The rifle in question is a Stevens 200 centerfeed rifle. It was originally a .223 Rem it's now a 6mm-222 Rem Mag, and it doesn't matter if it's the only round in the magazine or if I've loaded four down the last cartridge won't strip from the magazine. The bolt just rides over the top of the cartridge. The cartridges strip just fine with more than one cartridge in the magazine.

I can't see what the problem is when I had the action out of the stock. The magazine seems to be properly seated in the well, and my action is making even contact with the pillars. Should I be thinking magazine spring and follower or something else?