On my most recent AR-15 upper receiver build, the American Built Arms (ABA) nickel boron BCG’s gas key will not engage with the end of the Spikes Tactical mid-length gas tube.  It’s a full impact between the gas key and gas tube; not even close.  Prying gas tube up/down, left right no help.  Not that that’s a solution anyway:  The gas tube is perfectly centered on all axes.  Upper Receiver is Aero Precision.  (http://www.avantlink.com/click.php?tt=cl&pw=55963&ctc=j0zys5gg2b00zk8a00y51&mi=10077&url=http%3A%2F%2Fwww.brownells.com%2Frifle-parts%2Freceiver-parts%2Freceivers%2Fupper-receivers%2Far-15-m16-stripped-upper-receiver-prod54395.aspx ) Barrel is new Daniel Defense 16-inch Gov’t profile (chrome-lined); gas block is Sadlak Industries low profile; rail is Daniel Defense DDM4 12-inch; charging handle is Vltor BCM Mod 4.  Removing the charging handle is no help.  This is a mid-length gas system, all parts are new, and this is the first such problem I’ve encountered after building twelve (12) identical complete uppers like this always using the exact same new components.  Visually, everything appears to fit perfectly.  However, the BCG abruptly strikes the doughnut end of gas tube and remains 1.75” out of battery.  Black ink marker not much help in revealing impact points.  Visually, the gas tube appears to perfectly center within the Aero Precision upper receiver’s recess track.  There is no reason the gas tube would not be perfectly aligned:  the DDM4 rail fastening system is not like a carry-handle’s modern rail attachment, i.e.  there is no knurled star on the barrel nut through which to align the gas tube, or shim.  That is, there is no potential for the DD proprietary barrel nut to alter the alignment of the gas tube entering the upper receiver.  And, I swapped BCGs out and inserted a new Brownells nitrided BCG:  same exact problem.  Thoughts?   What are the chances the Aero Precision upper is internally out of spec?

Pull the gas tube out and see if the the tube can be inserted into the BCwhen it is not in the gun?
See if the BCG and CH will go into battery with the gas tube, not  installed?

Well, that's the problem.  Brownells shipped me a carbine length gas port hole on this new 16" DD barrel.  I bought 4 DD barrels at once, and never noticed the difference on this one.  That "impact" was the gas tube bottoming out inside the gas key.  As you guessed, the end of the gas tube protrudes ~ 2 1/8" too far!  Thank you.