Service rifle competition.

For the past few years I've been shooting SR competition with a standard Colt HBar A2 with a 20" 1:7 barrel and iron sights.

My go to load included a Nosler CC 77g bullet.

I've recently completed a new build. A 16" M4 style rifle with a mid-length gas system and a 1:9 twist with the Hi-lux 1-4.5 optic.

Now I'm wondering with the shorter barrel and "lighter" twist, should I downsize the projectile.

Any insights would be appreciated.

You don't have to.  You might go to a Sierra or Nosler 69 or Berger 73 to stay at the same short-lines velocity as the 20-incher for wind-bucking, but it's not necessary.

I set up a carbine just like yours for my son to shoot in the military as-issued match at Camp Perry.

Use an A-Max 75 for the 600 slow fire (a 1-9 may not stabilize an 80-grainer).  What negligible speed you may lose in muzzle velocity you gain back using the optic and the sliding stock.

A true or fast 9 might barely stabilize a 75gr AMAX/ELD. The several barrels I've measured indicated button rifling to be slow. OP, try the 77s, but the 69gr SMK/CC is the sure thing.

Sinister, a while back, how did the 60gr TMK fare as an external ballistic substitute for M855A1?