6.8 SPC Build Short Stroking Help

Parts list:
Upper:
BCM blemished receiver
BCM Gunfighter CH
PSA Chrome BCG
Wilson 6.8 SPC II 1:11 16” lightweight barrel
Wilson Low Profile Gas block
BCM midlength gas tube
MI Free Floating G2 SS 12”
PSA A2 muzzle device

Lower #1:
POF receiver
POF LPK
Geissele SD-E
VLTOR A5 EMOD Kit

Lower #2:
Sun Devil receiver
PSA LPK with PSA trigger
POF Carbine RE
POF Carbine Buffer
POF Carbine spring

Range Trip 1:
Ammo: Federal 115gr Fusion MSR
Magazine: PMAG 20 rdn. I would only put 2-3 rounds of 6.8 in the PMAG at a time

Only brought lower #1 as this is the lower I would prefer to run this upper on. Initially had some FTE, the gun was short-stroking to such an extent that the brass wouldn’t clear the chamber and be ejected, this was for maybe 2-3 rounds then the rifle started ejecting but FTF.
Fix 1: Removed gas block and tube and inspected gas-port alignment, everything appeared fine, reassembled, and lubed up the BCG.

The extra lube appeared to help a bit, the rifle now chambered the next round most of the time but would not lock back.
Fix 2: used a known good BCM BCG in lieu of the PSA BCG.
No difference in operation

I fired 45 rounds the entire trip, the rifle locked back on an empty mag once.


Range trip 2:
Ammo: Fed 115gr Fusion MSR
Hornady 110gr VMAX
Magazine: PRI 6.8 15rnd (finally got it in the mail)

Tried Lower #2 right off the bat with Fed Ammo. Immediately had failures to feed the next round. Replaced with Lower #1 and it would feed the next round but would not lock back.

Hornady ammo did worse, failure to lock back guaranteed with approximately 50% failure to pick up the next round

Again I pulled the gas block and this time tried slightly different positions while firing a couple rounds. Noting the location of the carbon circle made by the gas block port at each position. I reverted back to the original position once I was 100% sure that circle was indeed created by the gas block port and the original position was correctly lined up.

At this point I used the PSA Bolt Carrier in my 5.56 build and it ran fine, also the gas rings on the PSA bolt are in good shape, a bit tighter than my other bolts, but its brand new while the others have a couple thousand rounds through them so I don’t think this is a BCG issue. Also the carrier without bolt slides forward into the receiver without any binding. So I don’t think gas tube alignment is an issue. I am not seeing any excessive carbon build up around the gas block so I do believe the seal is tight.


Anybody have any idea as to what else I can to narrow this issue down? I checked the gas port when I came home and there is no obstruction. And using just my eyeballs and a digital caliper it measures somewhere between 0.080-0.86. (Wilson advertises 0.082) I am surprised that it would not run with the carbine spring/buffer combination. Both these lowers run 100% with their uppers. 14.5” BCM middy on Lower #2 and 16” build with Noveske barrel on Lower #1.

I haven’t been able to find SSA’s hotter “Combat Loads” to try but it does suck not being able to run standard factory ammo.

Thanks guys

I'd try a different mag, one specifically designed for 6.8. (nevermind I see you did that)

I know you checked it, but also gas block alignment. I had the same thing happening on a MK12-ish upper I built. Single shot, even suppressed. Took it home, realigned the gas block, tightened and staked the gas key screws (unknown BCG) and it was fixed.

I've never had any issues with my Bison Armory 6.8 barrel, but I was having light primer strikes. It had a Spike's M16 carrier and an LMT enhanced bolt. Changed the carrier to a BCM and it fixed it.

Some things just don't want to play nicely together, its a bitch to figured out what the problem is but it sounds like you're doing it right. It just can get expensive, especially with 6.8 ammo prices.

Good luck. Sorry it isn't working and I don't have any better input.

Pyro,
Generally speaking, getting enough gas volume is not usually an issue with 6.8, but that certainly sounds like your issue. We find that standard gas blocks, carbine buffer and springs, will usually run fine with our barrels and a good bolt. If you can get your hands on another 6.8 bolt, I would try that next. If there is anything we can help with, send me a PM and we will be glad to help.

Thanks for the quick replys. I'll look for and order a new bolt and see if that fixes the problem.

Thanks again.

Most everything in your build looks good.

Check the alignment of the side hole in the gas tube to the transfer port in the gas block with the assembly removed.

You should be able to drop the shank of a 1/8" or 7/62" drill bit through the rear gas block set screw and get it to stick in the gas tube side hole.

It is possible for these to be misaligned, restricting gas flow.


The only other thing that may be suspect is the chrome bolt.

What is the diameter of the bolt tail?

Clint,
Thank you for your post.
1/8" drill bit fit in the gas block/tube assembly. There was slight resistance once it entered the hole in the gas tube but everything lined up pretty good.

The Bolt tail OD measures .249-.250 depending on the mood of my digital caliper.

Thanks again.

Gas block alignment with the gas port would be my guess. I just went through the same issues with my 6.8 build.