Tangochaser,
The one I carried in Central America in the early 1980s had the moderator screwed right down to the front sight base. Due to a typo on my part (leave 0.02 inches between front sight base and start of threads turned into leave 0.20 inches between front sight base and start of threads
) my repro is just a tad long.
My original XM177 also had a light-profile barrel beneath the handguards though I cannot swear this was the original to the weapon as many parts on all of our weapons were switched out.
As it was, even with the 10.2" barrel, it turned out pretty nice.
XM177 repro on far right: