I dunno if anyone is really interested in this sort of thing, but here's my XM607 build... Upper is true vintage heavy barrel (very heavy) Colt XM607 from Colt's prototype department. Proper vintage triangular charging handle and chrome "slick-side" BCG, all on an Sp1 lower. The wire stock is an M231 copy by DPMS, but I should have the real thing in time.
Fired it supported, but handheld, for 20 rounds @ 50Yds (10 were rapid fire), with iron sights only... The heavy barrel keeps them all pretty tight!