Amazingly enough, the Thompson is ridiculously easy to render California-compliant, if you don't mind losing some ergonomics. The Cutts compensator on the front of the barrel is a muzzle brake, not a flash hider. Unlike on an AR-15, the pistol grip does not play any part in the action of the firearm. Lastly, there are two versions of the Thompson's forward furniture. One of them is a forward pistol grip (which must be unscrewed), or just a plain handguard (which is perfectly legitimate). The good news is that, with the pistol grips removed, a Thompson can legally use any size of magazine which you legally owned in California prior to the year 2000.
My experiences shooting my gripless Thompson is that it takes some getting used to - but once you get used to it, it's fine. Now, with the negligible recoil of a semi-automatic Thompson, it really isn't bad to shoot once you mentally get over the "waa, it doesn't look like in the movies" point. As a semi-automatic 45ACP carbine, the Thompson is absolutely serviceable.
I shall have to post pictures of mine eventually, but for now it's at least useful to let folks know how to render a Thompson CA-compliant.
The other solution would be to have some kind of bullet button setup. A few different methods have been proposed, but none of them have worked just yet.
-- SeanNewton - 15 Aug 2008