Headspace degradation on AR-308 rifle

[Des_Draws]

I built a 6.5CM gas gun last year using a SR25 pattern platform. Actually a Mega Maten lower and a stag upper. The barrel was a a very inexpensive 20 in made by DPMS ( i think)

I originally planned on getting a 6.5 CM barrel for my LMT MWSE but they weren't available and besides quite expensive.

I assembled the upper/barrel gas block and then checked the headspace using gauges and stripped the bolt of the extractor and ejectors . The no go gauge did chamber but barely. the field gauge did not chamber. Thought ok this not ideal but should be ok. Had some issues with under gassing opened up the gas port a tad and starting getting proper ejection etc

Started shooting with ok results 1 MOA to 1.5 MOA . Found some good loads. Gradually I started seeing cratered primer. tried less bumping of the shoulder. Not much improvement. lowered the powder charge but still getting nasty cratering of the primers. Eventually it was blowing and piercing primers.

What the hell ? So I checked the headspace again - now the field gauge chambers. By this time the barrel has about 500 rounds through it.
I thought was it the barrel or the bolt ?
I checked several other of my AR10 rifles with the same bolt - They all looked ok. I finally broke down and bought a good ballistic advantage 22in . Night and day no piercing no cratering very good accuracy
So sorry for the long winded intro but my question for the community is:
Why would headspace degrade so quickly ?
Was this just a bad barrel/ barrel extension or is something else going on

Thoughts comments ?

 

[owtlaw]

I built a 6.5CM gas gun last year using a SR25 pattern platform. Actually a Mega Maten lower and a stag upper. The barrel was a a very inexpensive 20 in made by DPMS ( i think)

I originally planned on getting a 6.5 CM barrel for my LMT MWSE but they weren't available and besides quite expensive.

I assembled the upper/barrel gas block and then checked the headspace using gauges and stripped the bolt of the extractor and ejectors . The no go gauge did chamber but barely. the field gauge did not chamber. Thought ok this not ideal but should be ok. Had some issues with under gassing opened up the gas port a tad and starting getting proper ejection etc

Started shooting with ok results 1 MOA to 1.5 MOA . Found some good loads. Gradually I started seeing cratered primer. tried less bumping of the shoulder. Not much improvement. lowered the powder charge but still getting nasty cratering of the primers. Eventually it was blowing and piercing primers.

What the hell ? So I checked the headspace again - now the field gauge chambers. By this time the barrel has about 500 rounds through it.
I thought was it the barrel or the bolt ?
I checked several other of my AR10 rifles with the same bolt - They all looked ok. I finally broke down and bought a good ballistic advantage 22in . Night and day no piercing no cratering very good accuracy
So sorry for the long winded intro but my question for the community is:
Why would headspace degrade so quickly ?
Was this just a bad barrel/ barrel extension or is something else going on

Thoughts comments ?

It would not be the 1st time I'd heard about a barrel extension going bad.

 

[Des_Draws]

It would not be the 1st time I'd heard about a barrel extension going bad.

Thanks I have heard of a bad BE when first manufactured. but I have not found any mention of a BE going bad over only 500 rds. I paid very little < 100$ I wasn't expecting much - but " you get what you paid for" applies.
In any case thanks for your reply

 

[Bobby Ball]

So I was planning to do a write up on this for a while.. I have an Armalite AR-10T with the 24" heavy Krieger barrel. And I have thrown endless amounts of parts at it upgrading it, and it has just shot like total shit. With custom hand loads, without going into extensive details everything on it was dialed. Finally (and believe me it's embarrassing I didn't even consider checking sooner) I went to check the free bore and it was .264 thousandths. I called and spoke with the guys at Armalite, I just told them what I had going on, wasn't mad, wasn't looking for anything, other than just some wisdom. And they were as dumbfounded as I was. They asked me to send it in to look at. I haven't yet unfortunately because I just moved, and then have been working out of country for the last 2 months. But I poured a chamber cast of it just to confirm and have on hand. I mean this thing was sent out to Geoff Korn for custom work on Armalite's direct recommendation. It's nowhere near a high round count and all of us are just sitting here scratching our heads like WTF?