Re: .300 Ultra Mag vs. .338 Ultra Mag
Killing is shot placement, more than cartridge. I'm presuming that you're a young fellow, since you're talking about your father's rifle.
But please consider this. 300 and 338 RUM ammunition is running near $90 per box. You may want to look at some less expensive ammo to buy. The 300 WSM, for example, is $52 per box (gasp!) with the 180 TSM, and it shoots pretty fast.
Meat destruction depends upon bullets, too.
If your two choices are restricted to those calibers, there's nothing in the lower 48 that you can't kill well with the 300 RUM, and it kicks less. That would be my choice.
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