26.4gr of powder. Total Force generated is 9,202 N.

.357 Remington Maximum, 10" Contender, 180gr @ 1,800 fps with 23.8gr of powder. Total Force generated is 7,734 N.

.358 Herrett, 12" Contender, 180gr @ 2,000 fps with 27.6gr of powder. Total Force generated is 8,105 N.

.375 JDJ, 14" Contender, 220gr @ 2,200 fps with 51.7gr of powder. Total Force generated is 11,002 N.

.375 JDJ, 14" Contender, 270gr @ 2,000 fps with 48.0gr of powder. Total Force generated is 10,642 N.

.375 JDJ, 14" Contender, 300gr @ 1,900 fps with 50.8gr of powder. Total Force generated is 10,595 N.

.44 Remington Magnum, 14" Contender, 300gr @ 1,400 fps with 20.4gr of powder. Total Force generated is 5,254 N.

.45 LC, 10" Contender, 300gr @ 1,300 fps with 21.3gr of powder. Total Force generated is 6,360 N.

.45-70, 16" Contender, 300gr @ 1,800 fps with 54.7gr of powder. Total Force generated is 8,413 N.

.45-70, 16" Contender, 350gr @ 1,700 fps with 53.0gr of powder. Total Force generated is 8,526 N.


In addition, I'd like to add two of my own loads for my JDJ's.

.309 JDJ, 21" barrel, 165gr @ 2,550 fps with 50.0gr of powder. Total Force generated is 7,798 N.

.416 JDJ, 20" barrel, 350gr @ 2,100 fps with 56.0gr of powder. Total Force generated is 10,486 N.

To me this says that the limits for the Contender action are in the area of 11,000 N. Anything over this and you will spring the action. I have done it when working up loads for the .416 JDJ that must have produced 12,000 N. The action sprung but didn't stretch permanently. I was very lucky, but I managed to bulge ever so slightly the barrel under the first forend bolt dovetail.

If we use an XP-100 action, the allowable force goes up a fair bit.

From the same source we find the following:

.221 Remington Fireball, 10 3/4" XP-100, 55gr @ 2,700 fps with 18.3gr of powder. Total Force generated is 5,822 N.

.22 BR, 14 1/2" XP-100, 55gr @ 3,100 fps with 30.8gr of powder. Total Force generated is 6,660 N.

7x45mm Ingram, 14 1/2" XP-100, 120gr @ 2,400 fps with 31.2gr of powder. Total Force generated is 7,035 N.

7mm BR, 15" XP-100, 120gr @ 2,350 fps with 32.6gr of powder. Total Force generated is 6,581 N.

.35 Remington, 15" XP-100, 180gr @ 2,200 fps with 38.6gr of powder. Total Force generated is 8,262 N.

Just for fun, let's look at a short barrelled bolt action.

7mm-08 Remington, 18 1/2" M788, 120gr @ 2,800 fps with 43.4gr of powder. Total Force generated is 8,111 N.

What limits the use of high pressure cartridges in the Contender is the thickness of the barrel walls. The stress involved is referred to as "hoop stress" and is calculated with what is called Barlow's Formula.

Pressure = 2 x Tensile Strength (PSI) x Wall Thickness (inches)/Outside Diameter (inches).

For the pressure required to bulge a given barrel, use the Yield Strength of a material. For pressure required to rupture a given barrel, use the Ultimate Tensile Strength of a material.

Using this formula, a Contender barrel diameter (.810"), and assuming a Yd Str of 110,000 PSI and a Ult Str of 135,000 PSI (the strengths for 416 stainless at a Rockwell C hardness of 26), a .308" bore would require about 68,000 PSI to bulge the barrel and about 84,000 PSI to rupture the barrel. The problem is the weak point of the Contender barrel is at the first forend bolt dovetail which removes .125" from the wall thickness. Pressures for bulge and burst then drop to 49,500 and 61,000. Going to larger bore barrels increases the problem.

[Jeff]

Hit the button too soon...

The F=ma equation applies at a given instant. The force on the bullet base equals the pressure at that instant multiplied by the area of the bullet base. The chamber pressure is a non-linear function of the burning rate of the powder and other factors. The F=ma equation can be used to calculate the acceleration of the bullet at that instant after you account for losses due to friction etc.

[James H.]

Don

By your formula, the .45-70 at 40,000 psi has a thrust of 40,000 x .1979 = 7916. The .30-06 at 50,000 psi has a thrust of 50,000 x .1855 = 9275.

Why can't Buff Bore ammo be used if this is how you judge the thrust?

Let's use a different analogy to see how larger case diameters don't mean more pressure on the bolt. Think snowshoes. Snow with a crust can only hold so much weight per square inch. Walk on it in street shoes and you are likely to go through. Walk on it with snowshoes and it will hold you no problem. The Force is the same in both instances (your mass x the acceleration of gravity) but the larger surface area reduces the impact on the "breechface" rather than increasing it as "back thrust" calculations would indicate.

[Jeff]

One more example; next time you're in a place that gets a lot of foot traffic take a look at the floor. You'll notice that women walking across a slate or tile floor in high heels will chip or leave permanent marks, while men twice their weight leave no permanent damage. The women apply half the force on a smaller area, and the material of the floor is stressed past the elastic deformation zone into the plastic deformation zone (a permanent depression) or even further past the ultimate material stregth (a chip from the floor). Why? Because the factor that causes yielding or failure in any material is force per unit area... i.e. pressure.

[don]

James and Jeff,
the fundalmemtal problem with your analogy is you are taking a fixed weight, and spreading it over larger or smaller areas. The result will be more of less pressure over the area (PSI).
My analogy is based on pressure per area (PSI is pounds per Square inch)
If you load your snowshoe at the same PSI as your foot, they will sink just as far.
Try holding stoppers in two bottles with a large opening and a small opening with the same air pressure.
We could make all our hydraulic cylinders smaller, under your proposal, to do the same work.
don

[Jeff]

Don: using the chamber pressure and area calculation, if you increase what you call the "thrust factor" by 1.0669, at the same time that you change the pressure by a factor of .8 (40,000/50,000) the 45-70 load actually puts only 85% of the "stress" on the locking lugs that the 30-06 load does.

James: if I follow the force balance model for a 1.375 oz 12ga slug at approx 1450 fps from a 20" slug barrel I get about 8000 Newtons (plus or minus round numbers) that would seem to indicate that a single barrel 12 ga could be re-barreled to 30-06 without modifying the lock up mechanism. Yet, if you ask NEF, they have to tighten the lock up and heat treat differently to make the Handi-Rifle work compared to their Partner shotguns.

My theory goes as follows:

The breechface is retained against the cartridge base by the locking lugs. The pressure within the chamber is equal on all surfaces. The amount of force applied to the locking lugs is equal to the chamber pressure (force per unit area) multiplied by the area that the force is applied to (the cross sectional area of the i.d. of the cartridge base). The yield strength of the locking lugs is a constant based on the material yield stress and the cross sectional area of the lugs that the force is applied to. If the force applied to the breechface exceeds the yield stress of the lug material divided by the cross sectional area of the lugs, the lugs will yield, resulting in plastic deformation (a stretched frame) If the stress exceeds the ultimate tensile strength of the material, the lugs will fail.

The force balance equation on the base of the bullet is made up of components due to bullet acceleration, barrel friction etc. Bullet acceleration isn’t a perfectly efficient system where all of the force applied to the base of the bullet goes only into acceleration. In the short instant that bullet motion is first initiated, the distribution of the force applied among the various components is very different from what it is after motion has begun. Once motion begins, you’ve switched from the static coefficient of friction to the dynamic coefficient of friction, the deformation of the bullet to engrave in the rifling goes from yield stress (UTS) to flow stress etc. etc. etc.

The rate that powder combustion inputs energy into the system isn’t constant, it’s a function of powder “speed”. Bullseye dumps a lot of energy in a hurry, RL22 releases its energy a lot more slowly. The rate of energy input determines how the system reacts. High rates of energy input (fast powders) build higher peak pressures because you’re adding a lot of energy into the system while it’s essentially static, so the energy goes into building heat and pressure until the static friction and inertia of the bullet are overcome and some of the energy goes into expansion and the kinetic energy of the bullet. That’s why the force balance equations aren’t easy to model, and don’t predict the real word performance of the system very well without some complex simulation.

[James H.]

Jeff

Everything you write makes good sense. But what you are talking about is engineering, not physics and, as I have admitted, it is above my paygrade.

Physics is good at looking at the Force acting on the whole system but it takes engineering to determine the results of how that force will act on the parts (in terms of hoop stress, shear etc).

I use Force as a means of comparing different cartridges to determine their relative suitability for a particular action. I guess I need convincing to buy into the "bolt thrust" argument. So convince me.

[Jeff]

One last example of the failure of the assumption that acceleration can be treated as a constant...

The standard .22 L.R. reaches peak velocity between 16 and 18 inches of barrel length. Any longer and the muzzle velocity decreases. At the point of peak velocity, acceleration is zero. After that the bullet decellerates, i.e. the sum of forces on the bullet is negative. That's because the friction / drag exceeds the force on the base of the bullet (which is still chamber pressure multipled by the area of the bullet base). The energy flux into the system from powder combustion has slowed, so expansion and heat consume more energy than is produced, so the pressure drops. Taken to it's limit, if the barrel length is long enough, the drag will slow the bullet to a stop. At that point, Vf=V0=0, the average acceleration is zero and your force balance model would say that you've put zero stress on the locking lugs to get there. Similarly, at any barrel length longer than 16 inches or wherever the peak velocity is reached, the pressure balance model predicts that cutting the barrel length would increase the stress on the locking lugs. i.e. you make a given load more dangerous by cutting the barrel off shorter. That demonstrates that the assumption of constant acceleration fails to predict the performance of the system.

Put another way, yield and failure are predicted by a stress / strain diagram, and stress has no time component, it exists at an instantaneous moment in time, produced by the force at that instant. That's why the only way to predict the maximum stress that's applied to the locking lugs is to use the peak pressure and the area of the cartridge base exposed to the pressure.

[James H.]

Jeff

How do you do that effectively? If two cartridges operate at the same pressure but have different areas in contact with the breech, does increasing the area increase the force on the breech or decrease it? What about rebated rim cartridges or cartridges with extremely large rims? I have heard it explained both ways and don't find either version particularly convincing. You sound like you have these concepts pretty well thought out so I would appreciate knowing how you see this applying.

Also, what impact does case area in contact with the chamber walls have on the back thrust? Since the yield strength of brass is lower than that of steels used for actions does this even have an impact?

If anyone has that article by JD Jones about case head thrust I'd really like to read it. Please drop me a PM.

[Kevin B]

Ok, for the non physicists like me, lemme ask this in a simple way. The parts of the bolt/action are made of a material with certain discernable attributes. It can withstnad X pressures exerted on them before failing, period.

What pressure/force can this action withstand? Seems that they have made certain claims wiht respect to both 06 and 45/70 actions. Are these actions identical?

Someplace in a milisecond the maximum pressure applied via the brass to the action is reached. That # is caclulable for any given round. It may not be perfectly calculable to 10 decimal points, but I'd argue that some of this minutia is really unimportant.

Is the 30-06 action Identical to the 45/70? If so why would an identical force distributed over a greater area result in part failure? Is there something about the location or load of that force to the critical parts of the action that will be different in the 45/70?

I'm not a physicist. I just don't understand why two nearly identical force loads delivered in appreciably the same way, to the same set of equipment would deliver seperate and domonstrably different results unless the action is either dissimilar, or weaker by design in the 45/70 action. I'm not referring to the barrel just the action.

Can someone address this in lay?

[Sundles]

Kevin B,
Here is the truth. No one has to like it or agree with it, but it is the truth none-the-less.

SAAMI specs for the 45-70 remain at 28,000CUP because some of the older weaker action designs still exist in private ownership. So, because SAAMI specs for 45-70 remain at 28,000CUP, EVERY GUN MANUFACTURER that makes 45-70's has to state that the max pressure for their gun is 28,000CUP--never mind that they will make the same gun with the same action, with the same metalurgy for cartridges such as the 06 or the 308 Win. which has a SAAMI limit of 55,000CUP.

Example: Marlin uses thier 336 action to make chamberings such as the 307 Win. or the 375 Win.. Both of these cartridges generate 52,000CUP. However, when Marlin uses the exact same action to make the 45-70 chambering, they state not to use any ammo that generates more than 28,000CUP. I could go on with a more detailed explanation, but I wont.

[Jeff]

I hope I'm not coming across like I'm some kind of arrogant know it all. I'm just a guy that probably thinks about stuff too much, trying to figure out how it works.

James: I think that the calculation needs to be based on the area that the gas pressure is acting on. That would be the inside of the case. The force on the breechface should be the same whether rimmed, rimless or rebated as long as the area the gas is acting on remains the same. As far as case wall to chamber wall interaction goes, the forces there are normal to the surfaces, so you have a pretty high level of static friction at the interface. Combine that with the relatively low yield strength of brass, and that's why when you have high pressure loads, the case length stretches. The case walls stick to the chamber walls but the base moves with the breechface deflection and in extreme situations you get head separation. That's how I've figured to explain the "pressure ring" on the case wall. I've always read that the case should be treated as if it was just a gasket in most calculations because its strength is so low in relative terms.

Kevin: As far as the strength of an action goes, the important thing is the cross sectional area of the locking lugs and the strength of the material that they're made out of. When an action "stretches" or the headspace gets screwed up, it's the lugs or whatever locking mechanism that failed not a deformed breechface where it contact the brass. If the manufacturer uses the same size lugs made of the same material and heat treats them the same etc, it really doesn't matter what the cartridge case looks like, unless the size of the case causes them to have to modify the geometry of the lockup to make it fit.

When you're considering action strength, the chamber pressure is important because it gives the simplest way to calculate a reasonably accurate number for the force that's on the lugs. Chamber pressure by itself is a big concern for rupturing a barrel, but as far as stretching the headspace, it's a combination of both the pressure and the size of the case.

I hope that makes sense.

[bobmn]

Kevin: Please allow me to summerize this discussion and try to answer your questions in non-technical terms. For simplification, forget about front locking bolt action rifles. We are concerned about break open actions such as the Thompson Contender, Spartan double barrel, and actions with locking lugs on the rear of the bolt such as the Win. 94 and Remington 788. These actions, while made of the same materials as bolt action rifles with locking lugs on the front of the bolt, are more "springy" because when they are fired, they flex more than say a Remington 700 or a Win Model 70. More specifically, in answer to your question about the Spartan double rifle, yes the action is the same for 30/06 and 45/70. Sundles, Patrick and James H. say because you can shoot a 30/06 at 50,000 copper units of pressure (cup, an indirect way of measuring cartridge pressure) the Spartan is safe to fire with 45/70 at 40,000 cup and that the size of the base of the cartridge is irrelevent. Intuitvely, this would make sense. However, my position and some of the others participating in this discussion is that the size of the base of the cartridge does make a difference.
Now to the best of my knowledge, none of us have even fired the Spartan rifle yet so we are all assuming, speculating, extrapolating etc. about what this rifle will take. The manufacturer, Baikal, a Russian company and Reminton, the importer, state that only 45/70 cartridges loaded to 28,000 cup should be fired in the rifle. I think everyone will agree 28,000 is o.k., Sundles, Patrick and James H. say that 40,000 is o.k. because 50,000 30/06 is o.k. and that 28,000 is too low of a number based on lumping weaker 45/70 actions together with the stronger Spartan rifle.
Now that I am done speaking for everyone else, I will try to make my case. I think that you should be cautious shooting 40,000 cup loads because the break open Spartan is subject to stretching like the Contender action. J. D. Jones is a custom gunsmith who pioneered custom barrels for the Contender in more powerful cartridges. The following are some of his quotes from an article he wrote in the 9th edition of Handloader's Digest entitled "Case Head Thrust":
"The 45/70 must be kept at low pressure levels or permissable thrust will be exceeded."
"... if in 45/70 you load progressively heavier, case head thrust makes the action "sticky" and I assume would lock it up at pressures that are no where near the 'blow up' point."
"I feel quite sure many mathematicians will feel they can produce formulas to accurately determine C.H.T. I distrust these formulas and will only accept them when I see them proven by hardware to measure what actually happens when a round is fired. Numbers only go so far. Sometimes when the firing pin hits the primer an entirely different story is told."
"How did I prove the acceptability of the Contender action to the series of cartridges and loads developed for it? Simple. Keep pressures moderate or low and fire several thousand rounds through a few guns and than examine them for stress."
Kevin, I think you, along with James H., Patrick and Sundles should buy $600 Spartan Rifles in 45/70, send them to Sundles so he can fire "several thousand rounds" of his ammo through them and I volunter to "examine them for stress". /images/%%GRAEMLIN_URL%%/smile.gif
Seriously, though, testing is the only way to definively answer the question "Does case head thrust really exsist?" The following antedotal evidence in my own guns may explain my caution:
1. Remington 788 .243 Win.: Loads which were recommended in various loading manuals and proven safe in other .243 rifles with front locking bolts locked up the 788 bolt. The brazed on bolt handle seperated from the bolt body when I attempted to open the action.
2. Win 94 Big Bore 356 Win.: A recommended load cracked the extractor well below the maximum without any of sign of increasing pressure as the powder charge was increased.
3. T/C Contender 6mm 30/30 Bullberry: Locked up action at a much lower pressure load than a 7mm TCU load at 50,000 cup on the same action.

[James H.]

I agree with the idea of blowing up a rifle to see just where, when and why it fails. If I find myself with more money than I know what to do with I'll do just that.

Thanks all for a fun thread that has given me a chance to shake some of the cobwebs out from between my ears!

[don]

Since we are not talking a big difference in square inches of case head percentage, lets us speculate on the wild side.
Supose we have big beefy break open action, that will handle 50,000 CUP.
Now we chamber one for 7 MM mag and load it to that pressure. The second action we chamber for 12 Gauge brass cases- then load those to the same CUP .
Since CUP is just an indirect method of measuring PSI, It seems appearent to me that tht pressure over the larger 12 Gauge area produces X% more that the smaller 7mm head.
The area of the opening of the chamber is the only area to consider for the thrust. Rebated rims, or rimmed make no difference.
There was years back some discussion of rear locking lugs (eg Marlin)and front locking lugs (eg Mauser) in regard to action streaching. A break open would by its design have even more stretch.
If we are assuming the 30-06 @ 10,000 newtons and the 45/70+ @ 10.669 newtons - that is a factor of 1.07
then with the case head area of 1.06 more for the 45/70
would it not be 1.06 X 1.07 = 1.13 more thrust?
don

[Jeff]

"Thrust" is a measurement of force. When you talk about force you are usually talking in units of "pounds-force" or "newtons"; occasionally some other less common units.

The area measurement comes into play to convert the chamber pressure, measured in pounds per square inch, into pounds. (i.e.: p.s.i* square inches = pounds)

What we are usually thinking about when we think about the strength of an action is "stiffess" or resistance to force.

A particular action design can withstand a particular amount of force (or thrust if you prefer) on the breechface before it is permanently deformed (plastic deformation).

Every time you fire a cartridge the action stretches, or the locking lugs (on front locking) or the bolt(on rear lock-up) compresses some amount. At low levels of force the action springs back to near its original dimensions (elastic deformation). When the force is too high, the action material reaches its yield point and doesn't spring back (plastic deformation).

[kenner]

Just picture someone lying on his back holding a plywood square above his chest. If we state that for each 1 foot square a 100 pound person may stand. This gives us 100lbs/sq.ft. So if we try holding up a 2x2 square we now have 4 square feet and 4 x 100 pounds force we are attempting to resist. 100 pound force vs. 400 pounds force yet the same 100lbs/square ft. ---More area = more weight. .....So likewise more area at the base of a cartridge for the same psi (pounds per square inch) = more force applied along that vector.

Or more simply, when talking PSI (pounds per square inch), ---- more square inches = more pounds. ............. Kenner

[imported_gebuesch1]

The force of the gas pressure doesnt depend on the diameter/area of the base of a cartridge but it depends on the diameter/area of the bullet which is accelerated by the gas pressure. That´s the law of action and reaction.
Example:
Pressure of 30-06: 3000 bar=30000 N/cm²
Diameter: 7,82mm -> area on which pressure is applied: A= Pi* (0,782cm)²/4=0,48cm²
Force: A*30000 N/cm²= 14400N
Same with 45-70:
p=2000 bar
diameter=11,43mm -> A=1,03cm²
Force=20500N
Force isnt the only problem with breakopen guns like a double rifle. The real problem for its strength is the torque applied by the force of the gas pressure onto the locking lugs. This torque can cause that a much higher force than the force of the gas pressure onto the locking areas.
So if there is the same construction of a 30-06 and a 45-70 double rifle the 45-70 rifle has to be alot stronger than the 30-06.

[bobmn]

Gebuesch1: Are you familiar with Griefelt double rifles? In Ken Waters classic pet loads article, he states:" Unclassified but approximating Group II in strength is my Greifelt 45/70 double rifle."

[imported_gebuesch1]

No, sorry. Never heart of them.

[mhlmvl]

Gentlemen (and ladies),

I am new on this forum. I have been using and buying guns,primarily rifles since 1956. I have owned hundreds of firearms and still own over 135. Lets just say I know quite a bit about firearms, and being an att'y, am good at research. That is one thing they do teach you in law school.

I own both a 45-70 and a 30-06 in an SPR22 Remington/Baikal/Spartan Double rifle. I find them fascinating rifles for the money. I find it frustrating to spend $1000 on a rifle and then only let it keep my attention for no longer than a day. The SPR22s have done that for me. They are a lot of fun, the 45 more than the '06, but both are worthy, and I am immersed in them two months later. Mercy!

Are there better double rifles out there? You bet, many of them. But for one third the price of a very inexpensive SXS, I bought two. And they have walnut stocks, two triggers, two barrels and handle well, shot well, give two quick shots, and recoil like real dangerous game rifles as they are light. Are they a little crude? Sure, but the stock is no worse than my two Weatherby Lite Weights, machine made stocks with clunky lines. I know there was a real stock in the WBYs as I spent $600 with my srtock maker finding it. Its there. It is also there in the Baikals, most of which have very nice walnut.

Now, the reason for my note. After years of promises, Remington finally got this rifle to market and it has been snapped up by buyers, from $700-1000. I expect they will increase in value over time. While Remington seems to have lost attention, EAA says they will import them in the future. I'm hoping other cailbers like ,405 Win, 9.3X74R, .444 Marlin, and 7.62X54R will be added to the chamberings. In terms of operating pressures, they are all in line with the action's expected capabilities.

And in that regard, this site has posted here the absolutely best discussion I have been able to find anywhere on the Internet on operating pressures safe for the SPR22. It is wonderful. There are many people who seem threatened by an inexpensive Russian Double Rifle giving many access to an otherwise expensive forearm type off limits to many budgets. Unusually critical remarks abound from many who have not even seen the rifle for real. Interestingly, many owners report fun!

On this site, like many other sites, we are getting posts from persons of limited technical firearms knowledge challenging the conclusions of others who obviously understand the math behind operating pressures. Operating pressures are a mathematical execise. Given all the facts, a person of ability and knowledge can closely approximate the ability of a known firearm to handle pressures. Since the SPR22 is chambered for the 30-06 Sprg and Russia is a member of CIP, we know alot about the strength of the action. It was all laid out here in excellent detail. Still, "bolt thrust" or some other nonsense is bandied about by some as though there was some relevance there, even after the bolt thrust equation had been well explored.

I visit many sites and on this subject, operating pressures in the SPR22, the discussion here is the BEST. Well done.
MartyBoy