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?

 

[camyoung603]

At the range, I removed the upper and had a cleaning rod ready in my range bag. I used it to carefully push the round out. I have a picture I’m trying to upload.


When I got home, I stripped the bolt, marked the round with a Sharpie, and gently placed it in the chamber. To avoid accidentally rubbing off the Sharpie, I carefully reinstalled the bolt, making sure everything stayed in place. Dropped the bolt back into position, then pulled the bolt back out and had to use the cleaning rod down the bore to push the round out.

All hornady 108's factory ammo

 

[camyoung603]

These are the other ones I was having trouble sending

 

[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.

 

[camyoung603]

This is why most people should buy factory guns.

Thanks for the help

 

[camyoung603]

I like to build stuff i like to take stuff apart I like to know how things work im not asking for someone to fix this for me im trying to learn so if I run into this again I have new never jump off point. I appreciate the help thus far I really do

 

[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.

 

[camyoung603]

This is my third AR rifle build, and it’s the first time I’ve run into an issue like this. All my previous builds have been in 5.56, not 6mm ARC. I tend to be pretty analytical and probably spend more time than necessary researching before buying parts or asking questions.


I started with the gas setting at 6, based on advice from a friend who’s much more experienced than I am. The rounds I fired at the range and the one I tested at home both showed the same rifling marks. I’m using Hornady 108-grain ammo. My current thought is that the issue might be a short throat, even though I carefully checked headspace both before installing the barrel and again after noticing the problem.


The only reason I thought it might be a gas issue was because of the marking I noticed on the backside of the BCG lugs. Aside from running a stripped bolt test at home, the only other time I fired the rifle was at the range.


I don’t want to throw the barrel company’s name in the mud, as I’m still waiting to hear back from them. But is it possible to have a chamber that’s tight enough to pass a go gauge yet still have a short throat causing these marks?

 

[camyoung603]

Someone mentioned that there are two types of bolts, so I did a bit of research during lunch today. The correct bolt appears to be a Type 2 with a bolt face depth of .136, while the other type is around .125. Since I'm not home to measure my bolt, I was wondering — wouldn’t using the wrong bolt cause headspacing issues? I could see how it might create a problem, even if it somehow passes a go gauge.

I also talked to the manufacturer, and they said they’ve encountered this issue before and suggested it might be related to the ammo. They mentioned Hornady has some problems with the ogive, which kind of makes sense. However, since the round seems to be making contact pretty close to the neck of the case, unless the round itself has a diameter issue, I don’t see how that would be the cause. I have about 30 boxes of both 108 and 105 grain ammo that I recently purchased, and the COAL is consistently around 2.25 inches, which is under the Hornady max of 2.260. I guess I’ll have to go home and bust out the calipers to do some measuring myself.

 

[camyoung603]

After talking with the manufacturer, they confirmed it's not an OAL issue. The problem is that Hornady is using a bullet with a tangent ogive, and even though the overall length is within spec, the bearing surface extends far enough forward to make contact with the rifling.

I went home and checked rounds from about 15 boxes, getting consistent measurements across the board. At the neck it's .243, and there's about .135" of bearing surface before the ogive starts to arc.

I gave Hornady a call, and the guy I spoke with basically said, “Why would a barrel be designed around one specific bullet profile?” So now I’m kind of in a holding pattern. The manufacturer suggested trying some different ammo, so I’ll probably pick up a few brands and hit the range to see what works.

It’s frustrating, because I don’t want to be locked into using just one type of ammo. That said, the barrel guys have been great super responsive and even offered to take a look at the upper if the issue continues.

 

[357Max]

But is it possible to have a chamber that’s tight enough to pass a go gauge yet still have a short throat causing these marks?

Go gauge indexes off shoulder so yes it could have a tight free bore while gauging within spec.

Made me think of this notice I recently read at X-Caliber.

 

[supercorndogs]

After talking with the manufacturer, they confirmed it's not an OAL issue. The problem is that Hornady is using a bullet with a tangent ogive, and even though the overall length is within spec, the bearing surface extends far enough forward to make contact with the rifling.

I went home and checked rounds from about 15 boxes, getting consistent measurements across the board. At the neck it's .243, and there's about .135" of bearing surface before the ogive starts to arc.

I gave Hornady a call, and the guy I spoke with basically said, “Why would a barrel be designed around one specific bullet profile?” So now I’m kind of in a holding pattern. The manufacturer suggested trying some different ammo, so I’ll probably pick up a few brands and hit the range to see what works.

It’s frustrating, because I don’t want to be locked into using just one type of ammo. That said, the barrel guys have been great super responsive and even offered to take a look at the upper if the issue continues.

Did you spec a short throat or something? It seems a Sammi chamber should run any sammi ammo.

 

[camyoung603]

Did you spec a short throat or something? It seems a Sammi chamber should run any sammi ammo.

No this is an off the self barrel and it was just north of 400 bones. Yea so I guess I've got 30 something boxes of ammo i can't run. For you reloading guys what are you running for OAL i know my stuff is close to the max but its still within spec

 

[camyoung603]

Go gauge indexes off shoulder so yes it could have a tight free bore while gauging within spec.

Made me think of this notice I recently read at X-Caliber.


View attachment 8733449

Im just wondering if the bolt lug issue im having is a pressure issue when the round is seated or something idk

 

[supercorndogs]

No this is an off the self barrel and it was just north of 400 bones. Yea so I guess I've got 30 something boxes of ammo i can't run. For you reloading guys what are you running for OAL i know my stuff is close to the max but its still within spec

It really depends on the bullet but we're mostly contrained to 2.25 or 2.35 to stay in thr magazine. Some bullets of course woukd hit the lands at 2.25.

I don't see how this isn't a problem wi5h the barrel. Factory ammo should fit in a Sammi chamber. Who's 6 arc barrel is it? Ask them for a chamber print? Ask them who's reamer they're using? A lot of people are shooting 6 arc with Hornady ammo. I haven't seen a lot of complaing about this. That really makes me lean toward a problem with the barrel.

 

[camyoung603]

It really depends on the bullet but we're mostly contrained to 2.25 or 2.35 to stay in thr magazine. Some bullets of course woukd hit the lands at 2.25.

I don't see how this isn't a problem wi5h the barrel. Factory ammo should fit in a Sammi chamber. Who's 6 arc barrel is it? Ask them for a chamber print? Ask them who's reamer they're using? A lot of people are shooting 6 arc with Hornady ammo. I haven't seen a lot of complaing about this. That really makes me lean toward a problem with the barrel.

While I havnt slapped a set of calipers on each round of every box the few out of each box have been the same except for OAL between 105's and 108's. Now even with a shorter OAL on the 105's I would think I would have less bearing surface above the neck but I havnt messed with loading so im not sure.