stengun build

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.

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

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.

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...

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 ). 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 )

"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.

TCB

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 (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" 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

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

TCB

a lathe... Do you have one? Or do you know somebody with one?

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.

TCB

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 )

TCB