Jimmy, the 125g Sierra is a deer bullet.
Second, a youngster at 12 should not be exposed to a hard kicking gun. A 125 in the 2200 fps is what you would be hoping for.
With my 12year old nephew, we tried down loading Model 7 Remingtons in 308 and a 7/08, he could not handle the rifle or the recoil. The rifle was just too big for the 12 year old; 12 year olds do vary in size, stature, and ability to handle finding an object in a rifle.
We got my nephew a Thompson contender with a 20" barrel in 30/30. We cut the barrel to minimum legal rifle length. He handled the recoil just fine. The Thompson Contender has no weight in the front and was/is very easy for him to handle it. We shot Wal Mart Rem 150g Core Loct in the TC. 3x9 Scope was perfect for him. We took him to the range practicing with a bolt action 22 Mag (Marlin 25) which got him accustomed to the noise and finding objects in a scope.
We next got him a 7/30 Waters barrel for The tc which was flatter shooting than the 30/30.
Nephew has killed 12 Deer with the TC, two bobcats, and numerous wild dogs, and several hogs.
Good luck with your 12 year old! Lots of fun and memories to be made that will last him a life time.
