AR10 Brass Forensics

[cuervo]

A new-to-me RRA BT3. I'm still troubleshooting the root problem, but I noticed this about all of the spent brass.

The two scratches appear to be from the lugs on either side of the feed ramp. The dent appears to be happening during extraction. It's hard to tell from the picture, but it is smoother near the base and then sharper near the neck, like the metal is being gouged deeper as the brass extracts. It appears that way on every round, so it looks like the scratches don't matter which side the round loads from, but the dent is always on the left scratch.

The bigger issue, whether or not related to the brass, is I've been having rounds not want to chamber. This did not happen at all on my first outing, where I was sighting in the irons. I was using a mix of FN and MAL ammo. In most cases, I loaded 5 or 10 rounds per magazine for sighting. I used two PMAGs and a steel factory magazine.

In the second outing, it was only MAL ammo, and I had installed a scope, torquing the cross screws to 60in-lb. In all cases, it's happened where I push the bolt release on a fresh magazine of 5 rounds. Normal firing and cycling had no problems. To get the round out, I had to mortar the rifle on the bench. Same magazines as the first outing.

The first outing was a mix of FN and MAL ammo, with the same marks. The second outing was just the MAL ammo, maybe 5 rounds out of 80. Same mix of magazines. So, the only real difference between the two outings was adding the scope.

Plans for my next trip, whenever that may be, will include trying different ammo and loading rounds with the charging handle instead of the bolt release, and also trying single feeds directly into the chamber. I've measured the next set of MAL rounds and compared them to some Hirtinberger, and found no differences.

 

[dcloco]

The BEST way to diagnose this, record as the round is fired/extracted. Playback in slow motion.

More than likely, cases are being slapped against the side of the brass deflector or side of upper.

 

[Demi-human]

In all cases, it's happened where I push the bolt release on a fresh magazine of 5 rounds. Normal firing and cycling had no problems. To get the round out, I had to mortar the rifle on the bench. Same magazines as the first outing.

First, that’s what ARs do to brass. The dent will be pressed out on the next firing, only to be replaced upon ejection.
The gouges are from the feed ramps. This can be stoned down smoother, and it will be worn in smooth with use. All my new ones gouge brass, the ones I haven’t stoned.
That’s just the way it is with ARs, they hate brass sometimes.

There are many ways a small brass cone can get stuck into a larger steel cone. The cartridge can be stuck by the body, the shoulder, the bullet in the rifling, and even a fat neck and bullet combination. And it it takes so very little. A longer case will also be a wider case at some point in the shape.

Since this is not handloaded ammunition, don’t buy that kind anymore. Be sure the rifle is properly lubricated, it’s harder to get a case out if the bolt is dry too. Be certain the chamber is clean as well.

To find where the cases are becoming stuck, take a recalcitrant case and completely paint it in black sharpie marker. The points that stick will remove the ink, showing where the interference is. Be sure to do the bullet to find if it is being stuck into the throat.

I hope that helps some for understanding the issue, but the solution is different ammunition.
(Unless the solution is a new barrel.)

 

[Charlie98]

It sounds like you have a tight chamber, or, remotely, an improperly cut one. As was mentioned, make sure that bolt and carrier is lubed up... new AR's can have teething pains until they are broken in a bit, too.

As a control device, I would go grab another box of ammos from a different manufacturer to see if you have problems with it as well. If you continue to have problems with that... it's the rifle.

You can also check chambering manually...

1) Separate the upper (it's easier...) from the lower, pull the BCG. Manually insert a cartridge in the chamber. Does it fit? This would probably be a good time to take a strong light and look at the chamber and throat.

2) Disassemble the bolt... remove the firing pin. Put the BCG back into the upper and with your pinky (or other appropriately sized finger...) stuck in the rear of the BCG, push the bolt into battery. It should go in and rotate into battery with slight but firm pressure, and pull out the same way. If it's binding, something is wrong.

 

[cuervo]

Thanks for the replies. I guess my two biggest concerns are that I got lucky on one box of ammo, but not the following two, which doesn't seem too likely, or that somehow mounting the scope is putting some stress on the upper and hence the barrel. Again, doesn't seem likely. I'll run some various controls the next time out.

The bolt does lock in when putting the carrier back in place after disassembly. I can try to remove it my finger pressure. I typically have just used the charging handle when removing the carrier.

 

[Charlie98]

I typically have just used the charging handle when removing the carrier.

Pulling the bolt open with the charging handle might hide the fact that it's dragging or binding up somewhere... that's why I suggested using just your fingers. I find it unlikely mounting a scope on the upper torqued it somehow... enough to cause feeding issues.

Truthfully... it shouldn't matter what ammo you get, any quality factory ammo should chamber without issues.

 

[cuervo]

An update. I took it out today with three brands of ammo (all surplus 7.62x51 including more Malaysian), and three brands of magazine. No jams of any kind.

One thing I did notice after trying a slow chambering with the charging handle is in the picture below. There seems to be some impact between the bullet and the bottom of the barrel as the round chambers. The picture shows the brass marks on the left loading ramp (I couldn't get the camera to show it, but there are identical marks on the right as well), and above it are the marks on the barrel.

Also, I was able to unlock and lock the bolt with just my finger pulling/pushing the carrier, so no binding there.

 

[Demi-human]

That mark is from the FMJ bullet nose impacting the ramp as it is stripped, lead Soft point bullets leave a grey mark. 450 Bushmasters leave a huge mark.

The mark is a bit low, showing where it hits the vertical face, instead of the actual ramped surface.
Check that your magazine catch is holding the magazines high enough in the receiver. A new one may be in order.
Check the magazines for worn catch holes, maybe compare them to other mags for proper placement of the cut.
Check the feed lips to be sure they release the round properly during feeding.

A small bit of brass chewed off the case by sharp edges is a great way to get a case stuck in the chamber. Hopefully it was just that.
If the rifle was purchased used, it doesn’t look like it was used very much. The low mag issue may be why. Might just need a good breaking in.

Hope that helps some, glad that it ran well last time.

 

[Charlie98]

The mark is a bit low, showing where it hits the vertical face, instead of the actual ramped surface.
Check that your magazine catch is holding the magazines high enough in the receiver. A new one may be in order.

That's my immediate thought as well.

The other thing that came to mind is bullet setback. If the bullet is piledriving into the frame like that... and your (handloaded) bullets don't have good neck tension or a crimp, you also might be looking at bullet setback.

 

[bfoosh006]

Interesting feed ramps on your RRA 308.

None of my 308 / 6.5CM barrels have center feed ramp locking lug "removed" like your photo shows.. All DPMS pattern barrels, Krieger, Criterion, Faxon, PSA, and BA.

 

[cuervo]

Another update, maybe the last if all goes well after this. I've narrowed it down to two items, possibly a combination. After the last outing above, when I started cleaning it, I could barely pull back the charging handle to remove the carrier. Once I got the bolt out of the bolt carrier, I looked inside and it looked like someone had poured wet talcum powder inside the carrier and then it had dried.

So, two possibilities
1. The MAL ammo has an additive that was getting things a little gummy
and/or
2. The lubricant (Miltec 1) that I put inside the carrier was cooking and that's what was left of it

As part of my last cleaning, I used some RemOil on the gas rings and left the rest of the inside of the carrier dry. I took it out this weekend and had no problems (but I did shoot fewer rounds than before). When I cleaned it again, I saw no signs of residue.

I'll stick to using oil only inside the carrier and just grease on the support rails on the outside. I may try the MAL ammo again later just to see how it goes.

I'll return to the bullet impact/ramp in the future. I was focused on just functioning this weekend. When doing slow loads, I never saw the bullet tip hit the lower, just the bottom of the barrel. I wonder if the two marks on the lower are from firing and movement of the rounds moving forward inside the magazine until they bump against the lower and stop.