Model 12 center feed short action 223.
Rifle started out as a blind mag and a tupperware stock. Never had any feeding issues.
I got a accustock from another member here. The stock on the outside looked identical to the old one, except it had the aluminum bedding and had the factory flush DBM. The stock was from a 308 rifle.
I swap the recoil lug and reassemble the rifle with no issues. The mag, I take the 308 mag body, follower and spring off the floorplate and snap on the 223 mag body, follower, and spring from my old stock. It fit perfectly with no issues.
I go to sight in the rifle and everything is going well until the last round. It will not feed. The bolt just jumps over the rim. I remove the mag, look at it, everything looks fine, put it back in and still, it will not feed the last round. I push it about halfway out of the mag with my finger and it feeds with the bolt the rest of the way fine.
I reload the mag and the same thing. Rounds 1-4 feed fine, but the last round, will not feed.
I load the mag and look at how much of the rim is exposed, then unload all but the last and it "looks" to be at the same height as the rest.

Anyone have any suggestions?