tommy gun semi auto version malfunction

[winchets]

I have a thompson semi auto and Im haveing a malfunction I cant figure out.  my 20 round mag (origanal ) wouldnt feed properly so I tried some 30 rnd mags and they seem to work. recently I started shooting at the matches the gun will lock back with 1 round left in the mag!!! if i pull the bolt the last round feeds perfectly. I have tried several new mags and still have the same problem. last month it happened during the first stage but the last 2 stages it fed fine.

any suggestions? I have cleaned the gun and oiled it.  could it be a weak spring?

[Pitspitr]

I'm reaching here and I know it, but is there a possiblity the mag is bent? That would seem like a stretch given how heavy built Thompson mags are.

[DeaconKC]

Could be the magazine spring or possibly something inside the mag [rust, grit, etc] causing the follower to drag.

[Pitspitr]

I have tried several new mags and still have the same problem.

OOPS! I missed this part of your description. That does make it seem like it's not the mag.

[DeaconKC]

I'm in the same boat with Pitspitr, I missed that it was other mags as well. Take the springs out, put some lapping compound on the races and then work the bolt back and forth for several hundred cycles. Something is rough in there, causing it to hang up once it gets hot and the metal expands.

[James Hunt]

I have a 20 rd mag that consistently will not feed the last round, manually working the bolt sends the cartridge home without problem. This mag is an unmarked 20 rd'er I bought from Kahr. Comparison with the other four (Seymour and AO) marked 20 rounders I own reveals that the spring is very much weaker.  Hopes this helps.

[Shenandoah]

The mags need to be modified to work in a semi auto. Maybe they have not been modified.

PM me if you need more info.