6mm ARC brass deformed

[dzz]

All,

I've just built 6mm ARC upper with 18" Shaw barrel and Sons of Liberty Gun Works SOLGW 6.5 Grendel/6mm ARC Bolt Carrier Group and an adjustable gas block. I adjusted gas down to get reliable ejection and empty mag lockup, ejection is going at around 4 o'clock. The rifle shoots sub-MOA 5 round groups on 100 yards with Hornady Black 105gn and just over 1MOA with Hornady Match 108gn. This is all good.
Now what concerns me is that I get ejector marks on most of the casings (see pics), there is lot of brass residue on the bolt face, and the throat on about half of the brass that has throat flat on one side. I understand that sharp edges on the bolt extractor and ejector could be causing marks on the brass and brass residue on the bolt. What could be causing flat throat on the brass?

Also, velocity is about 2520fps for Hornady Black 105gn and about 2480fps for Hornady Match 108gn, I have only about 80 rounds through this barrel so it may speed up little but. I was expecting little bit more velocity from 18" barrel.

Thank you for your help!
Dan

 

[dms416]

All of my arc brass looks like that. Out of a G$ gfr or my Hodge/GAP

 

[Bantam1]

The flat spots on the neck are from the brass hitting the brass deflector on the upper. Mine does it, my friends does it too. I added one of those stick on bumpers to the deflector and the brass no longer has the flat spots. I think it's just a characteristic of this cartridge in the AR upper that was designed around a different case. It's a non issue really if you load. The neck has to be sized anyways.

I have not had any ejector smears on my Hornady 108's. It's possible that lot was a little hot? Or maybe your barrel has a little shorter lead making the pressure jump?

 

[db2000]

I got 2540-2550 with Hornady black from that same barrel when it was new. A year later 2600.
I buy these as they stick really well and prevent neck dents like you’re seeing. Be sure to clean well with alcohol wipes prior to applying.

Deflector Brake 2 Pack | deflector-brake

The Deflector Brake eliminates brass burns by forcing spent brass to consistently drop 3-4 ft next to you instead of scattered all over. Great for left handed shooters or off shoulder firing because it pushes flying brass forward and away from your face. Our Deflector Brake works absorbing the...

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[Bantam1]

That is what I used also. I stuck them to a spray painted rifle. No issues yet.

 

[alamo5000]

As was stated, in the 3rd picture that is from a strong ejection and the brass hitting the deflector on it's way out.

If anything your rifle might be a little bit over gassed, maybe. If it bothers you try a little bit heavier of a buffer. It won't totally prevent brass damage but it can lessen it and slow everything down a little.

 

[FatBoy]

How much headspace do you have between new brass and the fired cases? I wonder if it’s still expanding during primary extraction. @alamo5000 touched in this with buffer weight, but I’d be curious if you have a generous chamber.

 

[alamo5000]

How much headspace do you have between new brass and the fired cases? I wonder if it’s still expanding during primary extraction. @alamo5000 touched in this with buffer weight, but I’d be curious if you have a generous chamber.

I wouldn't think it was still expanding, but rather just ripped out of the chamber with total prejudice.

 

[alamo5000]

If you look at the brass in photo 1, take note of the primers. None of the primers are deformed.

The damage to the heads are not pressure related but seem to point to rough handling by the rifle. Which that adds to my speculation about being over gassed.

 

[flogxal]

For the flattened case mouths, you can put some fuzzy-side velcro on the deflector if you have that handy.

Extractor or ejector related, only seen that in 6.5 Grendel as pressure signs on hand loads. Never on 6 ARC (Proof barrel, OdinWorks carrier, JP bolt) with any Hornady Black or Match loads.

 

[supercorndogs]

I woukd guess the flat spot on the brass is from the inside of the barrel extention. It's pretty normal with spring loaded ejector. My 700 actions do it on some brass.

 

[Bigjake83]

Those marks on the case head ARE high pressure indications anytime you see the extractor claw indentation thats a sure sigh of excessive chamber pressure.

I didn't read everyone remakes but its an easy fix, you need to add extra weight to your buffer to give the case just a little more time to unlock, also are you shooting suppressed?

 

[supercorndogs]

I would second that. You will probably get little ejector circles on 6 arc or 6.5 grendel. But you don't want to see flow into the extractor grove like that. Was wondering about a suppressor also. I assume rifle gas.

 

[dzz]

I would second that. You will probably get little ejector circles on 6 arc or 6.5 grendel. But you don't want to see flow into the extractor grove like that. Was wondering about a suppressor also. I assume rifle gas.

not shooting suppressed. Also, gas is reduced half way on the adjustable gas block

 

[DHDeal]

not shooting suppressed. Also, gas is reduced half way on the adjustable gas block

Then keep going down (close it up), and a heavier buffer and/or stronger spring. BigJake, Supercorndogs, and others pretty much covered it. That ammo is showing high pressure in your rifle in it's current configuration.

The flattened neck is of no concern, but the shaved brass from the extractor cut out is a big clue. Now that may because that area on your bolt has a burr, or is just very sharp. Many on here do some work to our bolts before the first round is ever fired with it. Radius and polish of the ejector, polishing the extractor ledge mentioned above (or whatever it is called officially), dehorning the extractor, polishing the lugs on the 3 o'clock side of the barrel extension, and polishing the feed ramps are mine. That is before the first round is fired. Some folk buy a complete rifle (WTH) so the barrel extension part is much harder to get to, but it helps make the Rifle kinder to the brass. The bolt work described helps keep a lot of the brass shavings in check (or not have brass shavings at all). In my opinion, any brass shavings is a bad thing and anywhere you see them, should be looked into and fixed.

At minimum, clean that bolt face, remove the extractor and clean under it, feel for sharp edges at the corner of the extractor (bet it will be sharp), IMO chunk the donut on the extractor spring, and if you have the know how remove the ejector and put a small radius and polish it.

I have fired that same ammo in my ARC's for barrel break in and it is warm, but never see that kind of build up or buggered up brass. In your case I believe it is a couple different things going on that the other posters have described.

 

[dzz]

Is there a factory bolt or a bcg that is not damaging brass or not damaging brass as much?
I’ll get heavier buffer as suggested here

 

[DHDeal]

Maybe, or maybe not. I use the Rexus Ultrabolt for 6.5 Grendel and 6ARC, and while all the "tricks" appear to be done, I still give them a polish under the magnifier. I use either a JP FMOS, Young Manufacturering NM, or most recently the Griffin Enhanced carriers and they only get shot suppressed.

I do not have any of the shaving or smear on my boltface on a new build, but I tend to keep these rifles clean. I hate money and spend quite a bit on the parts to build the .441" casehead rifles. They are not blasters and I don't treat them as such.

Parts tend to smooth out from normal shooting so what you are seeing may even out anyway. However, a dremel with good conehead felt bits, an extractor removal tool, and the correct size punch are handy to have anyway. If you want to do the minimum, dehorning the edges of the extractor will cut down on the shavings a bunch right now. Drag it gently across some 320 grit on a flat surface until it's teeth are smooth. I use a good fine file but finish it with the 320.