Help diagnosing 6mm arc brass

[rookie7]

Good morning,

I have a Ruger American Predator Gen 2 in 6mm arc. 30 rounds total fired. Attached are pics of fired brass. The rim at the bottom is raised slightly where I can feel it with my finger. Scratches alongside case I’m guessing from a burr in the chamber?

All brass used were new and unfired. Most were Hornady factory loads and I fired 4 loaded rounds where I used new Starline brass.

Is the chamber slightly oversized allowing the brass to expand too much at the case head? Should this be corrected by Ruger or live with it and shoot?

The purpose for the rifle is whitetail deer hunting and casual shooting enjoyment.
Thanks for the help.

 

[rookie7]

I’m trying to upload more pics but it’s not working at the moment.

 

[AWS]

The scratches on the body look more like from mag lips.

The light coloured ring at the head is easily checked with a paper clip to see if there is an indentation on the inside of the case. It could be a wear ring from ride it around in a plastic ammo box

 

[GLShooter]

The long scratch is from the mag. Polish the lips. The ring on the bottom is normal expansion but a quick polish with a tight patch slathered in JB BORE PASTE wrapped around a bore brush run on a drill back and forth for about 45 seconds will clean that up the radial markings. I've done this with a ton of barrels over the years. You don't want the chamber mirror bright as that can cause issues. Keep the polishing patch off the shoulder for extended times.

 

[gunsandgunsmithing]

I would go no finer than a 320 grit finish in the chamber. Maybe not even that. Too smooth increases bolt thrust...the biggest enemy of an ar15 on a ppc diameter case head. A good bolt gun can stand more but there's no point at all

 

[searcher]

Those radial scratches will not hurt your brass and likely won't shorten the lifespan. Having a few of those scratches might actually reduce bolt thrust. As gunsandgunsmithing said above - don't overdo if you attempt to polish them out. I have seen far worse machining marks in a chamber doing much worse to brass than that. I'm not sure there is really anything wrong with it from a mechanical standpoint. I'm not sure Ruger would do anything. If you smooth out your chamber, it comes at the expense of some increased wear on your bolt.

The gouges on your rims are a completely separate issue, caused by your extractor claw edge being very sharp. You can pull your bolt out and polish the sharp edge of the extractor just enough to remove the sharp edge. They don't need to be like a knife to extract. I do this on all of my offending A/R's and it usually eliminates the problem. Over-gassed guns are more pronounced - but being a factory gun like a Ruger, the odds of that being the problem are very low. If you have a very fine diamond file - several swipes across the edge will tame that swipe down. Polishing works best afterward.

 

[rookie7]

Thank you to everyone for the input and suggestions.

Just to clarify - this is a bolt action rifle not a gas gun. Unfortunately, it does use AR mags. Wish someone made a bdl conversion for the Ruger American.

 

[243winxb]

Is the chamber slightly oversized allowing the brass to expand too much at the case head?

SAAMI shows the maximum diameters for loaded ammo. Taking measurements may help?

 

[243winxb]

Fired brass would be larger, as it expands to the chamber. Brass will spring back after firing. Maybe .001" ?

 

[rsbhunter]

I'll follow this just to see if an answer comes up. On a different side, me and a friend built 6mm ARC ar15's a couple years ago. We both bought the same barrel, and he bought another make for a second rifle. All three rifles showed pressure from factory loads!! I'm talking primers coming out on first firing, flattened into the pocket. And, after resizing any once fired brass that DID survive, we both experienced a belt that formed at the base. I'm telling you, they honestly looked like mini 7 mag cases!!! From all three barrels. Not hot loads, top notch bolt assembly, proper gas blocks, spring and bolt followers... we both gave up....I know guys shooting gas guns now that work great, but I don't know their load data. I just built a 22 arc ar for a match gun that works great, so I don't have a clue...rsbhunter

 

[GLShooter]

It almost sounds like you were way over gassed on you Magnum ARC. I suspect an adjustable gas block would fix that.

 

[rsbhunter]

All rifles had adjustable gas blocks, even used Tungsten weights, had 26" bbls . We have both built numerous ar 15's, ar10's and PC rifles in the past. Remember, this was FACTORY ammo. Not internet loads , or "a friend told me" load. And , using Hornady's own load data, we still needed to lower powder charges. The " performance " was nowhere close to published claims....like I stated, now they seem to have everything worked out. I built a 243 LBC upper after that, and I am still running that configuration...at 1.0gr powder increase, and 0 pressure!!! Just my experience...rsbhunter

 

[GLShooter]

That's pretty interesting. I had a 6PPC that did that. I found out the bolt was Grendel depth but should have been a 7.62X39. It was one of the original Olympic Arms barrels.

I shoot three or four 243 LBC ARs and a bolt gun. 95 grain SMKs do great at didtance. The horsepower is there so why Rob yourself with the ARC. I can't imagine hoping that the factory ammo would be my only ammo source. Handholding is the only way to fly.

 

[rsbhunter]

That's pretty interesting. I had a 6PPC that did that. I found out the bolt was Grendel depth but should have been a 7.62X39. It was one of the original Olympic Arms barrels.

I shoot three or four 243 LBC ARs and a bolt gun. 95 grain SMKs do great at didtance. The horsepower is there so why Rob yourself with the ARC. I can't imagine hoping that the factory ammo would be my only ammo source. Handholding is the only way to fly.

What amazes me is that the difference between the 243LBC and the 6 arc is negligible, yet the 243 lbc , IMO, stomps the arc. I have noticed that alot of the bbl's that were available prechambered are not available now. I am liking the 22 ARC alot, cheaper to load!!! And with a 26" Craddock barrel, it hammers...rsbhunter

 

[GLShooter]

What amazes me is that the difference between the 243LBC and the 6 arc is negligible, yet the 243 lbc , IMO, stomps the arc. I have noticed that alot of the bbl's that were available prechambered are not available now. I am liking the 22 ARC alot, cheaper to load!!! And with a 26" Craddock barrel, it hammers...rsbhunter

The balance must be just right on the bigger case. I have a barrel company that does all the Grendel variants that shoot lights out and am happy as a clam with them. My first wildcat Grendel round was the 20 LBC and I learned a whole bunch on that one. When I got into the 243 LBC it was so simple as falling off a log. I've got a 30 caliber version I have not had time to work with but I'm sure it will be a hummer too.

 

[J Fess]

All three rifles showed pressure from factory loads!! I'm talking primers coming out on first firing, flattened into the pocket. And, after resizing any once fired brass that DID survive, we both experienced a belt that formed at the base. I'm telling you, they honestly looked like mini 7 mag cases!!!

Sounds like the classic "Grendel Belted Magnum Effect". Over the years on the Grendel forum, it has been seen fairly often. It is typically caused by overgassing and the case being partially extracted while still under high pressure. These cartridges are sensitive to the bolt unlocking too soon. Sometimes this is solved by reducing gas with an adjustable gas bloc. I have read about some people troubleshooting by turning the gas block so the holes do not align to see what happens without gas pressure opening the bolt.
Compared to most bolt guns, ARC/Grendel AR15's have less case support near the web from the deeper bolt (compared to 5.56) and generous chamfer on the base of the barrel for good feeding. Occasionally the barrel has too large a chamfer for support near the case head.
You can also check that you got the right barrel/bolt combination. The Grendel/ARC bolt is the same diameter but slightly deeper than the 7.62x39 version. GLShooter discussed that.
If indeed it is a pressure issue, it is possible that a worn reamer cut the freebore too small. The clearance is only half a thou, so it does not take much wear to get undersized.

 

[gunsandgunsmithing]

Sounds like the classic "Grendel Belted Magnum Effect". Over the years on the Grendel forum, it has been seen fairly often. It is typically caused by overgassing and the case being partially extracted while still under high pressure. These cartridges are sensitive to the bolt unlocking too soon. Sometimes this is solved by reducing gas with an adjustable gas bloc. I have read about some people troubleshooting by turning the gas block so the holes do not align to see what happens without gas pressure opening the bolt.
Compared to most bolt guns, ARC/Grendel AR15's have less case support near the web from the deeper bolt (compared to 5.56) and generous chamfer on the base of the barrel for good feeding. Occasionally the barrel has too large a chamfer for support near the case head.
You can also check that you got the right barrel/bolt combination. The Grendel/ARC bolt is the same diameter but slightly deeper than the 7.62x39 version. GLShooter discussed that.
If indeed it is a pressure issue, it is possible that a worn reamer cut the freebore too small. The clearance is only half a thou, so it does not take much wear to get undersized.
View attachment 1593032

Ok
I've loaded grendel based wildcats for ar's to the point of bolt failure but I've never once had that happen, with both bolt designs. It was a bolt strength test. I've loaded the same brass in bolt guns to well over 100,000psi(per rsi pressure trace), to where the brass did fail but it wasn't there in that case either.

 

[rookie7]

I plan to polish the feed lips on the mag as suggested, and I'm going to buy a 10 round magpul pmag and see if I can get it to work. I really dislike how the bottom of a 10 round AR mag is slanted. I also wish they made a small 5 round AR mag. This rifle is for hunting, and the less protruding from it the better.

 

[J Fess]

Ok
I've loaded grendel based wildcats for ar's to the point of bolt failure but I've never once had that happen, with both bolt designs. It was a bolt strength test. I've loaded the same brass in bolt guns to well over 100,000psi(per rsi pressure trace), to where the brass did fail but it wasn't there in that case either.

It seems that I edited out the line where I mentioned that I didn't know how much the problem was pressure vs early unlocking, but the first time this showed up on the Grendel site, Bill Alexander (one of the developers of the 6.5 Grendel) stated that he saw this with hot proof loads and was not surprised. Although it was nearly 20 years ago, I think he mentioned a number around 67,000psi would cause this in his test rifles. The "belt problem" has shown up maybe a dozen times on that forum over the years. One time, I recall, it was determined to be a chamber issue.
A quick search of "Grendel belt" or "ARC belt" will bring up a bunch of examples on forums.

Doing a quick check for more recent occasions of "belting" of this case, there is a thread on the 6 ARC on Sniper's Hide ("Grendel-like Belted Magnum Problem") where two people with extensive experience with the Grendel/ARC case state that it is a gas issue. One of those people is LRRPF52, who is Grendel forum admin and also posts on this site. Hopefully he will respond.

This video shows sectioned cases, some of which have the distinctive belt.

 

[gunsandgunsmithing]

It seems that I edited out the line where I mentioned that I didn't know how much the problem was pressure vs early unlocking, but the first time this showed up on the Grendel site, Bill Alexander (one of the developers of the 6.5 Grendel) stated that he saw this with hot proof loads and was not surprised. Although it was nearly 20 years ago, I think he mentioned a number around 67,000psi would cause this in his test rifles. The "belt problem" has shown up maybe a dozen times on that forum over the years. One time, I recall, it was determined to be a chamber issue.
A quick search of "Grendel belt" or "ARC belt" will bring up a bunch of examples on forums.

Doing a quick check for more recent occasions of "belting" of this case, there is a thread on the 6 ARC on Sniper's Hide ("Grendel-like Belted Magnum Problem where two people with extensive experience with the Grendel/ARC case state that it is a gas issue. One of those people is LRRPF52, who is Grendel forum admin and also posts on this site. Hopefully he will respond.

This video shows sectioned cases, some of which have the distinctive belt.

I've never seen it due to the bolt beginning to open but that doesn't mean it's impossible. I highly doubt it because you can see what appears to be the edge of the machined chamfer in front of that belt/bulge.

IME, I've been wildcatting grendel stuff for a long time and Lapua Grendel brass is NOT a weakness in any form or fashion...other than availability. Lol!

If it's opening, leaving too much unsupported case...it's still just that, and not a brass issue but a gun issue.

I load way, way hotter than gas gun data and way beyond what you'll find in any published data as well, in bench guns. You can certainly believe something else, but that belt is not a brass problem. I won't repeat myself on that. It's absolutely a gun issue. Why, is a different story but it is what it is. It's not arc/grendel limited either.

Bottom line..measure for yourself...the case is solid for approx .150". Anything above the solid area has got to be and remain supported while under pressure. By far, the most common cause of what you have in the pic is simply a very heavy chamfer at the chamber opening but it can also be from too much clearance between the bolt and bbl, too. Same end result...too much case left unsupported while under pressure.

You seem dead set on believing what you've read elsewhere and that's fine but I'm not the only person on here that knows what caused that belt. It's up to them to comment or not. The internet is full of good info...and bad.

The vid appears to be likely both, unsupprted case and over pressure. Yes, if the chamber is fat enough, a die can make a similar belt belt the vid doesn't look like a die caused it to me. Too sharp where it terminates. Die should have some amount of chamfer at opening.