Bolt Change Subsonic Cycling Issues

[themonk]

I have a random issue which you guys may be able to help me with. I wanted to try out 300 BLK so I built up an upper:
BCM upper receiver
CMMG 16" barrel carbine gas with a mounted FSB
BCM carbine gas tube

I started out testing the upper with a LMT full auto BCG that I have a from an LMT upper of mine. That BCG probably has around 2.5k 5.56 round through it. The 300 BLK upper with that BCG has been excellent with both super and subsonic rounds.

For Christmas my wife got me a BCM full auto BCG that shoots perfectly with 5.56, 223, and 300 BLK supersonic. Today I was doing some testing with Remington 220 gr subsonic and had issues with chambering the next round and the bolt locking open.

Do you think this is just a new BCG needing to be broken in? I weighed each bolt and they both weigh the same and both are full auto groups.

Thanks in advance for any advice!

[farm use]

Thats what it sounds like to me.. Just needs to be worn in, keep it wet and cycle it by hand a hundred times or so.

[smustian]

Give the new bolt gas rings a chance to wear in a little. You could swap bolts in the carriers to test my theory. If the problem follows the bolt then you know where the problem is.

[themonk]

Give the new bolt gas rings a chance to wear in a little. You could swap bolts in the carriers to test my theory. If the problem follows the bolt then you know where the problem is.

Great idea - I will test this tomorrow

[themonk]

Bolt break-in was the issue. After a couple hundred rounds the gas rings a much looser. Shot about 100 rounds of subsonic this morning with no issues, starting to really love 300 blk, just need my 762-SDN-6 paperwork to come back.

Thanks guys for the advice!!

[300Blk]

If gun parts needs to be broken in, then there is another problem. A properly designed rifle will work even when new.

Probably the barrel had too small a gas port or you don't have a carbine buffer.

[ThreeHundredTraveler]

If gun parts needs to be broken in, then there is another problem. A properly designed rifle will work even when new.

Probably the barrel had too small a gas port or you don't have a carbine buffer.

Even if every part is 100% compliant to a mil spec drawing, the outside envelope of tolerance stack up can make one part or the other tight or loose. I spent far too many years doing defense work to rule out a tight fit. And in many cases, no fit at all. Of course, being in the government contracting business that meant taking a $200 part and analyzing it until you found out what was wrong. Maybe spend $50,000 and then throw the part away and put in a good one. Almost without exception, a no fit was a tolerance stack problem. Min size hole with max size part to fit in.

[themonk]

If gun parts needs to be broken in, then there is another problem. A properly designed rifle will work even when new.

Probably the barrel had too small a gas port or you don't have a carbine buffer.

If that was the case why would it have fixed itself?

I had the issue with a brand new BCG and only with subsonic ammo, now after a couple of mags she runs like a top.

[300Blk]

If the gas port is undersizedsized or you have a heavy buffer, then the bolt most likely would have worked in other rifles and if so, I would not say that was the problem. If those other things were correct, then it was the bolt. I am only saying that you don't know which was the problem without checking out the other things too.

[themonk]

If the gas port is undersizedsized or you have a heavy buffer, then the bolt most likely would have worked in other rifles and if so, I would not say that was the problem. If those other things were correct, then it was the bolt. I am only saying that you don't know which was the problem without checking out the other things too.

I'm with you. Rifle ran flawlessly both with subsonic and supersonic with LMT bolt so my assumption was it was a BCG issue.