stengun build


December 16, 2013, 07:11 AM
So I finally started buying my parts to build a stengun. im using a mark 3 parts Kit, and a Indianapolis ordinance semi auto conversion. I was wondering if anyone has information on calibers, specifically the 357 sig and weather or not it could safely work in a build like this. I see that the sig runs about 10k psi hotter. could the action handle that? what would I need for barrel thickness. my goal is to end up with a 16" barreled carbine.

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Jim Watson
December 16, 2013, 09:36 AM
I don't think barrel thickness would be the problem. Balancing the blowback action for the hotter round might be. Getting the fatter cartridges to run through the magazine is another.

December 16, 2013, 09:58 AM
stick with 9mm,all parts are calibrated for the cartridge.going to 40 cal may mean different
spring rate and where are you going to get higher rated springs?then your talking about having a custom barrel made.,which will be expensive

Mauser lover
December 16, 2013, 11:17 AM
Why do you want to go to .357 Sig?

You are going to have big issues, not just with the pressure, but also bolt face size, magazine feeding problems, spring tension, possible extraction issues, etc.

If you really want a Sig, you could probably do it, but it will be pretty difficult. And expensive to shoot.

December 16, 2013, 11:40 AM
alright then. I'll stick with the 9mm. another question is if it would be safe to solder the trigger housing to the tube vs welding. based on my understanding the housing doesn't take a lot of pressure?

Mauser lover
December 18, 2013, 07:37 PM
Personally I would weld it even if it was safe to solder, but that is just because I would be confident in my welding skills, and I cannot solder to save my life. Yeah, I know, soldering is easier (I don't believe anybody who tells me that...)

Basically, I would have huge assurance issues if it was soldered, but most people (who have actually tried) can solder better than me.

I cannot really comment on the strength of soldering, but at least this is a free bump to the top...

December 18, 2013, 10:07 PM
IIRC, the original STENs were brazed, so there's that precedent. If you have no welding setup solder should be fine for everything but attaching the barrel/trunnion. Nothing sees pressure or significant force in a blowback tube gun but the barrel, trunnion, and end cap. The trunnion most likely sees its highest load when the bolt slams home on an empty chamber. The worst that will happen with silver solder instead of brazing or welding is that your trigger group begins to crack, hopefully slowly and not suddenly, and what little recoil the gun generates may knock the gun off your trigger hand and cause you to drop it. You'll probably get a misfire, though it is possible the misalignment of the cracked housing could reduce sear engagement to the point of dropping when the bolt chambers (if either happens, fix it immediately, duh :rolleyes:). Soldering is commonly used to permanently attach muzzle breaks, so it's capable of taking a good deal of shock --just not bending/fatigue since it will crack

Remember that the semi-auto closed bolt conversion can't fire without the fire control group installed, so if that part breaks away it's not like the gun will zip fire out of control or anything ;). If it breaks you'll probably just end up looking stupid (probably in front of a girl, no less :D)

"Why do you want to go to .357 Sig?

You are going to have big issues, not just with the pressure, but also bolt face size, magazine feeding problems, spring tension, possible extraction issues, etc."
Ditto. Bolt face mods are not insurmountable (just a single lathe operation and trim extractor to fit), but the 357 is quite a bit more powerful than 9mm. These guns are safe to use because the bolt is heavy enough compared to the bullet's momentum that its inertia keeps the breech closed long enough for pressure to drop; raise the momentum (bullet weight), pressure, or pressure duration (the SIG does both of the latter, I believe) and more bolt mass is needed to keep the pressurized case from squirting out and bursting. The STEN bolt is pretty heavy, so maybe it could be okay (I'd look to see if others have done this first), but you'd be eating into the margin of safety built into the gun. And having been modified to closed bolt, some of that margin is already gone since the bolt now has zero forward momentum when the primer ignites. Mag lips would need adjustment, too, FWIW.


December 18, 2013, 10:38 PM
I have heard of 40s and 10mm stems. but I've not actually seen one. the bolt itself, with my conversion is also made to run the 7.62x25 and the 9mm. I checked a 40case to my bolt face. and it fits. my trunions will be riveted, and the mags will stack the 40. so I guess now its a matter of can I take a 9mm barrel, and run a finish reamer in it to make a 357sig chamber or would I auctually have to cut the old chamber out and reprofile it? thanks for the help!

December 19, 2013, 10:25 PM
You'd need to compare the dimensions of the two cases. If the 357 is bigger in all of them, a piloted reamer trued to the barrel on a lathe should have no problems (doing it by hand is obviously more fraught). I would also suggest the 9mm Dillon or whatever it's called (10mm necked to 9mm, but original length, to surpass the SIG) if you will primarily be reloading since you would be guaranteed a clean chamber with a chamber that much longer. I forget how roomy the STEN mags are as far as the extra length, but 7.62x25 mags (PPSH, PPS43, CZ24/26) will fit the 10mm sized rounds without much change to the magwell.

"I have heard of 40s and 10mm stems. but I've not actually seen one."
Like I said, the STEN bolt is heavier than it absolutely needs to be simply to keep the full auto rate of fire down, but I'd still want to examine the brass off one of those guns before taking their mere existence as an affirmation. For all we know, they could have a swell all around the base like my semi-auto 22LR does :eek: (obviously with a greater potential for problems). If you reload you can load to whatever works and is safe, but I'll bet a necked round like SIG stretches quite a bit in a blowback action (for reference, 5.7x28 stretches something like .040" :what: in the five-seven semi-delayed-blowback pistol) so its life will be shorter than say, 10mm.

I love the 10mm case head rounds, since I think they have just big enough case capacity to actually take advantage of the 16" of barrel we're required to have by law (9mm is slowing down at that length, IIRC) as well as more rifle-like powders that burn slower to achieve higher velocities. Unfortunately I also worry that they are bumping into the practical limits of a blowback action when loaded hot, and that something like 10mm in a 40SW HiPoint carbine is needlessly close to the edge. But if you proceed with caution (as you will anyway) you shouldn't have any problems :cool:

357SIG will also reportedly feed from 75rnd Suomi drums --just sayin' :evil:


December 25, 2013, 11:22 PM
okay, my new bolt has a recessed bolt face, and I'll need to open it up a few thousandth to allow the 40casing to fit, is there a tool made specifically for opening up the face?

Mauser lover
December 30, 2013, 12:11 PM
a lathe... Do you have one? Or do you know somebody with one?

December 31, 2013, 05:39 AM
indeed I do know a couple guys who have a lathe, and they are willing toto help me with the bolt and barrel. is im planning on making my bolt face.002-.003 larger then the diameter of the357sig base to start off with then making slow adjustments as needed. anyone know what to do as far as coming up with 357sig headspacing gauges, they seen kinda hard to find, and preferably I'd like to buy a set, as far as the barrel goes, im making one from an old iwi timberwolf barrel, its beefy enough to fit the build requirements as far as my trunions go, and it looks clean

December 31, 2013, 11:47 PM
The STEN has no locking in the bolt operation, so can headspacing really be that critical? The bolt face will press against the cartridge in the chamber regardless, so it's not like the case will stretch to failure or anything like it would in a locked breech where the round would be "loose" in the chamber. I always thought chambering on a closed-bolt blowback tube gun was to cut the chamber as deep as possible (to best support the case head) with the bolt barely bottoming out on the barrel when a cartridge was present.

I have read that original barrels meant for open-bolt operation had shallower chambers that helped ensure the primers would go off since they would cause the bolt (and its fixed firing pin) to "squeeze" the round harder. That obviously makes pressures higher, which is detrimental in a closed-bolt build.


January 10, 2014, 11:26 AM
Thanks for the information, I did decide to go ahead and make it a 9mm. My reamer should be here tonight

January 12, 2014, 06:18 PM
ok, my trip lever is not engaging my bolt, how should I go about fixing it?

January 12, 2014, 11:05 PM
Trip lever? Do you mean the sear (striker build as opposed to hammer build)? Striker follow (not being caught by the sear) can be caused by a number of things you might check on;

-striker override (the striker moves vertically and shoots over the sear)
-sear protrusion (sear doesn't stick up far enough for sufficient engagement)
-faulty disconnector (sear stays pulled down after first shot)
-sear/striker geometry (sear bent [hook] is rounded or angled so the striker can slip off)
-probably not the issue, but striking the right balance between striker and return spring can alleviate all sorts of problems (but normally feed/extract related)

This striker follow issue is definitely something you want to take care of before taking it out for a test fire, since it can easily lead to anything from full-auto to out of battery primer strike (at best, it will simply not work and frustrate you :D)


January 13, 2014, 07:15 AM
it is not the sear itself, it is called a sear lever/trip leaver and it works like a disconnect, but when I charge it it holdsthe striker, but the trip lever doesn't disconnect, but the sear does release if I let off the trigger. m fcg is 100% original

January 13, 2014, 07:18 AM
it is a striker fired system now, and in the event that the sear fails the striker "should" ride with the bolt and prevent slam fire, the firing pin does not protrude from the bolt with the strike fully seated seated on the bolt

January 17, 2014, 10:42 PM
I was able to test fire it, I have some feed issues, hopefully a Lil polish will fix. I only got one round shot. but when I inspected my brass the case mouth got shaved down to a sharp taper, any ideas on what happened? I kinda think that because it didn't fully cycle the case got slammed back into the chamber.

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