Just finished up my first SBR

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alemonkey

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6.8 SPC, 12.5" barrel. It's a "dissipator" type build - the real gas block is under the handguards, the front sight just holds the handguards in place.

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I finally had time to take it out after work last week. I only had time to run about 10 rounds through it, and I had quite a few malfunctions that I'm thinking were due to short stroking. The case was fully extracted from the chamber, but kept getting stuck lengthwise in the ejection port, kind of the opposite of a stovepipe. I think the bolt wasn't coming back far enough to fully eject, and it was jamming the case mouth against the front of the port. I need to mess with it some more, maybe try a different buffer. I believe it's just a standard carbine buffer right now, since that's what came with the stock.

Accuracy wise it seemed ok, but I was just shooting offhand at 25 yards to function test. It's going to need a shorter front sight post. I turned the sight almost all the way down and it's still shooting slightly low.
 
I would first look at the obvious......the ejector. I assume your using a brand new bolt so it may not be the issue. But I would check to see if the ejector is flush with the locking lugs, and I would try to use a small punch to depress the ejector and check for a good strong ejector spring return. If all is well there, then maybe trying a lighter buffer might work.

On a side note, I had the same problem when I built my SBR where the case wouldnt fully eject and jammed in a fashion similar to yours. My issue seemed to be that the gas was so violent in cycling the bolt that it almost seemed to blow the bolt rearward with so much force that it was ripping off from the spent case and failed to fully extract it causing a similar type of jam like your having.

A heavy duty extractor and spring with a D-fender ring along with a H2 buffer seemed to cure it. Runs fine now.

A little break in may be needed also. Id look at a few of the cheaper simpler fixes first.

Good luck, I know how frustrated you must be, I was too..........
 
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What ammo? As said, verify that your ejector has good tension and double check your gas system that everything is tight (loose gas keys can be a problem).
 
Thanks for the suggestions. I'll start by looking at the ejector and double checking the gas system. It's a new bolt, and I'm using handloads - 28 grains of Reloader10x and a 110 grain Sierra Prohunter.

The flash hider is actually an ITS HF MCD comp (that's a mouthful!) from AR15 Performance. It seems to do a pretty good job of keeping the muzzle down without generating a lot of extra noise.
 
The guys over of the 6.8 forums suggested I cycle the crap out of the bolt by hand to break it in. I have noticed the bolt on this gun is a lot stiffer than my other AR. It takes a pretty good tug to rack it. I'm going to give that a try. I'm almost certain the gas block is properly aligned, the gas key is tight, etc, so if there is a gas system problem it might be the port size. The plan is to manually cycle the bolt a few hundred times, then take it to the range and load one round into a mag and see if it locks back. If I still have problems I'll try the bolt carrier from my other AR and see if that makes a difference.
 
When testing a new AR build, I'll load a single round into a magazine, slap it home, release the bolt, and fire the single round. If it locks back then I know the "stroke" is fine. If it doesn't lock back, then I know I have a cycling issue. If it fails to lock back, I would go ahead and try a different magazine. If we fail again to lock back, I can be reasonably confident that the issue is short stroking. (It should lock back weather you have a good extraction or not.)

If it extracts and expells the spent casing, then I know the extractor/ejector system is working as it should.

Next, I'll load 2-3 rounds. If they cycle then I'm good as far as the mag lips go. Next is to load a full magazine.
 
I tried the single load test last weekend and it locks back every time, so looks like it's not short stroking. I just installed an extractor upgrade kit from BCM, so we'll see how it runs this weekend. The ejector seemed like it had plenty of tension, but the extractor felt a lot weaker than my other AR. I'm thinking maybe the extractor wasn't holding on to the rim tight enough for the ejector to pop it out of the port, like bkjeffrey suggested.
 
The carrier is very well lubed, no problems there. It's definitely not short stroking.

I was at the range all day today but didn't find time to test the rifle with the new extractor spring. Spent the morning pouring concrete for a new range and had to RO a match in the afternoon. Maybe tomorrow.
 
Nope, I just never got around to updating. My ejection problems are solved, but now it's having a different problem. The bolt is failing to strip the next round from the mag. Ordinarily I would say it's short stroking, but it locks back if I load just one round in the mag. I think the problem lies with my mags (C Products). I've heard the 6.8 C products mags often need to be tweaked to get them to feed right. I bent the feed lips up just a little bit and now I need to go try it again. If nothing else, D&H has announced they're coming out with 6.8 mags that should be available in a couple weeks. I don't feel like spending $40+ on PRI mags.
 
The magazines and ammo cost are the only things keeping me from building a 6.8 myself.
 
rjrivero said:
The magazines and ammo cost are the only things keeping me from building a 6.8 myself.
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Completely understandable, that kept me out of 6.8 for years. I finally got comfortable enough with industry support and jumped in. Happy so far. All my mags run well, ammo isn't too difficult to find, performance is great. I still use the .223 for high volume shooting though.

I even ended up with a brand new 20" barrel that go into a 6.8 service rifle build late.r :cool: For now, I am just keeping the dust off of it.
 
alemonkey[/quote said:
I just got back from the range and it's running correctly now. The feed lips on the mags needed tweaked.
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Nice. Thanks for the update. Enjoy the new rifle.
 
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