i had the same problem with my 7.5" ar pistol. this fixed mine.
I took my 6.8mm SPC AR-15 SBR out to the range Sunday and had immediate failures to feed, meaning it started right away, not after a number of rounds had been fired.
Pertinent rifle specs are:
11.3" Wilson Combat barrel, carbine gas
Wilson Combat AR-15 BCG (not M-16)
Standard carbine buffer spring
Ammunition was hand loads using Hornady brass, CCI #41 primers, 27.5 gr H322, Hornady 110 gr BTHP bullets, and a COAL of 2.240+/-.005". These are not max loads by any stretch, but I didn't want to increase the powder charge more due to the use of the CCI #41 primers. Magazines used were a AR Stoner 25-rd, and a PRI 15-rd. I had just used the PRI mag in my 18" AR with no issues.
The failures were about 50/50 with the empty case left in the chamber and a fresh round trying to jam in behind it, or an empty case fully out of the chamber with a fresh round jammed against it trying to get into the chamber. It happened nearly every round, but every now and then I could fire 2 rounds before it jammed. The bolt would lock back on an empty magazine, and when it did eject cases, they were landing 6-8 feet away at the 3:30-4:00 position.
My initial thought was that it is overgassed, with the bolt unlocking and moving rearward before the chamber pressure drops to the point of "releasing" the case, and the extractor stripping over the rim. However, the ejection pattern would suggest that it is not overgassed, and there is no unusual damage to the case rims from the extractor.
I have a few things that I am going to try right away, including swapping out the AR-15 BCG for a M-16 BCG, swapping out the H buffer for an H2 buffer, and trying a few other types of ammunition. If it is overgassed, the correct solution would be a new barrel, adjustable gas block, heavier buffer, and heavier BCG, in that order by my thinking. Hate the idea of a new barrel, and dislike the adjustable gas block idea because the one I have now is also a flip up front sight. I haven't been able to find adjustable gas blocks with flip up front sights, so that would drive me to an adjustable A2 sight/gas block, or a low profile adjustable gas block, new hand guard, and new front sight. A heavier buffer and/or BCG might compensate for the overgassing, but are really just masking the real issue.
Any other thoughts from the forum?
i had the same problem with my 7.5" ar pistol. this fixed mine.
I'm not expert.......I read something about this a while a go......bolt dint move fast enough or moved out of battery before the fired case could let go of the chamber wall, but FTFeed & FTE .....I'd look at the bolt..... the failure to feed tells me bolt..........possibly a bad barrel.....feed maps out of spec or chamber out of spec.....
Good luck....keep us posted
If you load only one round, and it feeds, ejects and locks back, suspect extractor spring as mentioned before. Occam's razor would suggest the simplest explanation is right, which in this case happens to be the cheapest to fix. Start with the extractor spring and replace with one that has a rubber o ring or rubber insert. I bet when you remove the old one, you will find a wimpy little spring with no rubber. If the $3 fix doesn't do it, move on up in price and pita factor to the next most likely failing part.
take bolt out of rifle. Put empty case on bolt face (hooked under extractor,held in line with bolt), then let go. does it fling it? take a punch and push the ejector in/out, does it move freely/smoothly? clean under extractor.
what extractor spring? basic spring? rubber insert color? rubber washer around spring? I'm partial to the colt GOLD extractor springs myself...
COLT AR15A4 EXTRACTOR SPRING ASSEMBLY | Brownells
I've found it works on everything from 20" guns to shorties w/o damage to case rims like some of the rubber washer solutions (but I've not messed with non-std AR calbers like 6.8)...
brass full length resized or neck only?
since you are handloading but don't have an adjustable gas block, load a few at min charge and see if you can get it to run. Me, I'd prefer to fix an over-gas problem at the gas port vs by adding reciprocating weight, but the later could make it run, too. But I'm not so sure it sounds like overgassed. what if you shoot it with a loose grip, not pulled tight into your shoulder?
Ryan V. B. TY56060 Come shoot USPSA w Ft Wayne Area Practical Shooters: www.facebook.com/fwuspsa
Thanks for the feedback everyone. I cleaned under the extractor, and checked the ejector, and it moves freely. I also chambered empty casings and pulled the charging handle back, and they seemed to eject just fine. I've got another rifle and bolt carrier assembly I can try, swapping the BCG from one rifle to the other.
Thanks for the links. I kinda like that Colt extractor spring assembly, and think I may order a handful of those.
I'm going to try and get out tomorrow to do some troubleshooting, and will have o make a list of things to try, including different holds (loose versus tight to the shoulder).
Didn't get to go out today, but hopefully I can sneak out tomorrow before my daughter's recital.
I did break down the bolt today just to give everything a good cleaning, and to my surprise, the extractor is non-standard. It actually uses dual springs, and they are not the normal spring and insert. They are longer and smaller in diameter with no insert. I think I will still order some of the Colt springs for my other AR bolts.
Running the gun with another proven BCG that you have history with is going to be the best tell. If that doesn't work I would be shopping for buffers or an adjustable gas block.
It just so happens that I also have another Wilson Combat BCG with a 223/5.56mm bolt, and it has a standard extractor setup, so maybe this is something specific to them on 6.8mm bolts. I have two other 6.8mm bolts, and they also have standard extractor setups.
I was able to get out again today, and the good news is I had no failures using the same BCG as last time. I was using hand loads again, built with the same bullet, primer, powder charge, and overall length as the ones that gave me problems, the only difference this time was these were loaded in SSA and Federal brass. Magazines used were the same as last time as well.
First magazine was a PRI 15-rd, and I single loaded and shot three rounds (SSA brass) with no issues. The rounds ejected to about 4 o'clock, and the bolt locked back each time. Then I loaded two rounds into the mag and fired them. Both ejected to about 4 o'clock, and the bolt locked back.
Second magazine was the other PRI 15-rd, and I single loaded and shot three rounds (Federal brass) with no issues. The rounds ejected to about 4 o'clock, and the bolt locked back each time. Then I loaded two rounds into the mag and fired them. Both ejected to about 4 o'clock, and the bolt locked back.
Loaded five rounds (SSA brass) into the first magazine, and fired all five with no issues. All rounds ejected to about 4 o'clock, and the bolt locked back.
Loaded five rounds (Federal brass) into the first magazine, and fired all five with no issues. All rounds ejected to about 4 o'clock, and the bolt locked back.
Loaded fifteen rounds (SSA brass) into the first magazine, and fired all five with no issues. All rounds ejected to about 4 o'clock, but the bolt did not lock back.
Loaded fifteen rounds (Federal brass) into the first magazine, and fired all five with no issues. All rounds ejected to about 4 o'clock, and the bolt locked back.
Loaded twenty-five rounds (SSA brass) into the first AR Stoner magazine, and fired all twenty-five with no issues. All rounds ejected to about 4 o'clock, and the bolt locked back.
Loaded twenty-five rounds (Federal brass) into the second AR Stoner magazine, and fired all twenty-five with no issues. All rounds ejected to about 4 o'clock, and the bolt locked back.
Here is the ejection pattern. I'm sitting at the bench I was shooting from, and the target line is to the left.
Here is the rifle.