Can you manually lock the bolt carrier assembly back when you pull the charging handle back on an empty magazine? Remove the upper receiver assembly, insert an empty magazine into the magazine well and take a pic of how the magazine follower makes contact with the bolt catch, and post it please.  

CY6
Greg Sullivan "Sully"
SLR15 Rifles
TheDefensiveEdge.com
(763) 712-0123

From the pic it looks like the bolt catch is being pushed up by the magazine follower like it should.  Can you manually cycle the action back using the charging handle and get the bolt to lock behind the bolt catch, showing it can mechanically be done and nothing is hindering the bolt carrier assembly from going all the way rearward?    

CY6
Greg Sullivan "Sully"
SLR15 Rifles
TheDefensiveEdge.com
(763) 712-0123

The mag catch does not look like is coming up high enough via the mag follower,


So when you pull the charging handle all the way back, is the tension needed the same as from about the middle of the charging handle pull (read last inch of the charging handle pull is not harder like either the recoil spring is coil binding, or the butt stock bolt too long and now protruding into the back of the tube isntead.

When you pull the charging handle all the way back, does the face of the bolt stop about 1/8" to 1/4" in front of the back edge of the ejection port window (read well behind the bolt catch)?


On the mag catch, does it move smoothly in the receiver channel for it (just spring tension pulling it back down), or it is binding in the receiver channel instead.


If you installed the bolt catch your self, did you check the bottom of the spring channel for crap that may have forget to clean instead.  Also, spring goes in first, then the buffer on top of the spring, then the catch with roll pin.  Lastly here, some catch springs are too long/ the channels not drilled deep enough, and some tweaks needed to be made so the follower will raise the catch up high enough.


Short of that, make sure that you are cleaning the rifle correctly, including a chamber brush with CLP to scrub the chamber by hand, and then the upper receiver bearing areas are CLP lubed correctly before the rifle is short.

If all the above check out, then really need to check the chamber conditions and head space of the rifle (shot head space will cause short stroking), and that the gas system does not have any major leaks.  Hence quick way to check for short stoking even with a bolt catch problem, the ejection of the spent case are not about 10' to 15' out of the rifle, but instead dribbling out a much shorter distance isntead.

Key bolts may be tight, but the problem child carrier may have something (bur) under the key to carrier surfaces to cause the key to be leaking from the top of the carrier surface instead.


Its easy enough to check, and put some CLP around the base of the key to the carrier, hold the bolt in, and pressurize the front the key with 100lbs of air to check for a leak between the two.  If a leak is found, send the carrier back for a replacement.  Hence you can resolve the problem easy enough by removing the bur and lapping the two into each other, but you don't reuse key bolts, and would need new bolts, then have to stake the key back into the key bolt side of spines when you are done.

You can swap the bolts on the carriers to  live fire test,  to weed the problem down to either being a bad bolt or bad carrier alone.

If it runs off of one bolt carrier, but not the other, then it has to do with the bolt carrier.  On the bolt carrier that doesn't run, try dry fitting just the bolt carrier (No bolt, cam or firing pin) into the upper receiver, and see if it moves freely back and forth in the upper receiver with no binding, as it should freely move.  Also on the suspect carrier, inspect closely around the carrier key (gas key) and look for any carbon that may be coming out from under the gas key (jetting at a rear & downward angle) as this may be an indication of a broke carrier key screw (have seen this a couple of times).  Also does the rifle perform any different on American made high quality 5.56 ammunition?

CY6
Greg Sullivan "Sully"
SLR15 Rifles
TheDefensiveEdge.com
(763) 712-0123

What solvents are you cleaning and lubing the rifle with?  
Are you using a chamber brush to scrub the chamber by hand with CLP each cleaning?

Need to know if gas block, or if FSB with taper pins, and confirmed that with the bolt removed the carrier, the carrier dry fitted in  the upper receiver to confirm that the key is not binding on the upper slot, and the gas tube is correctly indexed with the key.

As for Bushmaster carriers, used to have a problem with them snapping key bolts to cause leaks of the key to carrier, but pretty sure that problem was solved a long time again. That is, if you have the correct  buffer in play for the length of the receiver extension, and the back of the key is not crashing against the back of the lower receiver each stroke to snap the bolts instead. Hence with the charging handle pulled all the way back, the face of the bolt stops 1/8" to 1/4" in front of the back edge of the ejection port window, and does not stop even with, or retracts back past the back of the ejection window edge instead.


As for building the rifle, lets call it more of assembling the produced assembly by someone else together instead.
The difference, is during an armory class, you have parts in the build bins that are out of spec, and check to see if the armor catches the bad parts during the build  on his checks during the build, instead of the rifle failing on a live fire isntead.

Couple in the rifle fouling out quickly from the wrong solvents use to clean and lube it, and was thinking the same thing.

The carrier with key (no bolt) dry fitted in the upper receiver would have told you if the carrier was the binding problem in the upper receiver.  Hence when you have a problem here, it normaly the sides of the carrier key that you need to lap to remove some width from the carrier so the key is not binding in the upper receiver slot.


Truth is, I think that I see the problem instead,


Take a good look at the tail of the bolt, notice that the start point to the rounded bugle section of the bolt is not a smooth transition,  but instead a larger OD instead.  Couple that with the wicked wear marks on the start of the larger step of the start point of the bugle section from where it was binding on the start channel of the carrier bolt tail section when the bolt was locked up, and this would explain why the B/C was having problem cam'g the bolt forward at unlock isntead.





So on that note, would still try to have the bolt replace, or if that is not a option, then chuck the tail of the bolt up in a drill or lathe, and reduce the step section on the bolt tail just before the bugle section so it the same OD as the rest of the tail of the bolt instead.  In a drill, you can wraps some sand paper around a small flat jewelers file, and spin sand down the high ridge to flush it out with the rest of the tail of the bolt.  In a lathe, C6 insert will make short work of removing the high step back so the parallel distance of the tail of correct, and a little sand paper work will make smoothing the new produced edge back to flush with the rounded bugle section isntead.