I'm having a problem where my 11.5" barreled Colt duty rifle is not locking back reliably on the last round. Sometimes it locks back properly, but sometimes it locks back halfway, with the bolt stop on the carrier instead of the bolt face.
There have been no other issues or failures whatsoever.
Details:
Rifle: Colt 6940, barrel chopped to 11.5" by ADCO, gas port should be .073. Rifle has seen a total of maybe 1000 rounds, and is in excellent condition. Gas rings not aligned, gas key not loose. Properly cleaned and lubricated. Lock back issue did not exist when the barrel was 16".
Magazines: Multiple Lancer L5AWMs in working condition.
Buffers: Started with Spike's ST-T2, changed to H3 on recommendations. Did not solve issue, MIGHT have made it slightly better.
Using stock Colt spring.
Ammo: Tested with M855, Federal Champion .223, and Federal .223 T223E duty ammo.
Problems: Rifle has fed ammo just fine while firing, no short stroking with any ammo, even during rapid fire.
The issue is that with the .223 ammo, the bolt fails to lock back on an empty magazine almost every time (seems slightly better since I switched buffers from ST-T2 to H3, but I'm not sure). Locks back almost every time with M855, but not always.
When it fails to lock back, the bolt is NOT going all the way forward on an empty chamber, rather, it is locking halfway back with the bolt stop resting on the carrier instead of the bolt face. So far, through hundreds of rounds, this is the only type of failure that I have experienced.
Rifle ejects to the 3:00 position without much force, brass goes about 2-4 feet.
I'm thinking that out is slightly under gassed. The gas port was supposed to be .073, but I have no way of confirming.
Rifle has not been shot suppressed, but it will be later when I get a suppressor, so I don't want to open the gas port more than I have to.
Please let me know what you think.


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