Well, since nobody has a reply...I'll post my latest findings.
ArmaLite suggested to my Gunsmith that the UPPER be sent to the guy that does their work...
The factory muzzleBRAKE was hydraulically press installed over a "serrated nubbin" <img src="/ubbthreads/images/graemlins/ooo.gif" alt="" /> and DOUBLE PINNED into place!!!
This requires a special procedure to UN-DO... <img src="/ubbthreads/images/graemlins/mad.gif" alt="" />
I'll need to send the UPPER back to the 'guy' who does this work, or 're-work', and he'll thread the 'NUBBIN' to accept the Yankee Hill Machine Phantom Flash Hider...
This is the only way I can accept the "Infantry Profile" bbl. within the confines of the Quad Rail, etc., that the bbl. ride in.
My only other choice is to purchase a 'T' bbl. and have it cut down to 20".
Not acceptable, especially cost-wise.
SO?
I'll be sending the UPPER in for the 're-work' and I'll be happy with whatever Professionally done work they do...! <img src="/ubbthreads/images/graemlins/cool.gif" alt="" />