We have been doing the 357 Sig DI uppers for a while. Lately we have gotten a surge in requests for BB version of the Sig and have made a few. It appears that other barrel makers have halted production of the 357 Sig blow back barrels due to case failures. We always suggest doing the DI upper vs the BB. The Sig (and 10mm for that matter) require some real heavy buffer spring combinations in blow back operation.
We also got a case failure on the 357 Sig BB and are doing an autopsy to see if there is a root cause or random act. Until we figure that out we are putting the 357 Sig BB builds on hold.
Picked up at Glock 357 Sig mag yesterday and ran it in the PSA glock lower with our DI upper. The unit ran well, my loads were a bit soft so had a couple of bolt over jams. We'll pump up the loads a bit and test some more. Also ran some 357Sig loads in our SP mags and the extra case length didn't do us any favors. It ran some rounds but I think I'm going to shorten up the SP mag from 1.4 to 1.28 and try it again. Then again, with the glock lower, its a no brainer...
Another test day at the range with the 357 Sig. Ran the upper on the modified Glock lower (the one I cut for TCM mags) and function was good the first couple of mags but then got worse as the upper got dirty. Using Heavy Pistol powder might be a bit slow and dirty so we'll check into that. Carrier drag is another issue on the Glock lower so we need to check into that some more. I did not bring the TCM mag to try it.
Also brought a SP mag for testing on stock lowers, but the larger dia case didn't play well with the ramp profile.
Another DI Sig build and we modified the gas system to reduce back pressure and operation was better. The previous build needed our Light / 1W buffer spring set up and this last build locked the bcg back with authority. Loading up more ammo to test with standard b/s set up.
Performed the autopsy on the BB sigs we got back and didn't find anything glaringly wrong. Put some extra polish on the chamber and test loads fired and ejected 100% with no failures. I do have to admit, that is a powerful round in a blow back and with the new Glock lower set ups, I see no reason to beat yourself up with a blow back Sig. Yes it is a simple operation, but the weight and springs you have to put behind the round makes for a jackhammer in operation.
BB Update: From what I can tell factory 357 Sig loads are 7 gr of Bullseye with 115gr JHP. That is some fast powder and generates around 42,000 psi of pressure according to Quick Load. That's an awful lot on a blow back AR.
I'm out of bullseye so I started ramping up loads of Unique to try and push the Sig in blow back. Started with 6gr loads 1500 fps est from my 16" test barrel, standard spring and 5.3 oz heavy buffer. Load was vary tame, est at 25ksi 6.5 gr woke things up a bit, but very pleasant to shoot. Heavy buffer set up slams home hard, so you have to deal with that. Est 1600 fps @ 27.5 ksi 7.0 gr and the recoil is lively. Est 1700 fps @ 32.3 ksi 7.5 gr and the upper is starting to bark. Est 1750 fps @ 37.6 ksi
8.0 gr and got a split cases on 2 rounds tested. Est 44 ksi. Now, I do have to admit I don't have any factory ammo..... I was using 40 S&W brass which is only rated for 35,000 psi so definitely pushed the limits. But, it also shows the limits of the Blow Back AR's
UPDATE: Got two boxes of factory ammo, S&B 115gr and Win White Box 124 FMJ. Pulled one of the Win loads and got 9 gr of a ball/semi flake powder. QL puts pressure through the roof for 231 or Bullseye, so it has to be a slower powder than that. No Chrono so can't work back from that.
Other issue is the cases are longer (to the shoulder) than my reloads by .005 - .010" So that means my go gauge needs to be checked against my die set.
Put in my custom 9 oz buffer and the BB upper ran fine. Like swinging a sledge compared with the DI, but it ran....
I made a set of gas blocks this week for the Tok / Sig / 10mm builds. Working with the 357 Sig and doing another DI barrel. Because we had a case separation failure I double checked my gauges and did some re-research on the 357 Sig.
The round still seems to have people asking: How do you headspace the 357 Sig?
Normally, any regular bottle neck case headspaces on the shoulder...… normally. The 357 Sig headspaces on the MOUTH. Lots of people, myself included, have run 40 S&W brass through the Sig dies and created 357 Sig rounds.... in error. They are short. The Sig case is .865" and the 40SW is .850" This, with the case headspacing on the mouth, puts you .015" short or, wherever the shoulder hits.
Having 4 different brands of ammo, I did some measuring:
Longest case I found: .860" (.005" short) Shortest case I found: .853" (.012" short) 40 S&W "sig" case: .843" (.022" short)
So, how does a case that is .012" or .022" short function in a firearm? Per the internet, there are some people claiming to headspace off the extractor.... Having a case that is short, I am sure that some rounds fire under this condition. I certainly would never chamber a gun off the extractor.
Having a .007" difference in case length and all being shorter than the .865" Spec, with a bolt or locked breach gun, things would function. So my question now is: where is the shoulder in relationship to the mouth? To test that, I measured my longest case (.860) against my shortest case (.843) and checked the difference. Mouth to mouth is should be .017". In the chamber the short case measured .011" short. (or net .849") To do a true test, I need to cut the neck off a same sized case to get a real comparison.
Now lets apply this to the Blow Back AR:
In my limit work with the blow back, the rounds need resistance from the bolt to prevent case damage. If the round had .010" of headspace between it and the bolt, there would be zero weight resistance to its rearward travel at the moment of ignition. If the neck/shoulder joint expands and there is no resistance to the base, the case is going to stretch and fail if given enough room. When I chamber a blow back, I always make sure the bolt seats on the case and not the barrel. This ensures that the round always has some weight resistance from the bolt at ignition. Now, take the Sig with its higher pressure and (from my small case sample) up to .011" head space, this could ( and apparently has ) created case head separations with the 357 Sig.
From what I've gathered with seasoned 357 Sig reloaders, they use their barrels as the case gauge and modify the die or shell holder to make sure the shoulder is .003" short of the case mouth. Then if the case is excessively short, the round will headspace no more than .003" short on the shoulder. Others modify their dies so that everything headspaces on the shoulder only. Issues people take with that has to do with neck tension which is important with the Sig, especially since there isn't much of a neck there.
We are going to research this some more and make a set of gauges to chamber with. And then send this case length information with each barrel so the shooter knows what they are getting in to.
Just did a 5 round test fire on the GenII Sig / 10mm G (glok) set ups and both ran 100%.
This is the NFA 80% lower I finished the other day with our new DI Sig set up and new gas blocks. I managed to run a batch while I was at the main shop a while back. New gas blocks use a straight custom gas tube so set up is much quicker than when we bent them in a jig.
May think about adding a gas adjustment screw to the gas system to help tuning, but we'll do some more load testing and see how it goes. Maybe back off on port size a bit.....
I do know that there is no reason for a BB 357 Sig or 10mm anymore! Bye Bye Jackhammer!
Range tested today with the 357SiG (glok lower) and it ran 100%. Ran a box of S&B through it and it needs a good cleaning! Glok lower and mag set up fed 100%. Didn't do any chrono work, I'll have to save that when we get to our range, hopefully next weekend. Good News!