Contrary to popular belief, the 300 HPs are not varmint bullets, and considering your sons are young you won't be loading them up too hot. I shot a 250# Alaskan blackie with a Hornady 300HP loaded to about 1750 fps in an old H&R Trapper - one shot behind the shoulders and complete penetration.
I had great luck with the Hornady 350RN in my own M98 Siamese Mauser, which I built from a mail-ordered Thai rifle in the mid-70s. Loaded to 1850 fps it expanded well judging by the wound channels and I never recovered one from a caribou or moose. I also used the 'varmint bullet' on deer with this rifle, and the only bullet I ever recovered was from a Kodiak blacktail shot at 225 yards. It entered under his chin and I found the mushroom under the hide on his rump. Perfect expansion too, although impact velocity was probably only 1150 fps.
As the above demonstrates, after 36 years of using the .45-70 on game I found that the moderate loadings do just as well as the .458-style loads, and I don't have to worry about losing any teeth from recoil and report. The .45-70 doesn't need magnum velocity to kill very very well.
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