At the All Thompson Show and Shoot last summer, one of our members was having an unusual problem with his 1928AI, the trigger would lock "back," particularly in semi-auto fire.

NOTE: As it turned out, the problem could occur with any Thompson, and could occur in full auto as well.

He asked if I could take a look at it and see what the problem was and subsequently sent me the offending grip frame. Fortunately, the problem did not cure itself enroute. I was able to simulate the "lock back" by pulling the trigger (C) and raising the trip (B) as if actuated by an empty magazine. I could also cause the "lock back" not to happen by pulling the trigger back and to the right. (See attached exploded drawing for letter identification).

The trigger was sloppy on its pivot plate (E) pin, I removed the internal parts and found that the trigger pivot hole was worn. That little wear was evident on the mating pin. The disconnector (K) was also very loose in its pivot hole in the trigger, enough that the disconnector spring due to its normal off center position tended to push the disconnector outward at its lower end.

CONCLUSION: The looseness of the trigger on its pin allowed the bottom of the disconnector to sometimes rise above the bottom of the well in the grip frame. The disconnector, due to the looseness in the trigger, was forced outward by its spring and latched itself to the edge of the bottom of the well. A new better fitting trigger and disconnector cured the problem.

One can only speculate on the cause of the wear. Perhaps a trigger that was not properly heat treated. the offending grip frame was from a "parts kit" and there was some indication that the whole assembly had been bead blasted and parkerized while assembled. Perhaps residual grit or the parkerizing itself had etched the parts. In any event, looseness little more than .00 I" in two places was enough to cause the problem. In handling and shooting dozens of TSMGS, I've never encountered this before, but suggest all of us check our guns at least for trigger wobble side to side. While I don't have enough information to calculate actual limits, anything appreciably over 1/32" might be considered suspect and merits further investigation.