My dad gave me his old 30 carbine to see if i could get it back functioning correctly. It will not go into battery with a full mag or one thats mostly full. Im using a 15 round mag and 110 umc fmj. If the mag has only 8 or 9 rounds in it it works fine.

I put a new recoil spring in and that did not help. Seems like the full mag is putting to much pressure on the bolt and slowing it down so that it will not go all the way forward.

Is a us postal meter gun. WW2 I assume.

What do I need to do? Try new mags? Weaker spring in mag?

I think you need a new Op-rod spring.

Go to this web site and they will be glad to answer your questions. They know more about M-1's by accident than most people know on purpose. A US Postal Meter carbine is a real treasure. Protect it. kwg

http://forum.shuffsparkerizing.com/

I would try pinching the magazine feeder lips a tiny bit tighter.

i tweeked the mag spring to make it a little weaker, seems much better now but maybe not 100%