Step U7: BCG and Charging Handle Installation

If the BCG will not seat completely or correctly into the upper receiver, STOP.  Verify the following:

• Ensure that the forward assist is not stuck in the depressed position or otherwise protruding into the upper receiver.  If so, remove the forward assist per Step U1 and inspect.
• If the dust cover is closed and will not pop open, it may be stuck.  You may force the dust cover open with the BCG or upper receiver action rod and correct the issue per Step U2.
• Ensure that the gas key is not striking the tip of the gas tube.  If so, the gas tube may be bent or the gas key may be installed crooked.  You can correct the alignment of the gas tube using a gas tube bender.  If the gas key is misaligned, it will need to be reinstalled on the bolt carrier.
• Ensure that the dimensions of the gas tube ferrule and gas key are correct and compatible (i.e. the gas tube fits inside the gas key).
• Ensure that the bolt is fully extended.  You will not be able to insert the BCG very far into the upper if it is not (the cam pin will strike the back of the upper).
• Ensure that the bolt is not rotating before it is in the forwardmost position in the barrel extension.  If the bolt is rotating early, the lugs will not align with the cutouts in the barrel extension.  Over time, the cam pin will gouge the inside of the upper receiver.  Normally, the cam pin is retained in the forwardmost position by the inside of the upper receiver.  When the cam pin head reaches the semi-circular cutout in the upper, it will be able to move out of the forwardmost position.  This allows the cam pin to ride in the carrier cutout, which forces the bolt to rotate and lock inside the barrel extension.  Under normal operation, the bolt will meet resistance when it strips the next round from the magazine.  This puts rearward pressure on the bolt, pushing it back into the carrier.  As it does so, the bolt puts rearward pressure on the cam pin, which tries to ride to the rear in the carrier cutout.  As it does, the edge of the cam pin presses against the inside of the upper receiver.  As the edge of the cam pin rubs the inside of the upper receiver, it progressively gouges a groove.  As this groove deepens, it allows the bolt to rotate slightly before it reaches the forwardmost position.  This can cause the lugs to strike the barrel extension.  This will generally take MANY, MANY rounds to occur.  However, if this scenario occurs, you will need a new upper receiver.
• Ensure that the barrel is properly aligned.  The indexing pin should be seated in the corresponding slot in the upper receiver.  If the indexing pin has broken off, the barrel may be seated too far into the upper receiver or may be misaligned in the upper receiver.  In this case, the barrel or barrel extension may need to be replaced.  If the indexing pin is intact and correctly positioned, and you used the recommended upper receiver action rod, the barrel alignment should be correct.

If you are still unable to trace the problem, try a different BCG (is available).  If the problem resolves with a different BCG, the problem is probably with the original BCG.  If it doesn’t resolve, the problem is somewhere inside of the upper receiver.  Carefully inspect for obstructions.