AR-10 Eating Brass for Lunch

[celltech]

Finally got around to trying to trying some handloads for my AR-10. Loaded up some groups of 168 Amax and 155 TMKs at magazine length.

The first group of 168s averaged 2597 fps and the 155s were 2814 fps. By comparison some Magtech 168s I used to zero the scope were 2483 fps.

So yes, my loads were hotter than some factory ammo, but were within the range I expected for the charge weight. Yet on my loads the ejector just ate the brass for lunch. The factory Magtech looked beautiful and loads I have made for my bolt guns also look fine.

What is it about an AR that would make it do this? I can understand if there is some smearing but don't get the gouging and burrs it left. Will overgassing contribute to this? If I need to back them off then so be it...but just perplexed as to why this occurs.


Attachment 5891

[CFJunkie]

I don't know if you brass was well used or once fired, but if it was once fired, I can see some head stamp flattening even though the primers are not flattened.
That could be caused by hotter than normal loads when the primer is tightly seated and the bolt face is tight against the brass, so the primers didn't back out of the pockets and flatten against the bolt.

I also notice that the extractor seems to have gouged deeper from the top left brass to bottom right.
If that was the order the rounds were fired in, I would say the problem is getting worse with firing.
That would disturb me more than if the effects were random because that says the problem is getting worse with each firing.

One thing you can conclude is that this rifle is going to be very hard on brass if you push for velocity.
You didn't say what length the barrel of your AR-10 was.
Federal Premium Gold Medal Match with SMKs 168 grain ammo has an mv of 2650 fps with a 24-inch barrel.
In a 20-inch barrel, that drops to 2550.
In an 18-inch barrel, that drops to 2485 so I would think your barrel might be 18 inches.
To get 2597 fps out of an 18-inch barrel with 168 gr A-Max bullets, I estimate that you probably were loading just about at PMax.
The gouging of the extractor and the flattening of the head stamp may be caused by the brass being forced back into the bolt face by the high pressure.
I had that occur with my 18-inch Les Baer .223 when I got close to Pmax trying to achieve a specific bullet exit time with the certain powders.

I think it would be prudent to back off at least 0.5 grains of powder and see if the flattening and extractor gouging goes away.
Shooting at high velocity close to PMax will not only reduce the life of your brass with this rifle a lot, but it also isn't doing your bolt any favors either.

[Robinhood]

Along with what CFJunkie stated, pay attention to where your brass lands. It should land to your right and a little rearward like 4:00 to 4:30. If it is throwing them right forward like 1:00 and you need to regulate it down. You may need to look at a better buffer to smooth it out when you get the gas right. I am no expert but I have listened in on a few conversations.

Good luck

[bearcatrp]

Have owned multiple AR 10’s over the years. Most are over gassed. With you loading them hot makes it worst on brass. I only use LC brass. Thicker. Takes the beating beater. Good info above to follow. Back off on your loads, watch where brass ejects. Maybe even invest is a good adjustable gas block. Especially if your going to run it hot. You won’t lose my velocity backing off .5 and should show improvement.

[bigedp51]

Some AR type rifles have a adjustable gas block and you can reduce the port pressure.
If you do not have a adjustable gas block, you reduce your load or switch to a faster powder, both will lower the port pressure.

Case-Head Swipe
https://www.shootingillustrated.com/...se-head-swipe/

[celltech]

Thanks for all the info guys...

The rifle is homebuilt. It's an 18" barrel with rifle length gas system, gas block is non-adjustable.

The brass was once fired. Top ones are CBC/Magtech and had the 168s, the bottom Starline brass had the 155s. Not sure how you can tell the headstamp looks flattened?

I wish I had been paying better attention to how the brass was slinging out but I think it was heading more towards the front of the rifle.

Both rounds were using H4895. It's just my stupidity in gauging the load and I really didn't take the barrel length into proper consideration. Guess it's time to pull the rounds and back off the juice! I understand the concept of the brass getting pushed into cracks and crevices but didn't get why it burred. That link helped to explain it....there is a lot going on when that hell is released. I might look into an adjustable block so I can dial it in. I love how soft an AR-10 shoots compared to a bolt gun.

[JeepsAndGuns]

Things that might help (as well as backing off the load a little) is a adjustable gas block and a heavier buffer.
I put a SLR adjustable gas block, a heavy buffer from slashes heavy buffers, and a slightly stiffer spring in my DPMS G2 and it made a world of difference. A heavy buffer and a stiffer spring can help slightly delay the bolt from opening enough to allow the pressure to drop some. Then adjusting the gas to where it is only what is needed to cycle can help things a lot.
Mine would mark the brass like that until I did the above and now it looks normal.

[celltech]

I ordered an Odin Works adjustable block this morning. Will start with it and see what results I get...

[bigedp51]

JeepsAndGuns above also gave good advice about a heavy buffer spring. the brass shearing you see is caused by the bolt rotating as it starts to open.

And Lake City 7.62 cases are made of harder and thicker brass in the base of the case with less chance of the brass flowing into the ejector hole in the bolt face.

Also in a semi-auto rifle the case body should be .003 to .005 smaller in diameter than its fired diameter. This allow the case to spring back from the chamber walls and extract reliably. This can make the difference between a just having a ejector mark or a case head swipe.

[mudpig]

I've recently changed my AR10 over to the Tubb flat wire buffer spring and adjustable gas block and they have made a ton of difference in brass life and also felt recoil.

[243winxb]

Reduce the powder charge. High pressure.

[celltech]

I already have a Strike flat wound spring that is supposed to be heavier than stock. I pulled the buffer and it's a 3.8oz H1. I might get some tungsten weights off e-bay and fatten it up a tad. The weight weenie in me is cringing...I was careful to keep the build at 8 lbs on the nose and now between the gas block and weights I will be adding ~3oz.....arrggghhhhh!!!

I am going to start collecting range brass and just treat it as disposable junk.

[Dave Hoback]

Nm

[JeepsAndGuns]

The Ar10 is notorious for bustin’ Up brass.

I'm guessing you have never had a M1A. Those things are murder on brass. My AR 10 makes brass look like it came out of a bolt action when compared to the brass out of a M1A.
One of the many reasons I ditched my M1A for a AR10.

[Dave Hoback]

Nm

[flowj]

I have and Armalite AR 10 in .260 Rem. I had the same problem . I was trying to load it like my bolt guns in .260. I finally figured out the powder I was using was to slow causing a timing issue. Changing powder and adding an adjustable gas block helped, but I still won't shoot my Lapua brass in it!

[bearcatrp]

What powder are you using? You may not have a totally flat primer but the raised ring where the firing pin strikes is a good indicator your to hot. 4064 worked great out my dpms 16 inch 308AR. Didn’t work as well for my 12 inch barreled 308AR though. Had to switch to 8208 since it’s a faster burning powder.

[celltech]

Was running H4895. I have since installed an Odin adjustable gas block. I didn't realize the carbine length buffer only had 2 weights and both are already tungsten. So I would have to get one built with a heavier casing. I cut back the powder and the brass looks a lot better with just a tad bit of smear here and there. I need to keep playing with the block as the setting that allows consistent cycling still throws the brass forward. I am just going to load nothing but range brass for the AR and treat is as a disposable item. There is always a ton of Federal .308 shells lying around.

[JeepsAndGuns]

I have found that mine likes faster burning powder. Both in function and accuracy. I have got some sub MOA groups with IMR 3031 and AR-comp with a 168 SMK.