I built this 10mm AR pistol with a 12.5" barrel. It's the fifth one I have built but is the only 10mm, the others are 9mm. This 10mm is having a problem I have never seen. One out of 10 cases will separate leaving the front half of the case in the chamber. One out of three cases that don't separate show signs of getting ready to separate. This was with new ammo Sig, GA Arms and Hornady. I also used my loads with new Star Line brass.
The chamber looked rough so I polished it and got the same results.
These pistol caliber AR's are blow back operated so it is not an over gas issue where the bolt is trying to rip the case out. The case is pushing the bolt back.
So what is the problem? I need help

I would check the chamber size to be sure that it was in-spec first. Have you tried this load in any other guns? signs of over-pressure or case separation in those guns?

__________________
On the Oak Savannah

Sounds like not enough bolt/buffer weight or the recoil spring is soft.

Is it possible the chamber is slightly short and it is firing out of battery?

__________________
Release the Kraken

I'd check chamber dimensions too.

I have used this same ammo in a Smith 1066, Smith 4010 and Glock G20. No issues. When I get home I will insert my 10mm chamber reamer and check depth. I am running an extra heavy duty spring and 10 oz buffer.

H&K and CETME engineers have a solution.

Attached Images

flutes.jpg (41.5 KB, 79 views)

I checked the depth and it appears OK. Inserted a fresh round into the chamber, looked good. By the way the barrel is off the frame now. Next I put a round in the bolt and slipped the round into the chamber, no problem. Then I took a spent case that showed sign of separating and it would only go in as far as the separation mark. Could there still be pressure enough to separate the case on the way out? This would lend support to light weight recoil components. I am using components from the same supplier for 10mm. 10 oz buffer, XP spring and heavy 10/40 bolt. I am going to look for a heavier buffer and spring. You guys have a supplier suggestion? By the way, the barrel is 12.5" long. Maybe its too long?

I had the same problem with a 9mm AR. Buffer spring was too light.

Attached Images

6503F850-F5A8-4D4A-9251-2965D0747845.jpg (109.1 KB, 18 views)

__________________
213th FBINA

I agree that you should try a heavier buffer.

Heavier buffer and spring.

KAK Industries makes a modular buffer that comes as a kit with 4 buffer weights made with different materials. Using all four Tungsten buffer weights you can get the buffer up to 13.4oz if needed.

Could be your headspace , I’d check it. Go no-go gauges not expensive
Also don’t know whose brand of 10mm barrel and parts your using some a plaqued with issues. I always using quality buffers and springs on my pistol caliber AR builds and never had issues. I have an MGI Buffer that would work great with Sprinco 308 carbine spring it’s the only mechanical buffer on the market I never had to worry about buffer weight ran 9mm, 5.56, 7.62x39mm and 50 Beouwulf with it. The heavier the recoil the more the buffer works, soft shooting , no more twang on your war from buffer springs etc.

Great ideas and comments! I will go with a Sprinco spring first, $18. I supposedly was sold an XP spring before but who really knows because it had no special markings. Sprinco has a great website that explains the conditions and they color code their springs. I ordered orange as recommended plus a spare red spring to test in my 9mms. Thanks to all who commented.

Did you find a solution that solved the problem?

When they leave a thread that long, you need to PM them. Ask them to come back and reply so we all get the follow up.

Ivan