Home Forums Hunting & Shooting AR and Tactical Rifles Help? Having an issue with a new build. Advice needed!

So I was out today for an initial run with a newly built AR 308. I’m having an issue getting it to cycle correctly. It’s built mostly with Aero Precision parts Including M5 Upper and M5 Lower, 308 Carbine buffer tube, buffer, and spring set, Ambi charging handle, Nitride rifle-length gas tube, and ATLAS S-One rail. I used a Faxon 18” Pencil Barrel (rifle gas length), a VG6 Epsilon muzzle brake, and a Superlative Arms Adj. Gas Block. Finally for the all important BCG I chose a Sharps XPB DLC (the 308 sized Relia-Bolt).

Now for the problem, I have it running partially. The rifle will fire and the bolt will lock back on an empty magazine. However, the rifle is not extracting properly. For most rounds fired, the bolt ends up locked back fully to the rear and the fired case is still in the chamber. Sometimes you can tip the barrel up and the case will fall out. Other times I can release the bolt back forward and then pull back with the charging handle and get the case out.

I have dialed the Adj. gas block up and down some, but can’t seem to hit paydirt (consistent cycling and ejection). I have built a few other AR’s and this is the first time I have run into this.

Is this a gas system issue, headspace, or a BCG or extractor problem. What should I try next?

Mark in GA

The rounds that do eject, where do they land? (If the muzzle is 12 o’clock and the ejection port is at 3 o’clock)

I’m sure it’s just overgassed. You need to turn the gas down, probably a lot. A donut on the extractor can help.

What spring and what weight buffer are you using?

Its an Aero P. standard 308 Carbine Buffer Tube, Buffer, and Spring set I bought as a kit. Not sure what weight.

When I was turning the Adj. Gas Block down it would get to the point that the bolt didn’t lock back on an empty mag. At that point, the case either didn’t make it out of the upper, or if they did they were dropping just outside the ejection port onto the bench top. I only had one out of 25 or so that came out like I expected and it landed about 6 ft out the right of the rifle at roughly the 3-4 o’clock position. Most never made it out of the rifle.

Mark in GA

Take the BCG out and try to move the extractor. If you push on it with your finger can you get it to move?

Pretty hard to diagnose not see seeing all the parts but I think Blue is going the right direction with the extractor being the problem.

Never used a Sharpe BCG so no idea about their issues or not.

Take the bolt out of the carrier, remove the ejector & see if a case rim will easily slip under the extractor & if it appears to have enough surface gripped. Are there any extractor marks on the fired brass?

About all I can say at this point other than try another bolt or another extractor if you have on available.

MM

So the problem is caused by the extractor slipping off the case rim before the carrier moves back. And that's typically caused by two things...
There's too much gas.
The excess gas starts moving the carrier while the case is still expanded in the chamber and the extractor doesn't have enough tension to pull it out of the chamber.
This can be mitigated in a few ways, or a combination of them:
1-Increase buffer spring tension or buffer weight to slow the initial movement of the carrier.
2-Reduce the gas coming to the carrier.
3-Increase tension on the extractor.

If you're having the problem when the gas is turned down that low this likely isn't the problem, or the solutions.

There isn't enough tension on the extractor, period.
See if you can move the extractor with your finger. If so, you almost certainly don't have enough tension on it. I'm not sure if .308 extractors use the same springs or not, but I prefer Sprinco springs in my 5.56 guns and you can add a donut if you want or need.

Springs are an area where manufacturers seem to try to save a little money and it wouldn't be unusual to have a new BCG have bad springs.

________________

MM might also be onto something about the extractor being out of spec, too.

That definitely sounds like an extractor problem, either it's damaged or not enough spring tension. Remove the extractor and inspect the hook area, make sure it's not broken, chipped, or rounded off. If that's OK, add a little O-ring around the spring, what the other guys are calling a donut.

OK, so I pulled the BCG out of the rifle. With the bolt still in the carrier and the bolt in its rearward (ie like where it would be if it were locked in the barrel extension), I can push it out from the center with my with my finger pretty easily. I think this may well be the right track.

When you all say o-ring, what exactly do you mean? Just a rubber ring?

Mark in GA

"...When you all say o-ring, what exactly do you mean? Just a rubber ring?....."


Yes, it goes around the extractor spring under the extractor.

Here's some,

https://www.midwayusa.com/s?userSearchQuery=extractor+o-ring&userItemsPerPage=24

https://www.sprinco.com/ar-extractor.html

They’ve got an option for .308s.

Don’t just get an O-Ring. Replace that crappy spring too.

Parts ain’t parts.

An o-ring and an H3 buffer will probably get you sorted out.

Look for aggressive extractor marks on the rim of fired brass, that's a telltale sign that the extractor is jumping the rim.

The heavy buffer will retard unlocking, and the o-ring will give the extractor a stronger grip on the rim.

I was able to swap the extractor spring over from the bolt in my other 308 AR rifle today and give it a try. Runs like a champ now! So the issue is a weak/bad spring under the extractor.

Thanks for your help guys!

Mark in GA

Ejector or extractor? I think you mean extractor since that's what we've been talking about?

Yes sorry, meant extractor. I have corrected my post.

Put a heavier buffer in it to slow the cyclic rate, I quit using standard weight buffers and either use an H3 or a custom from Slash's Heavybuffers.com depending on what I want from a rifle.

Unless you're running something suppressed or very unusual, it really shouldn't need an H3 buffer.

Glad it worked.

No heavy buffers! Dang, a 308 carrier is heavy enough!

Every one of mine has a heavy buffer, I have heard the same thing repeatedly from fellow shooters/lease members until they shoot my rifles. from 243 up to 358 Winchester, they are a dream to shoot and they don't beat up brass.

I chased this with one rifle until I was ready to part it out, read an article by Slash on ARF.com, ordered his buffer kit, no other changes and the rifle hasn't missed a beat since I installed it.