Different powders are needed when trying to maximize velocities on 55gr, 69gr, and 77gr .223/5.56 loads. Be sure to recognize if you have a 5.56, wylde, or .223 barrel. They will handle different pressures. It is your responsibility to understand this.
I'm in the process of developing loads to replicate Lake City M193 (55gr FMJ) and IMI MK 262 Mod 1 (77gr SMK). To heck with accuracy. I want to duplicate military loads in my garage. I'm quite sure that accuracy will not suffer much, or any.
I'm using a 14.5" 5.56 SBR AR-15 for the development with a Labradar to measure velocity.
I am resizing with a Redding small base full length die. REM 7-1/2 primers, WCC brass.
Here's the mil-spec ammo velocities that I've measured in my 14.5" barrel with the Labradar:
Lake City M193, 55gr, FMJ (20 rounds): 3149 fps
IMI MK 262 Mod 1, 77gr, HPBT (5rounds): 2712 fps
For the 55gr FMJ max power blasting load, I cannot reach the Lake City velocity with Varget powder. I run out of case capacity at 27.4gr with a velocity of 3001 fps. So I gave up on Varget for 55gr target ammo and moved to H335. I'm still working upwards with H335 powder and have reached 3092 fps with 26.0 grains. I'm about to try 26.1 and then 26.2 grains of H335, and then stop, even if I don't see pressure signs.
77gr MK 262 Mod 1 loads have proven to be much easier to replicate. 24.9gr of Varget has produced an average velocity of 2715.83 fps with 10 rounds. 23.4gr of Alliant AR-Comp has resulted in 2710.39fps with 5 rounds. I am going to load more rounds of Varget and AR-Comp to establish accuracy and std-dev, but I got a 9.0 standard deviation with AR-comp so that may be the "go to" powder for 77gr match type bullets.
These loads are in excess of the existing manuals. Reloading manuals are based on .223 and not 5.56 max pressures. You are responsible for safe reloading. Do no duplicate these loads without working up to them with knowledge about pressure signs and your gun's chambering.
I'm editing this because the charges listed above for 77gr SMK with Varget and AR-Comp proved to be overpressure, resulting in blown primers and enlarged primer pockets. I've settled on 22.8gr of AR-Comp with 77gr SMK.