What ammo, since I guessing that they are reloads, or if factory ammo, your riding the charging handle forward; with both causing the bolt to not fully lock up.

Bolt not fully locked up, means that the tail of the bolt is not out the back side of the carrier, and when FP is struck, it's just banging off the back of the carrier, with just enough energy to lock the bolt up instead (not enough impact to the FP once the the tail of the bolt protrudes out the back of the carrier to ignite the primer).  As for when the bolt is locked up (before you pull the trigger) the face of the carrier should be tight against the face of the barrel extension/


As for ammo, run it through an ammo gauge.  Could be that the cases where not fully sized to begin with, or even too much bullet crimp used to budge the should of the case.
Hence when you are cycling in the first round, may not be enough B/C to get the bolt locked up, but when the rifle is self cycling rounds, it has enough speed to lock up the less than stellar rounds isntead.


Also, when you where building the rig, tell me that you pulled the bolt of the carrier and used the carrier with key dry fitted into the upper receiver to confirm that the gas tube was correctly indexed with the carrier key.  If the gas tube was not indexed correctly, them it should be tweaked over the center of the barrel to align it correctly with the key.
And no, you don't rotate the gas block on the barrel to index the tube to key, since the gas block passage has to be indexed with the barrel port instead.

Once you check the alignment of the key to gas tube, and even to make sure that the sides of the key are not binding up on the inner U slot of the upper receiver,

Load a live round via the mag with the safety on, and take a good look inside the ejection port to make sure that the face of the carrier is tight against the face of the barrel extension.

If the key is not aligned with the gas tube, then it could be enough to either cause the bolt to not fully lock up, or worse, the carrier does a slight bounce off the barrel extension at lock up, and the gas tube to key bind is enough to not allow the carrier to move back forward instead.


Short of this, make sure that the hammer spring is installed the correct orientation,


And you have take the time to pull the bolt/extractor, and clean the inner slot and channel for both in the bolt.
Also, if you do find that you have a O ring around the extractor spring, remove it.   The O ring is more of a band aid, and it can keep the bolt from fully locking up since it creates too much extractor tension, and does not allow the extractor to slip over the case rim at loading cleanly; to allow the bolt to fully lock up.  If the mags are new, they too should be stripped down and CLP cleaned as well.

As for 300 blk out mags, would help to know which type are in play.  With magpul 223 mags, the inner ribs has to be slightly modified to allow the rounds to sit cleaning against the feed lips.

[youtube]https://youtu.be/wkwJ5DT9Uhs[/youtube]

Still sounds like a lock up issue (bolt not locked home), so before you pull the trigger, confirm that the face of the carrier is tight against the face of the barrel extension.


Also, pull the B/c out of the rifle, drop a round into the chamber and fully seat it home in the chamber with a wooden dowel, then tilt the muzzle upwards to confirm that the round is dropping free of the chamber.  If the round binds in the chamber/does not drop free, figure of it the problem is the bullet embedding into the rifling, or just the case not fully sized (even after bullet crimping) that is causing the problem instead.