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

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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?