quote:

Originally posted by Boanerges57:

Also the twist rate was 1 in 9 for M16A2 and 1 in 7 for M16A4 and M4 (there was an earlier post talking about 1:14 and 1:12 referencing the SS109 and M193 rounds, SS109 is obsolete and M19x series are designed for optimal twist around 1:7).

You need to get your stats right. The M16A2/A3/A4, M4 and M4A1 are all 1 in 7" RH twist rates. This is due to the higher twist rate needed to help stabilize the M856 tracer ammunition. When the M16 was first designed (first known as the AR15 by Armalite and Stoner) is had a 1 in 14" twist rate which was later changed to 1 in 12" twist due to arctic testing showing that the slower 1 in 14" twist would not stabilize the M193 55 gr round. The 1 in 9" twist rate is a civilian creation by the manufactures due to the wide variety of grain weights encountered in the civilian world. The 1 in 7" twist is best for 60 gr and up, the 1 in 9" is good for about 69-50 gr, and the 1 in 12" is good for about the 55 and lighter. The 55 gr. in a 1 in 7" twist will over stabilize the round, it is best used for the M855 round. Further more the SS109 is the NATO designator for our M855 round which tends to run at hotter velocities.