Let me begin by saying that I do NOT have a statistically relevant sample, however, between a buddy and I, we have three 16" 1X9 Colts and one 20" 1X9 Colt. All have shot their best groups ever at 300 yds with 75 gr .223 specification TAP. Temperatures were in the 30's (F) on all range days, and instrumental velocities from the 16" guns were 2550fps, give or take. Group sizes were consistently 1 to 1.5 MOA at 300 yds, with enough sub-MOA groups to be scary. This stuff shoots folks.
POI has been within the margin for error at 300 between 55gr XM193 and the 75 gr TAP. It shoots higher than the 69gr ammo in our guns at that range. The ballistics calculations say the TAP should track within less than an inch between the muzzle and 300 (zero range) with the 55 gr, and hit 50% harder based on kinetic energy.
The Hornady tech rep I spoke to said 1X9 was fine down to a 16" length. I did not believe him, but based on that encouragement began what became a very succesful experiment for me, YMMV.
I have never checked the twist on my Colts, they say 1X9, but tolerances and nominal dimensions may vary among manufacturers. Mine work, I would suggest trying yours so you do not miss out on good ammunition.
That said, new barrels for me have been and will be 1X7 for 5.56/.223 until something better comes along in widespread usage. I started down the heavy bullet path in the 1980's and lost my way for a while. 55 grain bullets make for cheap practice these days.