@RickB, Thanks for the correction on the twist of the M16 and M16A1.
@scsov509, The trend as the rate of twist is going to be as you say. The magnitude is another question. Assuming my freshman physics and integral calculus is right, adding a typical twist makes a small negative correction to the velocity that is proportional to the twist squared times the radius of the bullet squared. Since the number of turns per inch of the twist and the radius of the bullet in inches are pretty small numbers, your meter might not be able to detect the change.
In which case what I said about velocity is correct but kind of irrelevant.
