Quoted:
Bigger question, who built it, and was it test fired before it left there shop in the configuration you are going to be running it (back pressure of the bore with a suppressor attached).

Short of this, then make sure to clean it correctly (including scrubbing the chamber with a Chamber brush with CLP by hand), make sure  the upper receiver bearing surfaces are correctly lubed), and not a bad idea to bring some super sonic factory ammo with you as well.

Quoted:
Because if the gas port is too small for the sub ammo, even with the back pressure of the can, it may not cycle correctly.  This way if the rig is acting up with the sub ammo with the bolt unlocking too slow/late, at least you don't loose range time by not having super ammo that my cycle through it fine so you can at least shoot the rig.


If is does not lock the bolt back to the bolt catch on the  first single loaded round in mag, the empty mag left in the mag well as the single round is fired test, go find the spent case to take a very good look at the rim of it.

If the rig is under gassed with the bolt unlocking too late, the rim should not be bent.  

If the rifle is over gassed with the bolt unlocking too soon isntead, then the spent case rim should be bent to hell isntead.

As for getting the bolt to unlock faster, lighter recoil spring, and to slow the bolt down from unlocking too fast with the residual bore pressure to high and causing the spent case to be too pressure welded to the chamber isntead, stronger tension recoil spring.


And again, the gas system is going to have more gas with the suppressor in play, since the suppressor creates more back pressure to get more gas down the gas tube to the carrier.  

And yes,  you really need to take a look at the spent case rim to tell the difference on a short stroke, since both the bolt unlocking too slow, or unlocking too fast, will both cause the same short stroke of the action (to not allow the B/C to retract far enough for the bolt catch to catch the bolt face and lock the bolt back).

Lastly, since this is a 300blk, make sure that you either have 300blk mags, or you have modified 233 mag into 300 blk mags.  Hence the bullet is much wider on the 300 than the 223, and with a straight 223 mags, the rounds are not feed cleaning up and out of the mag, and will cause all kinds of jam problems on the feeds.

https://www.youtube.com/watch?v=https://youtu.be/wkwJ5DT9Uhs

To add, an good shop that is building a one off rifle should be test firing the rig to make sure it going to run before it leaves the door.  Not only will they catch a gas port sizing problem, but leaks or blockages in the system that may cause the rig to no run correctly before it leaves the shop.   Also, gives them time to do a test group pattern to make sure that the rig is printing correctly and not a problem with the rig in this area as well.

Quoted:
Just a FYI, but if needed with the JP Enterprises buffer, you can use quarters as shims to get the bolt face to stop in the correct loaction.


Hence pull all the way back on the charging handle and take a look at where the bolt face stops against the back of the ejection port window with the handle all the way back.
The correct place the bolt face should be stopping is 1/8" to 1/4" in front of the back edge of the ejection port back edge.  

If your bolt face is stopping flush with, or retracting behind the back edge of the ejection port window, then pull the JP Enterprises buffer out of the receiver extension, and start dropping quarters into the tube as shims to get the bolt face to stop in the correct position against the back edge of the ejection port window.
Note, the bolt face has to retract at least 1/8" behind the bolt catch, so if you are shimming forward, make sure that the bolt on the CH pull is still retracting back past the bolt catch.

Granted that it may not stop the hard close up problems (guessing that the rig is going muzzle down on the lock up, and more of a lighter rig problem), but will help to soften the lock ups with the buffer-B/C  not having a longer run at stripping the round out of the mag and then locking up the bolt.

Quoted:
You lost me, and if the problem is drifting the rear take down pin over to shotgun the upper open so you can remove the buffer and just clean the rig between shootings, then a chop stick or Golf tee works great as a punch.  Hence if the rear take down pin is really snug to drift it over to allow the upper to be unlocked from the lower, the non maring chop stick or Golf tee will blow up long before you do any damage.   Also to note, the take down pins only get drifted over until they clear the upper take down lug channels.  If you try to punch the detent retained pins all the way out, your going to crack the lower receiver .

A detent pin rides in the channel, so once it starts to drift over from the closed position, just push on the pin until the detent rod has traveled to the far end of the channel/ the farthest that the take down pin is going to travel until the detent pin is removed.
http://cdn3.volusion.com/zwlmq.eyhrp/v/vspfiles/photos/GATDP-2T.jpg?1446204273


As for removing the buffer once you have the upper open, then just need to depress the buffer retaining pin, and slide the jp assembly over it.  As for the fun part due to the Jp design, its getting the last of the buffer out.  For this, small dental pick works great, since you can get under the back of the buffer spring section to press the buffer retaining pin the final time to get the back section over the back part.

http://cdn0.thetruthaboutguns.com/wp-content/uploads/2013/10/silentcapturedspringcollage.jpg

Quoted:

I have no buffer retaining pin.  It was left out to make changing the buffer springs easier since the qd mount was staked to the castle nut.