Need help with 6AR ejector swipe

[SG4247]

So this is a 24” custom 6mm AR-15 that has been a steel plate hammer. Won lots of hardware with it this year. Great rifle.

So, the barrel was originally a used Bartlein 6mm 7.5 twist bolt gun barrel, with unkown round count.
Using my borescope, I decided to cut off 3-4” off both ends and made it an AR-15 barrel with my 6AR reamer. Used a new JP Grendel bolt assy, and new WOA bbl extension. Adjustable gas block at rifle length.

Using 6.5 Grendel Lapua brass necked down to 6mm. Std Grendel shoulder location. Shot this cartridge for many years with the same reamer.

At about 750 rounds I started getting this eyebrow shaped burr on the case head.

At 990 rounds I installed a new JP Grendel bolt, thinking there was issues with the ejector pin or pin bore. Dang it - still have the same problem, maybe even worse than before.

Any ideas why this is happening? Its a pain to stone this burr off 95 rounds for reloading, before every match. The eyebrow burr is so pronounced, the case wont go into the shell holder without stoning first.

#450 primers look fine to me. See pic attached.

Shot this same ammo - in my 6AR back up rifle -and it looks perfect. So brass is okay I think.

 

[gunsandgunsmithing]

So this is a 24” custom 6mm AR-15 that has been a steel plate hammer. Won lots of hardware with it this year. Great rifle.

So, the barrel was originally a used Bartlein 6mm 7.5 twist bolt gun barrel, with unkown round count.
Using my borescope, I decided to cut off 3-4” off both ends and made it an AR-15 barrel with my 6AR reamer. Used a new JP Grendel bolt assy, and new WOA bbl extension. Adjustable gas block at rifle length.

Using 6.5 Grendel Lapua brass necked down to 6mm. Std Grendel shoulder location. Shot this cartridge for many years with the same reamer.

At about 750 rounds I started getting this eyebrow shaped burr on the case head.

At 990 rounds I installed a new JP Grendel bolt, thinking there was issues with the ejector pin or pin bore. Dang it - still have the same problem, maybe even worse than before.

Any ideas why this is happening? Its a pain to stone this burr off 95 rounds for reloading, before every match. The eyebrow burr is so pronounced, the case wont go into the shell holder without stoning first.

#450 primers look fine to me. See pic attached.

Shot this same ammo - in my 6AR back up rifle -and it looks perfect. So brass is okay I think.

Some possible causes are excess headspace, lube left on cases or chamber, pressure, chamber finish too smooth and case head being a little soft.
The more general root cause is the case slamming into the bolt face.

 

[jelenko]

I would add a carbon ring to the possible causes - goes with 'pressure' in the above post.

 

[SG4247]

Shoulders are sized -.005” from fired length. Tried a few at -.003 and it did not improve.

Same brass/load shoots and looks perfect fired from other guns made with same reamer.

Load is a little bit high on pressure, however I have shot this load for years in several rifles, but primers look fine in this case.

No lube on cases they are dry.

Chamber bore finish is mirror smooth.

i will check for a carbon ring today with the Hawkeye borescope. I shot a match yesterday and ran 95 rounds, so it should be visible.

 

[69mach]

i went through this on a compass lake build. i think the problem is excessive chamber pressure. you can clearly see the ejector mark in the brass and the only way i’ve got that as defined as yours is loading the rounds to hot. load the same round with a grain or grain and a half less powder and i’ll bet it improves

 

[SG4247]

New JP Grendel bolt with 95 rounds on it from yesterday. 4 times fired Lapua brass. Match air Temp yesterday was 40F to 65F. No carbon ring present.

 

[SG4247]

Honestly, I am now thinking the ejector pin dia to bore clearance may be excessive, on these JP Grendel bolts. Going to measure this now.

 

[troupe]

Has you adjustable gas block been cleaned. Some adj. GB can build up enough carbon and it will change your settings. I used to shoot 3 Gun and I noticed it with 223 ammo. The higher pressure, it would not take much build up to affect it. Just a thought.

 

[6fatrat]

I shoot a 6 fatrat as my match rifle, I can do this to if my pressure gets to high.
Steve Bair

 

[troupe]

I have seen the adj screws carboned up so bad that they will not even turn. If there was a ball in the block, not even move.

 

[SG4247]

Ok well measured the JP Grendel ejector pin and bore, as well as another manufactures Grendel ejector pin and bore. Both were basically identical with my 5 place Mitutoyo mic.

Ejector bore dia = .1108”
Ejector pin dia = .1068”

Clearance is .004”

Seems like a lot, but probably typical for AR stuff.

 

[SG4247]

So, I cleaned the gas block at 158 rounds ago. Didnt take it off the bbl, just removed the adjustment screws and ran q-tips and compressed air in there. Wire brushed the adjustment screw and put it back in.

What gets me is the problem just started recently with this rifle. The first 750 rounds on it were great then this brass damage starts going on. I ran this exact same brass/bullet/powder charge in my other 6AR rifles with out this problem. Put in a new JP bolt and did not fix it.

I have always had an ejector circle on the case head with this load, but never a burr and brass everywhere on the bolt like in the pics.

Something has changed.

 

[SG4247]

So, I think it is fixed!

Noticed thst the JP ejector has a large radius on the head, vs .556/.308 mil spec ejectors. Found an AR-15 cheapo ejector, that basically has a flat top with a very small radius on the edge. See pic.

All ejectors I have on hand, measure the same diameter. So, I swaped in the cheapo flat top ejector into the new JP Grendel bolt.

Pulled the worst 5 cases from the match loads yesterday, with big burrs on the case head. Stoned the burrs off them, and loaded them up and headed to the range.

Fired all 5 in about 30 seconds.

They look perfect!

Apparently the large radius on the JP ejector creates extra space for the brass to flow into during extraction/ejection, when the ejector spring is compressed.

 

[DaveTooley]

I still think pressures are a bit high in this rifle with this load. Ejector pin marks on the case mean you have exceeded the yield point of the brass.

 

[10at6]

What Dave said. Too much bolt thrust

 

[SG4247]

Understood and agreed.

Quickload plots this 6AR load right at 60ksi max.

Lapua brass goes in the trash at 6-8 firings. And bolts go in the trash at 600-1000 firings, depending on inspection condition. Barrels wont go beyond 2000 typically.

BTW, all the 6ARC shooters have similar ejector marks also, with commercial barrels and factory ammo.

I appreciate all the comments guys!

 

[troupe]

Great job on working through it and thinking beyond the box, you may have figured this out for others, as well as JP.

 

[DaveTooley]

Everyone wants more out of a smaller case. You can't fool mother nature.

 

[Deleted Dewey]

Have you checked whether the swipe appears on a chambered but unfired cartridge?

It seems odd that enough pressure to extrude the solid portion of the case head into the ejector bore
wouldn't also flatten the primers.

 

[fkimble@charter.net]

I realize this is an 11 month old thread, but the thing that jumped out at me was this "Chamber bore finish is mirror smooth". I always thought the chamber wall was not supposed to be that slick?

Frank