[zahc]

I've heard about Thompson SMG's having the firing pin fixed on the bolt face, so it is basically slam-firing. But, how do you stop firing? Seems it would shoot the mag dry no matter what.:confused:

[CMcDermott]

It's called the "open bolt" system, when you release the trigger the bolt is held in the open position, and the gun stops firing. Yes, this lets in dirt, crud etc fairly easily, which is why these weapons generally have generous clearences to allow plenty of space to hold the dirt and let it get thrown/fall out of the action when the gun is fired. Not a good system for a target weapon, but it works just fine at close combat ranges.

[cdbeaver]

The M3 .45 caliber Grease Gun used in WWII and Korea worked on the same principle. It had a flip-down cover that served as a safety (to hold the bolt in retracted position) and to protect from rain and/or dust.

[telewinz]

IIRC the first thompsons fired from a closed bolt but when the Army took over in WW2 they changed it to an open bolt to save costs.