Ripping case rim off

[HCICVOGT]

I rebuilt my AR308 recently with a new Wilson Combat Super Sniper 20" barrel. I have a superlative arms adjustable gas block, Surefire RC2 can, JP Enterprise silent capture carbine buffer. Balistic Advantage upper /lower and Midwest industries hand guard. So far with this new barrel I have around 60 rounds fired.

With cheap 147gr ammo I tuned the gas block to eject at the 4o clock position (cycles like it should)but when I shoot 168 FGMM or 175gr Prime I'm ripping the case rim apart with the extractor. I've tried adjusting the gas block to be 3 clicks open and same thing happens. With the 147gr ammo I'm 7 clicks open. I only shot 3 rounds without the can and it did the same thing with the Prime ammo. Do I need to get a heavier buffer and higher weight spring? Live ammo will cycle manually from the mag and eject when pulling the charging handle. It's almost like it's trying to unlock while still pressurized or the brass isn't shirnking back quick enough after being fired. Any guidance is appreciated.

 

[SmartDonkey]

That's what makes ar-10's so much fun to trouble shoot...... last year I was trying to help a buddy with a temperamental one--- but it would shoot all the good ammo just fine - but as soon as he tried the 147 it would rip the case rim off. Sounds about like your issue- adjusting the gas block didn't fix the issue.

Our conclusion (probably wrong.... not an expert) was that it was asking to much to expect it to shoot such a vast variety of ammo/pressure combinations- so he found the one it worked great with and stuck with it.

I'm interested in what the experts can tell you. Good luck.

 

[FredHammer]

You should be like 30+ clicks out on the SA gas block shooting with a can. 18 clicks out is normal gas block operation.

 

[B'N'F]

The case rim being ripped off is indicative residual high pressure during unlock/extraction. The case is also likely sticking to the chamber, and doesn't have time to contract allowing for clearance between the chamber and fired case.

Have you experienced the same issue when the suppressor isn't used and the gas block is adjusted for function?

 

[BCP]

Pics of brass would help.

Is it JP BCG too or just spring?

 

[HCICVOGT]

I'm on the road this week for work amd do not have pictures of the brass. It is a Balistic Advantage BCG as well.

The case rim being ripped off is indicative residual high pressure during unlock/extraction. The case is also likely sticking to the chamber, and doesn't have time to contract allowing for clearance between the chamber and fired case.

Have you experienced the same issue when the suppressor isn't used and the gas block is adjusted for function?

No I onlyfired those 3 rounds without the can just to see if there's a accuracy difference. The cases woukd not eject and still ripped the rimm off. I also had to keep a cleaning rod handy to knock the brass out the chamber.

 

[BCP]

I'm on the road this week for work amd do not have pictures of the brass. It is a Balistic Advantage BCG as well.

No I onlyfired those 3 rounds without the can just to see if there's a accuracy difference. The cases woukd not eject and still ripped the rimm off. I also had to keep a cleaning rod handy to knock the brass out the chamber.

That sounds almost like a headspace issue. Did you check with go/no go gauges?

 

[mettee]

7-5/8" receiver extension?

I would make the move to the slash heavy buffer if you have that extension.

XH Carbine Buffer + .308 Rifle Spring

Yes the rifle spring works very well.

Once you have that. Try shooting again with the gas off, open it until you get reliable function with the surefire can(they have a lot of back pressure).

Then remove the can and get your unsuppressed setting.

I wouldn't shoot it again until you get the buffer. If you do get decent extraction it's likely it is ruining your brass.

Look for the cleanest possible brass, that will tell you it's not trying to unlock before it should be.

 

[bfoosh006]

In for pics of the fired brass. Both the Ball brass and the FGMM
Edit to add... "maybe" a short throat is raising pressures ?

 

[91Eunozs]

Had a similar issue in my AR10 when shooting 7.62x51 instead of .308 ammo. Pretty sure my chamber was cut with a worn out reamer and the throat‘s just a bit too tight, so the slightly longer NATO ammo was getting stuck.

Note the burnishing on the bottom of the case too; likely too tight of a chamber for the NATO ammo.

I hit the chamber with a new .308 finish reamer about a year ago and it’s better, but still sticks occasionally with the 7.62x51, so I only shoot .308 for now. Should probably get a 7.62x51 reamer and see what it does…though I’d likely lose some accuracy.

I’m no expert by any means though, and would appreciate any tips or advice before potentially ruining an otherwise decent barrel. It would be nice to shoot cheaper ammo for just doing drills at the range though.

 

[Ex E6]

BY No means any expert when it comes to firearms ,however I've been around the block more than twice and have seen things .

I knew a guy that shot at our range who had similar issues and remember someone telling him about Peak and secondary pulse pressure ,the Gas timing might be ? . Why some powders work well in gas guns and others that don't .

This fellow has gone into complete details regarding issues related ;

https://www.ar15.com/forums/ar-15/H...d-how-everything-works-in-harmony-/66-266108/

 

[Cascade Hemi]

7.62N has less pressure than 308 Win.

 

[Ex E6]

Also came across this scenario which fits HCICVOGH's problem ; Could be Extractor and Gas issue ?.

In the AR15 and AR10, the gas port tends to erode under normal use as unburnt powder wears the area around the port. This happens more quickly in shorter barrels as the gas port is closer to the chamber and gets more of a sandblasting.

As this happens, the rifle starts to feed more gas back to the carrier and the cyclic rate of the weapon increases. Eventually, the rifle will try to extract the brass while the brass is still stuck to the chamber due to heat and/or residual pressure. When this happens, you can see some of the following symptoms:

1) The extractor slips off the rim before it can complete extraction and the case stays in the chamber.
2) The extractor bites or bends the rim (usually when it has an upgraded spring or an O-ring/D-fender type mod).
3) The ejector hole has tiny brass flakes in it that eventually cause the ejector to periodically jam. Basically the ejector is pushing on the brass while it is still soft and nipping tiny chunks off as the brass flows into the ejector hole.
4) In extreme cases, the rifle acts like it is short-stroking because it is starting to pull way too early and running out of energy to finish the job.

The usual fix for these on AR15s is to increase buffer weight to delay cycling a bit and add an M4 extractor spring, insert, and an O-ring to give the extractor more bite. Giving the extractor more bite while not delaying the faster cycle time works for awhile but eventually you get bent rims or even entire chunks of the rim ripped out. A heavier buffer addresses the main issue. Corrosion or gunk in the chamber will make these problems worse.

It sounds to me like you may be seeing something similar. Notice any bent rims on the empty cases? I’d advise upgrading extractor and ejector springs regardless. It is a cheap fix and a part of normal maintenance. You can also see if your ejector is nipping brass while swapping them out. If the issue continues.

 

[BCP]

Had a similar issue in my AR10 when shooting 7.62x51 instead of .308 ammo.

Tight chamber/rough chamber/what is the headspace?

You can try polishing the chamber but you don't want a "too polished" chamber or to change the dimensions too much. I suppose a ball hone on a drill might help.

 

[91Eunozs]

Tight chamber/rough chamber/what is the headspace?

You can try polishing the chamber but you don't want a "too polished" chamber or to change the dimensions too much. I suppose a ball hone on a drill might help.

Chamber is correct according to my Forster .308 go/no-go gauges (took extractor off the bolt before measuring), as well as the PTG go/no-gauges I borrowed from a friend. Haven’t measured the actual headspace.

It shoots .308 all day long just fine, so haven’t really worried about it…but saw this post and was reminded that it sure would be nice to be able to shoot the cheaper 7.62x51 NATO ammo without having to worry about stuck cases.

 

[Jgault]

Having built a few ar 308’s and chased more than a few problems I have run into this one. At first I thought it was a tight chamber and wasted allot of time and ammo chasing a bad theory. For me it was the dwell time. My carrier was moving back to fast and the shell was still fully expanded, and adjusting the gas won’t fix this. The only solution for was increasing the buffer weight and adding a spring with more tension. It’s possible the chamber might be a little tight, but for me that wasn’t the case.