Barrel ramp vs unsupported chamber….

[Jtm35a2]

It’s BRP’s Thompson 1928 configuration built around a stemple tube. It uses a atandard 1928 lower receiver with a modified sear (taller) to reach up inside the stemple tube that’s encased in the sguare aluminum thompson shell. The lower reciever, furniture, and rear sight are the only real thompson parts. Everything else is made by BRP to make a John Stemple reciever tube look like a Thompson 1928. In 45 it uses unmodified thompson stick and drum mags. The parts I changed are:
Bolt with face id reduced to 9mm
9mm suomi barrel, virtually a drop in part
9mm BRP recoil spring
Suomi drum with rib welded onto the back to slide into the thompson lower’s magazine slot

Here’s a link to their product page:

Stemple 76/45 Takedown Gun (STG) M1A1/1928 setup

Stemple Takedown Gun (STG), Transferable Machine Gun, Transferable machine guns for sale

www.brpguns.com

 

[Jtm35a2]

Empty trunnion


Trunnion with 45 barrel for reference


Trunnion with mildly throated suomi barrel and before extractor cut widened


Trunnion with heavily throated suomi barrel, still needs polishing. As of this posting I haven’t fired this iteration

 

[A&S Conversions]

Thank you for the pictures. So how much unsupported shell casing do you end up with and how far back from the shell base is it? I am guessing that you bought a second Thompson bolt that you modified or did add a 9mm face to your original 45 bolt.

That ROF sounds really good. I had a Savage built REWATT M1A1. A project I have had in the back of my mind was to make a Thompson upper for a M10 based on a 80% M1A1 receiver, bolt, barrel assembly, and the mag rail. What is your ROF for your BRP 45 Thompson configuration? Very cool project. Have you thought about welding the spine of Thompson mags to Uzi mags so you would have stick mags? Thank you again for sharing your project.

Scott

 

[Jtm35a2]

Thank you for the pictures. So how much unsupported shell casing do you end up with and how far back from the shell base is it? I am guessing that you bought a second Thompson bolt that you modified or did add a 9mm face to your original 45 bolt.

That ROF sounds really good. I had a Savage built REWATT M1A1. A project I have had in the back of my mind was to make a Thompson upper for a M10 based on a 80% M1A1 receiver, bolt, barrel assembly, and the mag rail. What is your ROF for your BRP 45 Thompson configuration? Very cool project. Have you thought about welding the spine of Thompson mags to Uzi mags so you would have stick mags? Thank you again for sharing your project.

Scott

Yes, I bought a spare brp 1928 bolt for the bolt face conversion.
The gun in 45 runs around 745~770 rpm depending on ammo.
Philadelphia Ordnace sells solid T shaped material for adadpting other mags to Thompson. It’s better than trying to cut the formed sheet metal ribs off of thompson mags.
Dan Block said he could never get uzi mags to work. He used sten mags. I am assuming double feed mags combined with small 9mm rounds in the big 45 feed ramp/funnel and the extremely long jump from magazine to barrel equated throwing a hotdog down a hallway. He did have luck with sten mags, I am assuming due to being single feed. I would rather only have drums than load those dang things.

 

[Jtm35a2]

First two pics are opposing side views of a chambered round. Bottom pic is my caliper set to the .120” of bolt face depth the gun has. I am hoping that it’s not too much unsupported case. I will know in a couple of hours if the full diameter throating feeds better and is enough case support.

 

[Jtm35a2]

So it ran like a sewing machine until the bolt bushing failed 😞

 

[A&S Conversions]

Thank you for the pictures. That looks pretty good to me. I personally am a little “gun shy” ( pun intended) of unsupported shell casings. If you are only going to use these 9mm rounds once, hopefully there is enough support.

My out of battery/unsupported shell extension has mostly been from closed bolt medium caliber rifle rounds. The one exception was with a 9mm conversion in a M16 at a local machine gun shoot. The problem was the owner wasn’t familiar with the mass requirements. He only had a H buffer. On his second or third mag he had an exploded casing. Luckily the shell let go away from the ejection port. We got to looking at his spent brass and all of them were very bulged. The H buffer is only 3.9 ozs, where the standard Colt 9mm buffer is 5.5 ozs. That extra mass of the 45 bolt would certainly help with your conversion.

Thank you for the info about the the shaped steel for the mags. I had not heard of that. Again thank you for sharing your project.

Scott

ETA what happened to the bolt bushing? BTW, since both the 9mm and 45 bolts go into the same receiver, couldn’t you use a 9mm bolt?

 

[Jtm35a2]

Spent shell casings from this evening. I am still not seeing any signs of bulging

 

[Jtm35a2]

ETA what happened to the bolt bushing? BTW, since both the 9mm and 45 bolts go into the same receiver, couldn’t you use a 9mm bolt?

Dan brazed in a ring. Then cut the ejector and extractor slots. The cuts basically created two half circle pieces. The braze didn’t penetrate and was only on the face edge of the bolt and bushing.
I looked at brp 9mm bolts but the 1928 bolt guide assembly is different than all but one style of 9mm bolt. And the charging handle hole is a half hole from being in the right location, besides being a totally design of handle. Also it’s about 10~15% lighter than the 45 bolt.

 

[A&S Conversions]

The brass looks great.

I could be wrong but brazing is low temperature. I would think that with enough fire, I would think that the brazing could become warm enough to soften. If the steel face of the 45 bolt was built up and machined back down to 9mm dimensions with TIG, I would think steel, that was welded, would be much more heat resistant than a brazed on part.

Scott

 

[Jtm35a2]

I could be wrong but brazing is low temperature. I would think that with enough fire, I would think that the brazing could become warm enough to soften. If the steel face of the 45 bolt was built up and machined back down to 9mm dimensions with TIG, I would think steel, that was welded, would be much more heat resistant than a brazed on part.

Scott

Agreed. Dan wants me to send it back for another braze, but I am leaning towards either tig buildup and machined out or tig welding another bushing. My concern about the first option is can this bolt be chucked up in a lathe. It’s not round

 

[A&S Conversions]

It doesn’t need to be. The bolt can be a fixed point and the tool can be the moving part. It is more difficult to set up, but can be done.

 

[Jtm35a2]

It doesn’t need to be. The bolt can be a fixed point and the tool can be the moving part. It is more difficult to set up, but can be done.

Ah, I hadn’t thought of that. We talked about a recessed center mill bit, but finding one in .400” has proven difficult.

 

[Jtm35a2]

I thought about scalloping the face of the existing lip, tig welding the bushing to it in the scallops and smoothing it out

 

[slimshady]

Brazing doesn't penetrate, it's basically high temperature soldering using a much stronger metal than lead Alloy. If the entire contact surface of the inserts were covered with the remnants of the braze metal I would say the the bond of the brazing is just not strong enough with that little surface area. The problem now becomes if you try to weld it and there is even a microscopic bit of the old brazing material present it will contaminated the weld puddle and give you porosity and brittleness.

If I was doing the job I would opt for TIG welding, using the lathe first to remove everything down to the bare metal, just barely taking a few thousands off of the original metal. With a four-jaw chuck you can fit a lot of different offset shapes in there and adjust it out to get the front end of the bolt running true. If the inserts are small I would just leave them out and build it up with the weld metal before machining the final shape.

 

[Jtm35a2]

Brazing doesn't penetrate, it's basically high temperature soldering using a much stronger metal than lead Alloy. If the entire contact surface of the inserts were covered with the remnants of the braze metal I would say the the bond of the brazing is just not strong enough with that little surface area. The problem now becomes if you try to weld it and there is even a microscopic bit of the old brazing material present it will contaminated the weld puddle and give you porosity and brittleness.

If I was doing the job I would opt for TIG welding, using the lathe first to remove everything down to the bare metal, just barely taking a few thousands off of the original metal. With a four-jaw chuck you can fit a lot of different offset shapes in there and adjust it out to get the front end of the bolt running true. If the inserts are small I would just leave them out and build it up with the weld metal before machining the final shape.

Here’s my dilemma:
I 100% was pursuing tig welding up the rim and machining it back out to 9mm.
BUT…….
The highly experienced tig welder I chose for the job needed to know what steel he was dealing with. So I emailed BRP and got less than good news. The bolt is made of S7 tool steel and hardened further after machining. Applying the heat from tig welding can really frig up the metallurgy, way worse than normal stuff. Think like extreme brittleness and possibly even shattering the bolt while welding.
I have been driving Dan bonkers trying to work through the solution. His position is that he didn’t get a good braze. All of the material is at the very front edge of the bolt, it didn’t “flow” down between the bushing and bolt rim. He assures me that’s why it failed and that we should try it again
Now I really don’t know what to do…….

 

[A&S Conversions]

So let him try. What have you got to lose? The only other thing that I can think of is, have a new bolt face made with a slip fit with a roll pin to hold it in. If that works well, I would think that you could have a set made. Have a 9mm, 40 S&W/10mm, and 45.

Scott

 

[Jtm35a2]

So let him try. What have you got to lose? The only other thing that I can think of is, have a new bolt face made with a slip fit with a roll pin to hold it in. If that works well, I would think that you could have a set made. Have a 9mm, 40 S&W/10mm, and 45.

Scott

So i have two spare 1928 brp pattern bolts, besides the original 45 bolt that came in the gun.
I am sending the one back to Dan for another brazing attempt.
I went over to my machinist friends house and we confirmed we can center the bolt on a lathe using a 4 jaw chuck. So I am going to try having the second spare bolt welded and machined out to 9mm.
I need to build two of these guns anyway so we’ll see how it goes

 

[slimshady]

Yeah it does look like it didn't get drawn into the joint like it should have. Looks like he is using a stick of filler metal instead of the brazing paste that has the flux and ground up metal in it. Commonly used for furnace brazing things like a carbide insert to a tool. Everywhere the paste is, metal is there too, you don't have to rely on capillary action to draw it into the joint like you do with separate filler metal sticks.

Even the heat required by brazing is likely going to get the metal up over the critical temperature, so the heat affected zone from welding or brazing is going to change the hardness. S7 is air hardening, meaning when it cools off in the air it retains its maximum hardness instead of requiring it to be dunked into water or oil. At that point you need to temper it at the correct temperature to get the hardness you require, and it's recommended you do it twice.