I’m a 10mm fan and finally picked up a New Frontier Armory AR in 10mm. I shot about four different types of factory ammo in it as well as some hand loads to see how they grouped at 50 yards. I was shooting five rounds and letting the barrel cool. I was at a bench rest range where I could only fire one shot every five seconds so I was firing slowly. I was also checking my hits after every shot.

I bought some Buffalo Bore 10mm heavy factory ammo i intended to use hog hunting and I was done shooting but wanted to fire one round of the BB ammo before leaving so I loaded one round into the mag. I pulled the trigger and could see a cloud of dust and felt some debris hit me. That one round of BB ruptured, blew out my Glock mag (it blew it about 7 feet from where I was sitting) and blew out the mag catch. The bolt closed on the brass.

I’m positive my barrel was clear so I’m thinking it was the ammo. The head of the brass was bulged on one side and blown out on the other side. Check out the pictures in the link and tell me what you think. I’m sure there are some AR guys on here that can give me some insight. FYI the pistol caliber AR is direct blowback and it was assembled at New Frontier Armory and was stock. I had fired about 200 rounds prior to this with no issues.
Pics

Glad you are okay.

If I had to guess, it would be an overcharged cartridge. Second guess would be that it wasn't fully in battery when fired.

quote:

Originally posted by sigfreund:
Glad you are okay.

If I had to guess, it would be an overcharged cartridge. Second guess would be that it wasn't fully in battery when fired.

I use a brass catcher attached to my rail so I charge the first round and make sure it’s charged then put the brass catcher on. I had some feed issues when I first got it so I’ve gotten into the habit of looking at the bcg after charging the first round then attaching the brass catcher. Thanks for the input, I’m trying to make sure I’m not missing something before I shoot it again. I always wear eye protection and this is one time I’m glad I do.

I figure there are three possibilities in addition to Sigfreund's point; none of them particularly attractive.

First, could have been an out of battery discharge. Test and see if you can get the hammer to fall with the BCG anything short of fully seated.

Second, could have been a consequence of a much more powerful round than the direct blowback was designed to handle; i.e., the case and BCG were moving back before the bullet had left the barrel, and consequently there was still significant chamber pressure involved and the case burst when the case head became unsupported. I can't think of a way to safely test this, other than calling BB and asking if they have any experience with use of their 10mm in blowback AR designs. You might need a heavier spring and buffer for BB loads.

Third, it's possible that there may be an unsupported portion of the case head in your system. You might check and make sure that either a case is fully supported, OR, that headspace is proper such that cases are fully chambered. Might not matter with "regular" 10mm, but might with the BB stuff.

Good luck with the search for answers and please let us know what you find.

quote:

Originally posted by ArtieS:
I figure there are three possibilities in addition to Sigfreund's point; none of them particularly attractive.

First, could have been an out of battery discharge. Test and see if you can get the hammer to fall with the BCG anything short of fully seated.

Second, could have been a consequence of a much more powerful round than the direct blowback was designed to handle; i.e., the case and BCG were moving back before the bullet had left the barrel, and consequently there was still significant chamber pressure involved and the case burst when the case head became unsupported. I can't think of a way to safely test this, other than calling BB and asking if they have any experience with use of their 10mm in blowback AR designs. You might need a heavier spring and buffer for BB loads.

Third, it's possible that there may be an unsupported portion of the case head in your system. You might check and make sure that either a case is fully supported, OR, that headspace is proper such that cases are fully chambered. Might not matter with "regular" 10mm, but might with the BB stuff.

Good luck with the search for answers and please let us know what you find.

Thanks, I’ll check those things out. BTW, I thought about trying a heavier buffer and ordered a Kaw Valley Precision 10 ounce buffer (made for pistol caliber AR’s) this morning.

To add, if it had been due to a barrel obstruction, that would have been obvious from examining the barrel, and it almost certainly would still be obstructed.

quote:

Originally posted by sigfreund:
To add, if it had been due to a barrel obstruction, that would have been obvious from examining the barrel, and it almost certainly would still be obstructed.

Agreed on this.

If your barrel looks clear with no damage, it wasn't an obstruction but one of the possibilities already mentioned.

What buffer was in it???

I have a NFA Lower that I am collecting parts for my 10mm build.
In my reading the 8.5oz is the lightest one should use and should use a Wolff XP spring.
I plan on using the KAW Valley 10oz figure the more the better.

quote:

Originally posted by Black92LX:
What buffer was in it???

I have a NFA Lower that I am collecting parts for my 10mm build.
In my reading the 8.5oz is the lightest one should use and should use a Wolff XP spring.
I plan on using the KAW Valley 10oz figure the more the better.

It comes with an 8 ounce buffer from NFA so that’s what I had in it.

quote:

Originally posted by RHINOWSO:

quote:

Originally posted by sigfreund:
To add, if it had been due to a barrel obstruction, that would have been obvious from examining the barrel, and it almost certainly would still be obstructed.

Agreed on this.

If your barrel looks clear with no damage, it wasn't an obstruction but one of the possibilities already mentioned.

I cleaned up the upper and lower and did not see anything amiss other than the mag catch being gone. I checked the barrel and it’s fine, I’m confident the barrel had no obstructions. I just checked and the rifle will fire with the bcg out of battery. As a matter of fact the hammer will fall with a 1/4” gap between the face of the bcg and barrel. Even though I looked at the bcg before firing I suppose the bcg could have been out of battery. I’ve been conversing with an NFA gunsmith so I’ll ask him about that.

The hammer may be able to fall, but the back of the bolt below the firing pin shouldn't allow the hammer to contact the firing pin when out of battery.

If you split the gun, pull the bolt, and float it above the lower (lined up with/touching the buffer) with the hammer in the fired position you should be able to tell how far back the bolt can be and still allow the hammer to push the firing pin out of the breech face.

My problem is due to an unsupported chamber as was mentioned above. I was looking at that and then Buffalo Bore emailed and said that was what they were seeing from the pics. It all makes sense now, I’m contacting NFA to get the upper replaced. Thanks for the help guys.

quote:

Originally posted by mrvmax:
My problem is due to an unsupported chamber as was mentioned above. I was looking at that and then Buffalo Bore emailed and said that was what they were seeing from the pics. It all makes sense now, I’m contacting NFA to get the upper replaced. Thanks for the help guys.

Does NFA state their uppers are a supported chamber?

I am planning on using a stripped Rainer Upper I have, with a Brownells barrel and an NFA BCG. I actually had not taken into account the the supported vs. unsupported chamber aspect.
I know in pistols it is how the barrel is designed, is that the same for the rifle?

quote:

Originally posted by sigfreund:
Glad you are okay.

If I had to guess, it would be an overcharged cartridge. Second guess would be that it wasn't fully in battery when fired.

What I don’t understand is, if you are firing that slow why wasn’t the bolt closed ... the AR isn’t like an open bolt or even designed to fire unless the bolt is closed ... or are you saying that the case ruptured while being extracted?

Is the 10mm design that much different from the 9mm? On my Colt, even if the hammer were to follow the bolt while it was closing there isn’t enough force to strike the primer which on the M16 design there is that slight delay before the hammer is released by the auto sear ... unless you had a build up of crud that was holding your firing pin forward like an open bolt gun.

quote:

Originally posted by Black92LX:

quote:

Originally posted by mrvmax:
My problem is due to an unsupported chamber as was mentioned above. I was looking at that and then Buffalo Bore emailed and said that was what they were seeing from the pics. It all makes sense now, I’m contacting NFA to get the upper replaced. Thanks for the help guys.

Does NFA state their uppers are a supported chamber?

I am planning on using a stripped Rainer Upper I have, with a Brownells barrel and an NFA BCG. I actually had not taken into account the the supported vs. unsupported chamber aspect.
I know in pistols it is how the barrel is designed, is that the same for the rifle?

My situation is not normal but it never dawned on me until I got ideas here from members. I had feed issues so NFA took the barrel back and did some work for better feeding, that extra work appears to be removing material causing the neck to not be supported. My low power loads seemed to work fine but yesterday I looked over my brass and had numerous used pieces with bulging around the neck. I usually only inspect primers when firing so I didn’t notice the neck buldge while firing. The full power BB round being unsupported at the neck due to the extra material removed caused the neck to blow out meanwhile destroying my magazine and mag catch. I’ll see how the new barrel looks when I get it.

That would likely explain it.
I fugured most AR barrels would be made with a supported chamber. But go messing with it a bit and I can now see your issue.

I had a problem with Buffalo Bore ammo in this exact same load and caliber while shooting my GLOCK 20. I had it tested and I was told the ammo’s OAL was all over the place. In short, their manufacturing was not consistent and some rounds were short, some were overly long and some were within specs. For a hard cast bear load, I’d look elswhere - especially as BB charges a premium for their ammo.

quote:

Originally posted by kimberkid:

quote:

Originally posted by sigfreund:
Glad you are okay.

If I had to guess, it would be an overcharged cartridge. Second guess would be that it wasn't fully in battery when fired.

What I don’t understand is, if you are firing that slow why wasn’t the bolt closed ... the AR isn’t like an open bolt or even designed to fire unless the bolt is closed ... or are you saying that the case ruptured while being extracted?

Is the 10mm design that much different from the 9mm? On my Colt, even if the hammer were to follow the bolt while it was closing there isn’t enough force to strike the primer which on the M16 design there is that slight delay before the hammer is released by the auto sear ... unless you had a build up of crud that was holding your firing pin forward like an open bolt gun.

Look at this picture, this is a factory round that is fully seated. The bcg will sit against the outer surface but there is a good portion of the brass outside of the barrel and that is the area where it blew out. Other used brass I fired prior to that had a bulge in the same area that case blew out. I’m sure BB is loaded at the high end and was at max pressure. I’m pretty sure the bolt face was seated forward, I can think of a couple of ways it ruptured but I’m not sure what exactly caused it.

https://flic.kr/p/2ato2F4

Here is the explanation from Buffalo Bore, I never mentioned to him that NFA did work to the barrel to correct feeding issues.
“A lot of gun makers machine out the “feed ramp” area to facilitate feeding, but if that is done very much to the 10MM, it will leave too much unsupported case web for those 10MM pressures. Many years ago, I got two calls in the same day from folks saying they had a case web rupture with our 10MM ammo. Both were shooting 1911’s. I pulled out my hair trying to figure out what was wrong. I spent part of a day testing our 10MM ammo that was in stock, etc. I could find nothing wrong, so I called both of them back and discovered that they both had Clark Custom barrels in their pistols. I contacted Clark and told them what had happened and they looked into it and discovered that the company making their 10MM barrels, was relieving the feed ramp the same as they would for a 45 acp, which works well for 45 acp ammo loaded at or under 18,500 PSI, but the 10MM operates at or under 37,500 PSI and doing things the same way for a 10MM as you would for a 45 acp, was a mistake. Anyway, Clark fixed the problem and all was well again “

quote:

Originally posted by sigfreund:
Glad you are okay.

If I had to guess, it would be an overcharged cartridge. Second guess would be that it wasn't fully in battery when fired.

My first guess would be an Out Of Battery Event. Would want to see some better images of the Bolt Carrier firing pin interface and details on the trigger group before making that a firm call.

As for an overcharge it is possible of a fast burning powder were used for the load but the 10mm really doesn't lend itself to using a fast powder. Because the 10mm is all about the "Power" of this caliber and it's users typically do NOT shop for Powder Puff versions, they shop for the hottest load they can find, which means using a case filling slower burning powder.

I have no experience with the 10mm NFA gun’s, but have seen, handles, fired and own two in 9mm.
I have learned to really check them over when new. The last one I bought still had a 5.56 hammer in it (which causes issues down the road) and the castle nut on the buffer tube was 1/2 a turn loose.
A lot of the early guns wouldn’t feed 147gr ammo reliably because of just a touch too much OAL. We had to cut the throats on a few to get them to gee & haw with heavier bullets.
My 9mm pistol had to be cut, the rifle I just bought passed the “plunk” test and made it through my last PCC match w/ nary a bobble. Except for a junky Vism optic that I knew better than to put on it anyway.