So, here is what I have worked out for the Thompson market in California. This modification only works with the 1927 and 10 round drum to be California compliant. The new California law allows the magazine to be switched if the gun is disassembled (as in upper and lower on a Thompson moving apart). Foiling California Legislators like the new AR15 bullet button device people did just feels good.
The way a thompson works is that the drum is held in place by the upper, but the lower mag catch keeps it from sliding side to side. With the magazine catch welded in place, the drum won't be able to slide. To allow it to slide, the lower must be able to move backwards since the magazine catch won't move.
A bolt hold open device must also be used or the magazine won't slide either unless the bolt is locked back. The traditional bolt hold open devices that have been built and work with the lower and a full auto selector won't work as it would keep the gun from sliding apart. Thus a new top piece device was required. EZpull springs help out here too...
To allow the upper and lower to slide apart quick and easy, I have also devised a frame latch button lever (saw this idea at a TATA show).
With all these things working together, it works slick and easy. Drum reloads are very user friendly.
The other side is that if a guy moves out of CA the mag catch weld can be easily cut away with a dremil tool. Maybe without even seriously damaging the magazine catch if you are good. Worst case is you have to replace the magazine catch.
Unfortunately, this won't work with stick magazines as the magazine sticks up into the mag well, but is held by the lower (no sliding possible). That's a topic for another post as I'm working out a design idea for M1 type guns.
Offering this modification complete with EZpull springs for $250. I can do this on a new ordered gun (I can order you a gun at a discount) and ship to California dealers, or California guns can be shipped to me if you already have a version of bullet button gun but don't want to go down the road of registering as an assault rifle.