I shot my 20" AR today and after I shot a mag I looked at the ejected cases and I fould that 1 had a split neck and a few had powder burn marks as far back as 1/4" from the base.

Is this a problem with the rifle waiting to happen or is this due to the Ammo being reloads?

The ammo was Black Hills 55Gr.

Thanks

The Cheat

Is it an ASA upper?

Could be the ammo, but as BH reloaded ammo has given me no problems (the bit I've shot), I'd suspect "outta-spec" chamber.

Yes, could be a problem, best to have the chamber checked.

FWIW, more info on the upper, as asked above is most helpful in matters like this......

Mike

No, its a Colt upper.

The Cheat

The upper is a from a Unfired SP-1, the previous owner swapped it out for a CAR-15 style upper the same day he bought it NIB. So I was the first to fire it. It headspaced ok. Not sure if the chamber is .223 or 5.56, but it is a chrome bore and barrel.

The cheat

Remember that the chamber may “Head Space” correctly, but if the chamber was polished too much or the chamber was reamed incorrectly, this would cause a problem.
This would cause the neck to split at if head spaced with too much free run of the case shoulder expanding, but due to the neck it’s self flaring out as if shot threw a M-249 where the chamber is reamed loose to allow the rifle to run in the dirtiest of conditions.

Best way to check the chamber is to take one of the spent cases and mic it against a fresh/live round.  Check not only the distances from the rim back to shoulder, but the diameter of the case sidewalls due to blow out at firing.

Also, keep in mind that if the chamber was not fully cleaned (end of neck just before the throat), this fouling may cause the ammo to act as is it was loaded too hot and may be the reason that the neck split and the case blacked so far back.

If you find that the spent case has not been blown out too much, the chamber is fully cleaned, then use a few rounds of the ammo in another ar-15 to see if the problem just lies in the cases being on the way out (over worked).

P.S. Keep in mind that the black hills reloaded ammo is just that, reloaded.  If the case was first fired in a very loss chamber, and then resized, it may have been really worked/overworked to be pressed back to spec. This would cause the brass to split even in the best of chambers. Black hills loaded their ammo very carefully, so I seriously don't believe that it was the ammo that was overloaded with powder.

Thanks Dano here are the dimensions I came up with. All brass was fired from the same rifle, same day.

SPLIT CASE:
Brass thinkness as neck: .015"
Diameter of Neck: .2558"
OAL: 1.765"

UNSPLIT CASE:
Brass thinkness as neck: .016"
Diameter of Neck: .2563"
OAL: 1.769"

Seems to be a size difference between the cases, this is the case between all of the split brass and unsplit brass.

Any ideas on the powder burn marks?

The Cheat

Cheat,
I'm still MIA in Palm Beach-FL running from a remote laptop and can’t connect to my main server at the house (didn’t set the server up to log on threw a modem) or spec books that are sitting in the gunroom. If someone can list the specs on 5.56 chamber, it would help a great deal.

As for the burnt marks, my spent cases have a tendency to looks smoked/fouled, but keep in mind that I run ammo by the case. So by the time I police my spent cases, they all seem to be pretty well fouled.  Also your rifle is an auto loader, so the cases will foul up somewhat due to the spent gas on ejection. It’s nasty burn marks that you want to worry about, such as the burns that were caused by the cases splitting and the pressure flowing past the case, and not staying contained in the case cause by the case sealing off against the chamber side walls. This plasma cutting can run a chamber in a very short time, so I wouldn’t run the black hills ammo until you figure out if the ammo or chamber is at fault.

I noticed that the split cases were .001 thinner, but can't get to any of brass to check the thickness of a standard case (read new).  The dead giveaway to if the brass that split were over worked would be if the lot stamping of cases, both split and non-split were the same head stamp.  


As I pointed out, if the reloaded brass began there life as new Military ammo, then the some ammo may have been run threw M-16's and other threw a Mini (M-249). The Mini and Mag-58 (7.62 Nato) are hard on brass, due to the looser chamber in the sidewalls of the chamber and really aren't the best to reload.

To keep it simple, get a couple of boxes of XM-193 and take the rifle out for a spin.  The 193 ammo is the hottest stuff I have found, and still in Mil spec.  If the rifle/upper runs on the 193 ammo, and doesn't have a problem, then I would just write off the black hills ammo as less than favorable in the rifle. Also, Black Hills warranties their ammo, so you may want to touch base with them to see if anyone else has had a problem with the same lot number that you have.

P.S. Remember to give the chamber a good scrubbing and make sure that you have gotten all the fouling at the end of chamber out before you take the rifle out agian.  

It works best to just spin a 30 cal cotton mop in the chamber to check if you have missed that last bit of fouling at the lip before the throat. It fouling is left here, then it makes ammo act as if it’s loaded too hot, due to the constriction of the case not being able to stretch forward as it expands during firing (thinks as if the case was not trimmed and too long for the chamber on exspantion).

Hope this helps.

Thanks again Dano.

I'm heading to the range again later this week. I'll try a few different types of ammo mfg.s and see what I come up with.

The cheat

The_Cheat,

Got a range report for us?