Thompson

militants · July 12, 2006, 02:46 AM

Uzi installed blocking bars on their semiauto version of the SMG. I see a lot of parts kits for Thompsons and was wondering what parts could be used on a semiauto or for a semiauto build. What changes did they make to comply with the ATF?

tINY · July 12, 2006, 03:53 PM

I believe that the semi-auto fires from a closed bolt and the auto fires from an open bolt.

-tINY

Scorch · July 12, 2006, 04:50 PM

That's it exactly. The original 1921 Thompson fired from an open bolt, and used a fixed firing pin to fire the rounds. There is a toggle that grabs the bolt when the trigger is released. If you hold the trigger back, the action continues to cycle and fire. The WWII-era Thompsons were designed to fire the same way (different design trigger and receiver, but similar).

The semi-auto Auto-Ordinance Thompson fires from a closed bolt. This is to comply with the NFA and GCA. It has a hammer that is released by the trigger and a disconnect that disconnects the trigger. A sear holds the hammer in the cocked position after the action cycles and chambers a new round. You have to release the trigger for the trigger to re-connect and be able to fire the next round.

Dfariswheel · July 12, 2006, 08:04 PM

The semi-auto Thompson is a totally different design than any of the full-auto guns, and there is VERY little part interchangeability.

The semi-auto is a combination of original parts from the 1921/28 and the M1/M1-A1 guns and a newly made trigger group, receiver, bolt, and action parts..

The action parts on the semi-auto are totally different.
Full-auto parts will not fit the semi-auto since both the trigger group and the receiver are different sizes specifically to insure no full-auto parts will fit.

The full-auto parts you can use on the semi-auto are:
Barrels.
Front sights.
Cutts compensators.
Rear sights.
Rear sight bases.
Both the pistol grip and horizontal fore arms.
The M1 and M1-A1 rear pistol grip.
The M1/M1-A1 buttstock.
You can mount a 1921/28 detachable buttstock if you buy an adapter plate that screws to the trigger group.
Safety.
Magazine catch.
1921/28 ejector.
Extractor.
Trigger pivot plate (can be modified to work).
Magazines can either be modified to fit, OR the magazine catch can be modified to work with magazines, OR a full-auto magazine catch can be used.

NO other parts will interchange.

Again, you cannot use full-auto bolts or trigger group parts since the receiver is "lower" than the full-auto, and the trigger group is made differently.

You cannot install semi-auto parts in a full-auto gun since again, the trigger group and receiver are different.

James K · July 13, 2006, 02:33 PM

Hi, Scorch,

The M1921, M1927 (semi-auto)*, M1928 and M1 did not use a fixed firing pin. The firing pin was forced forward by a triangular shaped part they called the "hammer". The hammer fits into the bolt and when the bolt goes forward, the bottom of the hammer hits the frame and pivots on its pin, driving the top forward. The top strikes the firing pin, driving it forward. The M1A1 used a fixed firing pin like the Sten.

Open bolt firing takes advantage of something called "advanced primer ignition" (API), which means the primer is fired BEFORE the bolt comes fully to rest. The remaining forward momentum allows the bolt to act as if it were much heavier than it is, so the bolt can be lighter and the return spring can be lighter. That is why the current semi-auto guns require a very heavy spring and are so difficult to cock, where the normal TSMG can be cocked easily with one finger.

*The original Model 1927 is semi-auto, but fires from an open bolt; the only difference was in the selector. The modern SA gun is called the Model 1927A1.

Jim

**44 AMP** · July 14, 2006, 07:27 PM

The Auto Ordnance 1927A1 semi auto does not have a "hammer" that strikes the firing pin, I don't remember the proper names for the parts, but it is more like a "striker" type system. It has it's own spring, seperate from the recoil springs.

Also, the upper/lower reciever dimensions are 1/4" off from the SMG, to absolutely prevent the use of the full auto (interior) parts.

To make the full auto magazines work, you have to "egg out" the mag catch hole, to compensate for the mag catch being in a slightly different place. I don't think replacing the mag catch with a SMG one would work, as the SMG mags would be a little too low to feed correctly. I'm not sure, and will check this out, as I have several GI 30rnd sticks that have not yet been modified for the semi auto. Right now, I am having fun trying to get a new 50rnd drum to work reliably. Anybody have any tips?

Anybody have any tricks for getting a lighter trigger pull on the semi auto? I'm not looking for a 3lb let off, but something around 8-12lbs would be a lot better than what it is now.

It does take three men and a boy to cock the gun, though. I can live with that. I can even live with needing 3 hands to remove the empty drum, but I would really like a somewhat lighter trigger pull. The gun is capable of some good accuracy, I have gotten 5 shot one hole groups at 25yds, and regular hits on long range targets using the ladder sights, I just wish it was easier to pull the trigger!

James K · July 14, 2006, 11:25 PM

44 AMp wrote:

"The Auto Ordnance 1927A1 semi auto does not have a "hammer" that strikes the firing pin..."

I didn't say it did, but I can see it would be possible to read it that way. Sorry for the confusion.

The new semi-autos which they are calling the Models 1927A-1, 1927A-1c and Model 1927A-5 semi-auto pistol have a separate firing pin which is held back by a sear in the conventional semi-auto setup. There is a disconnector to prevent full auto fire. It might be possible to convert the guns to open bolt FA fire, but it would be at least as difficult as with other semi-auto rifles.

Jim

Dfariswheel · July 15, 2006, 04:54 PM

The Semi-auto Thompson has a really weird firing mechanism, designed by George Numrich to keep the ATF off his back.

The bolt contains an odd "dog-leg" striker/sear, AND A "HAMMMER".
This long, wide striker is held back by the trigger group's sear.
In the center of the bolt where a recoil spring would be is a cylindrical hammer that's a section of 3/8" rod and a large diameter spring to power it.

When the trigger is pulled, the striker is released and runs forward to strike the primer. The cylindrical hammer riding behind the striker "bounces" on the striker to give it enough energy to fire the primer.

To improve the Semi-auto trigger pull:
Cut one to one and a half coils off the trigger spring.

Lightly polish the top and rear of the trigger's trip lever.

Lightly polish the bottom front of the sear where the trigger trip contacts it.

Lightly polish the "step" area of the sear where the striker sear contacts it.

Cut one half to one coil off the sear spring.

Square up and remove machine marks from the front face and bottom of the striker's sear area where it engages the trigger group's sear.
Be careful to not remove any more metal than is absolutely necessary.

Lightly polish the pivot holes in the trigger and sear.

Lightly polish the large pins on the pivot plate.

Cut one to one and a half coils off the cylindrical hammer's spring.

**44 AMP** · July 23, 2006, 03:30 AM

After looking up the parts list, I see that the 1927A1 does indeed have a part called a "hammer", exactly as described. I was thinking of the small pivoting hammer of the full auto gun, which the semi does not have.

Thanks for the info on the trigger pull, I will work on it after I get a few replacement parts, in case I go too far.

July 12, 2006, 02:46 AM	#1
militants Junior Member Join Date: July 12, 2006 Posts: 1	Thompson Uzi installed blocking bars on their semiauto version of the SMG. I see a lot of parts kits for Thompsons and was wondering what parts could be used on a semiauto or for a semiauto build. What changes did they make to comply with the ATF?

July 12, 2006, 04:50 PM	#3
Scorch Senior Member Join Date: February 13, 2006 Location: Washington state Posts: 15,248	That's it exactly. The original 1921 Thompson fired from an open bolt, and used a fixed firing pin to fire the rounds. There is a toggle that grabs the bolt when the trigger is released. If you hold the trigger back, the action continues to cycle and fire. The WWII-era Thompsons were designed to fire the same way (different design trigger and receiver, but similar). The semi-auto Auto-Ordinance Thompson fires from a closed bolt. This is to comply with the NFA and GCA. It has a hammer that is released by the trigger and a disconnect that disconnects the trigger. A sear holds the hammer in the cocked position after the action cycles and chambers a new round. You have to release the trigger for the trigger to re-connect and be able to fire the next round. __________________ Never try to educate someone who resists knowledge at all costs. But what do I know? Summit Arms Services

July 14, 2006, 07:27 PM	#6
44 AMP Staff Join Date: March 11, 2006 Location: Upper US Posts: 28,820	1927a1 The Auto Ordnance 1927A1 semi auto does not have a "hammer" that strikes the firing pin, I don't remember the proper names for the parts, but it is more like a "striker" type system. It has it's own spring, seperate from the recoil springs. Also, the upper/lower reciever dimensions are 1/4" off from the SMG, to absolutely prevent the use of the full auto (interior) parts. To make the full auto magazines work, you have to "egg out" the mag catch hole, to compensate for the mag catch being in a slightly different place. I don't think replacing the mag catch with a SMG one would work, as the SMG mags would be a little too low to feed correctly. I'm not sure, and will check this out, as I have several GI 30rnd sticks that have not yet been modified for the semi auto. Right now, I am having fun trying to get a new 50rnd drum to work reliably. Anybody have any tips? Anybody have any tricks for getting a lighter trigger pull on the semi auto? I'm not looking for a 3lb let off, but something around 8-12lbs would be a lot better than what it is now. It does take three men and a boy to cock the gun, though. I can live with that. I can even live with needing 3 hands to remove the empty drum, but I would really like a somewhat lighter trigger pull. The gun is capable of some good accuracy, I have gotten 5 shot one hole groups at 25yds, and regular hits on long range targets using the ladder sights, I just wish it was easier to pull the trigger! __________________ All else being equal (and it almost never is) bigger bullets tend to work better.

July 23, 2006, 03:30 AM	#9
44 AMP Staff Join Date: March 11, 2006 Location: Upper US Posts: 28,820	Thanks for the info After looking up the parts list, I see that the 1927A1 does indeed have a part called a "hammer", exactly as described. I was thinking of the small pivoting hammer of the full auto gun, which the semi does not have. Thanks for the info on the trigger pull, I will work on it after I get a few replacement parts, in case I go too far. __________________ All else being equal (and it almost never is) bigger bullets tend to work better.

July 12, 2006, 03:53 PM	#2
tINY Senior Member Join Date: April 26, 2005 Location: Orygun Posts: 2,589	I believe that the semi-auto fires from a closed bolt and the auto fires from an open bolt. -tINY

July 12, 2006, 08:04 PM	#4
Dfariswheel Senior Member Join Date: May 4, 2001 Posts: 7,478	The semi-auto Thompson is a totally different design than any of the full-auto guns, and there is VERY little part interchangeability. The semi-auto is a combination of original parts from the 1921/28 and the M1/M1-A1 guns and a newly made trigger group, receiver, bolt, and action parts.. The action parts on the semi-auto are totally different. Full-auto parts will not fit the semi-auto since both the trigger group and the receiver are different sizes specifically to insure no full-auto parts will fit. The full-auto parts you can use on the semi-auto are: Barrels. Front sights. Cutts compensators. Rear sights. Rear sight bases. Both the pistol grip and horizontal fore arms. The M1 and M1-A1 rear pistol grip. The M1/M1-A1 buttstock. You can mount a 1921/28 detachable buttstock if you buy an adapter plate that screws to the trigger group. Safety. Magazine catch. 1921/28 ejector. Extractor. Trigger pivot plate (can be modified to work). Magazines can either be modified to fit, OR the magazine catch can be modified to work with magazines, OR a full-auto magazine catch can be used. NO other parts will interchange. Again, you cannot use full-auto bolts or trigger group parts since the receiver is "lower" than the full-auto, and the trigger group is made differently. You cannot install semi-auto parts in a full-auto gun since again, the trigger group and receiver are different.

July 13, 2006, 02:33 PM	#5
James K Member In Memoriam Join Date: March 17, 1999 Posts: 24,383	Hi, Scorch, The M1921, M1927 (semi-auto), M1928 and M1 did not use a fixed firing pin. The firing pin was forced forward by a triangular shaped part they called the "hammer". The hammer fits into the bolt and when the bolt goes forward, the bottom of the hammer hits the frame and pivots on its pin, driving the top forward. The top strikes the firing pin, driving it forward. The M1A1 used a fixed firing pin like the Sten. Open bolt firing takes advantage of something called "advanced primer ignition" (API), which means the primer is fired BEFORE the bolt comes fully to rest. The remaining forward momentum allows the bolt to act as if it were much heavier than it is, so the bolt can be lighter and the return spring can be lighter. That is why the current semi-auto guns require a very heavy spring and are so difficult to cock, where the normal TSMG can be cocked easily with one finger. The original Model 1927 is semi-auto, but fires from an open bolt; the only difference was in the selector. The modern SA gun is called the Model 1927A1. Jim

July 14, 2006, 11:25 PM	#7
James K Member In Memoriam Join Date: March 17, 1999 Posts: 24,383	44 AMp wrote: "The Auto Ordnance 1927A1 semi auto does not have a "hammer" that strikes the firing pin..." I didn't say it did, but I can see it would be possible to read it that way. Sorry for the confusion. The new semi-autos which they are calling the Models 1927A-1, 1927A-1c and Model 1927A-5 semi-auto pistol have a separate firing pin which is held back by a sear in the conventional semi-auto setup. There is a disconnector to prevent full auto fire. It might be possible to convert the guns to open bolt FA fire, but it would be at least as difficult as with other semi-auto rifles. Jim

July 15, 2006, 04:54 PM	#8
Dfariswheel Senior Member Join Date: May 4, 2001 Posts: 7,478	The Semi-auto Thompson has a really weird firing mechanism, designed by George Numrich to keep the ATF off his back. The bolt contains an odd "dog-leg" striker/sear, AND A "HAMMMER". This long, wide striker is held back by the trigger group's sear. In the center of the bolt where a recoil spring would be is a cylindrical hammer that's a section of 3/8" rod and a large diameter spring to power it. When the trigger is pulled, the striker is released and runs forward to strike the primer. The cylindrical hammer riding behind the striker "bounces" on the striker to give it enough energy to fire the primer. To improve the Semi-auto trigger pull: Cut one to one and a half coils off the trigger spring. Lightly polish the top and rear of the trigger's trip lever. Lightly polish the bottom front of the sear where the trigger trip contacts it. Lightly polish the "step" area of the sear where the striker sear contacts it. Cut one half to one coil off the sear spring. Square up and remove machine marks from the front face and bottom of the striker's sear area where it engages the trigger group's sear. Be careful to not remove any more metal than is absolutely necessary. Lightly polish the pivot holes in the trigger and sear. Lightly polish the large pins on the pivot plate. Cut one to one and a half coils off the cylindrical hammer's spring.