Please help me understand how select-fire works...


February 25, 2010, 07:43 PM
More specifically....the firing pin system. I have some questions so please bear with me.....

I do not nor ever have owned a FA firearm....never held or fired one either. Never the less I'm still fascinated by them. Now, I can fully grasp how a simple "slam-fire" type of weapon works (M3 grease gun, ex) as the firing pin is machined right into the bolt face. Very simple.

Now select fire.....I'm confused:banghead:

Lets use the classic Thompson submachine gun as an example because it is my favorite FA of all time.

When switched to "Semi-auto"....does the gun shoot from a closed bolt just like any other semi auto rifle?

What happens when the gun is switched to "full-auto"?

Is the firing pin then LOCKED OUTWARD into place so that it then potrudes out from the bolt face? Or.....does it remain "floating" loose in the bolt so that the bolts forward inertia slams it forward into the cartridge primer?

Lastly the firing pin always spring loaded and the system uses a HAMMER which closes the moment the bolt face strikes the back of the cartridge?

I appologize if these questions are stupid, but I hope someone can explain the system to me.

Many thanks in advance!

If you enjoyed reading about "Please help me understand how select-fire works..." here in archive, you'll LOVE our community. Come join today for the full version!
February 25, 2010, 07:48 PM
All full-auto weapons are different.

In the case of the Thompson gun, its a "slam fire" design.
The gun fires from an open bolt. When the trigger is pulled, the bolt closes, chambers a round and fires it.
This holds for full-auto or semi-auto.
The Thompson Models 1921, 1928, and M1 have a spring loaded firing pin, but the pin is free to move forward at all times.
These guns also have an odd, triangular hammer that strikes the firing pin the instant the bolt closes
The Model M1A1 has a firing pin that's simply a projection machined into the face of the bolt and has no hammer.

Other guns, especially many machine guns, fire from a closed bolt, and some guns fire from an open bolt in full-auto but a closed bolt in semi-auto.
Again, how it operates depends on the weapon design.

February 25, 2010, 07:58 PM
dfariswheel, thanks!

Ahhh. So then, on a tommy gun.....the firing pin does NOT get locked outward as I thought it did.....but remains spring loaded all the time.

I've seen diagram drawings of Thompsons and I always thought the hammer was strictly for semi auto fire only.

Again, thanks:)


February 25, 2010, 10:20 PM
If you want to understand how the M16 works, watch this. (

February 25, 2010, 11:47 PM
The Thompson fired in full auto as described above; the bolt, traveling forward, chambers a round and the triangular hammer pivot when the bolt hits the forward frame, firing the round.

In semi auto, you have to understand that the bolt is held back by a sear when cocked, and the trigger rotates this sear down, releasing the bolt. In semi auto, you flip a lever one hundred eighty degrees. This raises a cam and a clawlike device is raised into the path of the bolt. The bolt hits this and knocks it forward as it chambers the round. This action releases the sear independently from the trigger and when the bolt is pushed back, the sear rises and catches it.

Actually, the firing pin in the 1928 is NOT free to move forward at all times it is held back by a spring and is only forced forward by the triangular hammer .... atleast on the 1928 bolt I have, that's the way it works. Earlier Thompsons may be different, and the M1A1 as previously stated had no separate firing pin.

February 26, 2010, 12:10 PM
Thanks to everyone for their input. I have a better understanding now. I was always convinced that on ALL submachine guns with select fire....the firing pin got locked outward. It made sense to me.

Seems needlessly complicated to have a moving firing pin though (hence, the birth of the grease gun)


If you enjoyed reading about "Please help me understand how select-fire works..." here in archive, you'll LOVE our community. Come join today for the full version!