Diagnosing Case Separation and Bolt Lug Wear on 6 ARC Rifle

[camyoung603]

Hey all,
I’m pretty new to building rifles, so I might not have the right vocabulary to fully describe or diagnose everything here, but I’m hoping to get some input.
I just finished putting together a 6 ARC build and took it out for some initial testing. I’m running a RIFLESPEED gas block (set to 6) and shooting suppressed with an OCL Polonium.
I ran into a couple of issues right off the bat. First was a double feed—I'm pretty sure that one's on the mag.
The bigger issue came around the sixth round. When the range went cold, as I cleared the round in the chamber, the case separated and dumped powder all over the inside of the action, but the bullet stayed stuck in the barrel. Worth noting: I had just cleaned the barrel a few rounds before this happened.
I have some video and photos showing the chamber and the round, where you can see rifling marks indicating where the round is contacting the rifling. I’ll attach those for reference.
I’m trying to figure out if this is a gas issue or something else. Does the RIFLESPEED block only restrict gas, or does it also vent some off? I’m wondering if too much gas or pressure is being pushed back into the chamber and causing issues like this. I’m also looking into whether the throat on the barrel is too short—planning to contact the barrel manufacturer about that.
Ejection looked pretty normal—good pattern, not excessive—but when I tore it down afterward, I saw some noticeable wear on two bolt lugs (one above and one below the extractor). That wear has me thinking the timing might be off.
I've attached a couple pics of the bolt lugs as well.
Any ideas or similar experiences? Would appreciate any input.

 

[msgriff]

Are you aware of the two different types of 6ARC/6.5Grendel bolts, are you using the type which is matched to your barrel's chamber, and/or have you checked headspace with gauges?

 

[camyoung603]

Im ashamed to say I didnt know that. But yes I did check headspacing with both a go and no go.

 

[supercorndogs]

You pulled the bullet out of the case because the bullet was jammed into the rifling? That's what it sounds like to me. Was it a reload or factory ammo?

 

[DeathBeforeDismount]

This is why most people should buy factory guns.

 

[supercorndogs]

Gonna have to wait and see if this is a gun or ammo problem.

 

[Tictacticaltimmy]

This is why most people should buy factory guns.

Wow thank you for your insightful and valuable input to this discussion.



You are going to need to go back to the range and troubleshoot this by process of elimination.

My first thought is that your barrel chamber was cut too short, but I dont think that should be possible if it fit a go/no go gauge.

second thought is bad ammo. I would go back and try a few different brands of ammo to see if this issue occurs across different brands. Even if its a high end company, everyone makes mistakes in manufacturing from time to time.

Certainly I would try messing with the gas, but I dont see how that would cause the issue you describe. did you already go through tuning the gas block or did you just make an assumption as to which setting it should be on? the typical procedure is to start with the gas block mostly or fully closed, and fire one round at a time with an empty mag until you get lockback. then increase gas one to several more settings to account for variations in temperature and ammunition which may decrease the gas pressure.

would you mind posting the brand of barrel and ammo?

 

[TonyAngel]

If you tried to eject a live round and the projectile stayed in the chamber, I seriously doubt that gassing is an issue, or at least, a part of this particular problem.

When you knocked the projectile out of the barrel, could you see engraving from the rifling on the bullet?

This is why most people should buy factory guns.

I know this sounds like a jerk's response, but the AR community has really gotten to a point where most actually believe that ARs are just tinker toys. Even this forum is filled with threads posted by guys saying that they are "building" one setup or another and then asking what buffer and spring they should run.

I get it ... it's a learning process, but many would benefit from learning to walk before they run. Buy a rifle and keep it running for 25k rounds. At the end of that experience, you'll know something.

As for the present problem, assuming that the proper bolt and ammo is being used, my knee jerk guess would be a chamber problem.

In any case, is this the only problem that has manifested? The bullet getting pulled when extracting a live round? Have you tried repeating it to see if it happens again?

Pictures and a better description of the barrel and ammo would help.