Barrel ramp vs unsupported chamber….

[Jtm35a2]

First off this is not a mac specific/related project. But this seems to be the most active thread on the forum and I think the question is probably universal to smg feeding.
I am working on converting a BRP Stemple 1928 Thompson to 9mm using a suomi barrel.
I just got my bolt back from Dan Block. He converted it to 9mm bolt face. He also sent a converted suomi drum to try out.
I’m having some feed issues. Some are being caused by the drum feed angle. But while looking at it I noticed something else. The feed ramp built into the trunnion was designed very similar in shape and dimensions to a Thompson. Hence it was sized so that the small end of the ramp is sized to match a 45 bore that has a slight chamfer. The ramped portion of the 45 barrel is approximately .590”. The 9mm breech face is ramped very shallow and is approximately .490” and has a very pronounced lip at the same intersect point with the feed ramp trunnion. It’s only measuring.050 difference but looks like alot more.
How much can a 9mm barrel be ramped before unsupported chamber becomes a problem?
I figured I am not the first person wanting a smoother transition from the built in feed ramp into the chamber of an smg.

 

[Slowmo]

A picture might help. Supposedly a Suomi barrel and a Swedish K barrel are interchangeable. I’ve never messed with a Suomi, but a Swedish K has a very nice feed ramp.

 

[A&S Conversions]

So why haven’t you gotten the parts from BMP? If nothing else, get the parts and copy them for yourself. Why reinvent the wheel?

Scott

 

[Jtm35a2]

BRP doesn’t make the parts for 9mm Thompson

 

[Deerhurst]

They don't but they make a Stemple Suomi.

 

[Jtm35a2]

They don't but they make a Stemple Suomi.

You’re missing the point. I am making a 9mm 1928 thompson brp. I already have a suomi.

 

[Deerhurst]

You’re missing the point. I am making a 9mm 1928 thompson brp. I already have a suomi.

You said this in your OP:

"I am working on converting a BRP Stemple 1928 Thompson to 9mm using a suomi barrel."

This indicates you have a Stemple Thompson that is somehow in something not 9mm and making it a 9mm. See how we all have no clue what you are talking about? I believe BRP has all there parts to convert your Stemple from 45ACP to 9mm. From there you need furniture. The Suomi is already 9mm. Why the barrel and bolt work?

I am a bit interested and concerned what was done to convert your bolt to 9mm. Was it just welded up and re-machined? Probably could have bought another bolt.


You need to be clearer in conveying your question as I still have absolutely no idea what you are actually doing and what has been done.

 

[A&S Conversions]

That is great that you have the Suomi as a base. It is my understanding that 9mm has higher pressure than 45. So what are you going to use for a 9mm mag? I don’t know of a 9mm Thompson mag.

It would seem to me that the you could have the Thompson rear spine added to like a few Uzi mags. Since 9mm is a little smaller, I would think that the guide (the forward part of the Thompson receiver that guides the rounds from the mag into the chamber), would need to be resized for the smaller diameter 9mm rounds. Since there would be a small difference, it would be hard to add a small amount of material to the 45 guide to convert it to 9mm. I hope you have the skills or can hire the skills for such a custom project.

Scott

 

[Jtm35a2]

You said this in your OP:

"I am working on converting a BRP Stemple 1928 Thompson to 9mm using a suomi barrel."

This indicates you have a Stemple Thompson that is somehow in something not 9mm and making it a 9mm. See how we all have no clue what you are talking about? I believe BRP has all there parts to convert your Stemple from 45ACP to 9mm. From there you need furniture. The Suomi is already 9mm. Why the barrel and bolt work?

I am a bit interested and concerned what was done to convert your bolt to 9mm. Was it just welded up and re-machined? Probably could have bought another bolt.


You need to be clearer in conveying your question as I still have absolutely no idea what you are actually doing and what has been done.

Ok, let’s try this again.
1.I have a BRP/Stemple 1928. These come in 45 only. Brian does not make and has no interest in making 9mm parts for them.
2. i am using a 9mm suomi barrel for this conversion. Because the front trunnion is the one part of the 1928 that’s compatible with the other brp/stemple parts.
3.Dan Block made a bushing and welded it into the 45 bolt face to reduce it to 9mm size. His gun and Merle’s gun have run thousands of rounds this way. In fact Merle used his 9mm Thompson in matches quite alot. Again, because the brp 1928 uses no other brp/stemple parts. It’s very unique amongst all of Brian’s other offerings.
4.The gun is using a suomi drum with a thompson rib welded to the rear to fit in the Thompson magwell.
All I am trying to find out is how much a 9mm barrel can be ramped/throated before I run into problems with too much unsupported case. I just included details about the project to avoid the questions about why my suomi needs barrel work

 

[Jtm35a2]

That is great that you have the Suomi as a base. It is my understanding that 9mm has higher pressure than 45. So what are you going to use for a 9mm mag? I don’t know of a 9mm Thompson mag.

It would seem to me that the you could have the Thompson rear spine added to like a few Uzi mags. Since 9mm is a little smaller, I would think that the guide (the forward part of the Thompson receiver that guides the rounds from the mag into the chamber), would need to be resized for the smaller diameter 9mm rounds. Since there would be a small difference, it would be hard to add a small amount of material to the 45 guide to convert it to 9mm. I hope you have the skills or can hire the skills for such a custom project.

Scott

Yes, exactly. The face/throat of the 9mm barrel are smaller than the feed ramp of the 45 Thompson. As this is a conversion that I would like to use alternately with 45, i don’t want to modify the trunnion. I would prefer to throat/ramp the 9mm barrel. When it runs it’s awesome 716 rpm with cci 115. But when it chokes, it chokes bad. Cramming the 9mm projectile way back in the case and smashing the bullet nose.

 

[A&S Conversions]

…I would prefer to throat/ramp the 9mm barrel. When it runs it’s awesome 716 rpm with cci 115. But when it chokes, it chokes bad. Cramming the 9mm projectile way back in the case and smashing the bullet nose.

And there is the rub. A small peg going into a big hole, can tend to get sideways occasionally. That is the problem, right?

Champhering the chamber opening would certainly help feeding the rounds into the chamber. But like you had previously posted, what about the support for the round? The deeper the shell goes into the bolt and the higher up the shell is unsupported, the more likely the pressure in the shell can blow out and send brass shrapnel out of ejection port.

So without adding anything to take up , how are you going to guide the smaller diameter 9 mm rounds into the chamber? Could a guiding cone be added into the receiver that was held in place with a collar, by the barrel change mechanism?

Scott

 

[Jtm35a2]

And there is the rub. A small peg going into a big hole, can tend to get sideways occasionally. That is the problem, right?

Champhering the chamber opening would certainly help feeding the rounds into the chamber. But like you had previously posted, what about the support for the round? The deeper the shell goes into the bolt and the higher up the shell is unsupported, the more likely the pressure in the shell can blow out and send brass shrapnel out of ejection port.

So without adding anything to take up , how are you going to guide the smaller diameter 9 mm rounds into the chamber? Could a guiding cone be added into the receiver that was held in place with a collar, by the barrel change mechanism?

Scott

It would be tough to get anything in there. I can barely even get a picture of it. The round is probably a 1/8” in the bolt. So yeah, the unsupported area would be between the 1/8”and up to a 1/4” from the rear of the cartridge case.

 

[A&S Conversions]

So far you have not had a blown out sidewall of the 9mm? At one time I owned a Savage built M1A1. What are you using for magazines for the 9mm of your Thompson conversion? The Thompson mags were designed to be pushed straight out of the magazine into the guide in the front section of the receiver. Are you using a 9mm magazine that shoves the rounds towards the center of the receiver or are you relying on the guide that the Thompson uses, to shove the 9mm rounds towards the center?

Obviously, there is a big gap between the 45 chamber opening in the adapter for the Thompson and the 9mm barrel. IMHO it would not take much of a chamfer in the 9mm barrel chamber to cause sidewall ruptures of the 9mm shell casing. To me, I would think that you need a custom 9mm barrel that could fill the gap between the 45 Thompson chamber opening of the conversion and the 9mm chamber opening of the barrel but still have enough room for the bolt to fully chamber the rounds.

Something to consider, HK used a similar magazine for the MP5-40/10 that didn’t shove the rounds towards the center very hard, but they did use bullet guides inside the trunnion, to push the flat faced semi wad cutter bullets towards the center to enter the chamber of the barrel. Maybe if you had something that could clip in with bullet guides to push the rounds towards the center of the chamber.

Scott

 

[Jtm35a2]

I’m using a suomi drum which is center feed. So no left/right guidance is needed. Variable there is how much do the rounds wobble left/right after leaving the magazine before entering the chamber. The front of the drum is more than the length of a cartridge behind the chamber. So there is some “jump” space between the two.
I measured bolt face depth at .120”
A cartridge plunked into the suomi barrel leaves about .130” of the cartridge exposed.
So if the bolt fully closes before detonation. I have about a .010” area not enclosed by chamber or bolt.

 

[Jtm35a2]

Dan Block seems to think that the drum angle needs to be changed to give the round a little more up angle. He’s a Thompson expert, so i tend to listen to him. The drum he sent is angled downward, which he said his gun needed. Looking at the drum it is noticeable the top forward tilt. Although the round still has some upward angle in its presentation. I ordered some Thompson magazine adapter rail to build another drum.
Just figured I would ramp the barrel while I was waiting if I could figure out an acceptable amount of unsupported case that would be more than .130” forward of the rear.

 

[slimshady]

Early Glock, possibly later ones too, had 9 mm barrels with a rather aggressive feed ramp built into them. The cases had a bulge that expanded into that ramp, one of the reasons roll sizing once fired cartridge cases in 9 mm has become so popular.

Just Google Glock bulge and you will find hundreds of photos of damaged cases, some of them sectioned. It's easy to see where the ballooning of the case begins, measuring a few printouts with calipers should give you a a safe depth for the case to be unsupported.

 

[Jtm35a2]

I sent some pics and measurements to barrelxchange for his opinion on if the throat can be enlarged safely.

 

[amphibian]

The Ballistic Advantage EPC barrels have a pretty big feed cone and IME buldges brass too. I don't have a roll sizer. I modified a few of those barrels for the CMMG RDB system but the feed cone doesn't help at all with that system. So I prefer not to use that barrels since I reload.

 

[Jtm35a2]

Today I ramped and polished the barrel from about 4:00 to 7:00. Jams reduced substantially. I am seeing zero signs of case bulging. The only thing I an getting now is rounds that are hitting the face of the chamber around 10:00. I am kinda thinking the rounds are hitting the bottom of the feed ramp (it’s very long and gently sloped) and deflecting upward at a steep angle. Tonight I ramped and polished from 7:00 to 11:00. Tomorrow will test fire again. When the drum adapter material gets here from Philly ordnance I will build a drum with a little more up angle in the bullet nose. The drum i have is a loaner fron Dan Block and I am not altering his drum.
As a side note the rate of fire is clocking in at 690 with cci blazer brass 115 fmj. That rate of fire with 9mm in a 11 pound Thompson is amazingly controllable. There’s very little perceived recoil. I would put it on par with my U9SF BRP gun.
The long bursts I was able to enjoy today confirmed the project is a worthwhile venture.

 

[A&S Conversions]

Good to hear that it is running better. So what are you using for a magazine? Are you using the Thompson lower or something else? A picture would be great.

Scott