I found it interesting that Ramshot's published 5.56 data for TAC and 2460 show one full grain and ~100 fps more for the 75gr BTHP compared to the 77gr SMK.
That really doesn't make much sense based on the length of the bearing surface of the various bullets.
Here are the bearing surface length's on several of the bullets mentioned:
75 Hornady OTM = 0.408"
77 Sierra TMK = 0.425"
77 Nosler CC = 0.420"
69 Sierra TMK = 0.337"
69 Sierra SMK = 0.315"
Based on the bearing surface comparison, I can't see much reason for powder charges to be much different for the Hornady compared to either of the Sierra 77 grain bullets & I am using the same charge of AR Comp for all 3........velocities for all 3 are within 25-30 FPS of each other in the same gun.
However, that's a 3% difference in bearing surface, so straightlined on a 23 grain powder charge that could be a .69 grain powder weight increase for the Hornady..........maybe.
Varget will get as much velocity with 77's as just about anything else if you load it up to it's capability, so will RL-15..........highest velocity might always = best accuracy though.
8208XBR is an excellent powder but falls a little short on velocity compared to the others above but it's generally pretty accurate.
MM
