Full length resize with a small base die. For ammunition to be used in multiple rifles, size the case as closely as possible to factory dimensions and that requires small base dies. Use RCBS water soluble or Imperial sizing wax as lubes. Use case gauges to set up your dies
You will never get the proper base to shoulder lengths if you follow the typical die instructions that say,
"screw the die to the shell holder plus a quarter turn".
Trim case to minimum trim length. Trim each firing. Bevel the inside of the case mouth, and remove the flash from the outside. Cut the crimps out of military brass, various ways to do this. Clean the pocket out. Prime with the least sensitive primer for a gas gun, I recommend CCI #41's . The 41's are CCI's military primer, these primers are less sensitive than the standard commercial. Federals are the most sensitive primer and there are more slamfires with Federals than any other brand.
Prime by hand, and if the primers are not below the case head on seating, ream the pockets to depth, or figure out if a crimp is causing problems. The primer must be below the case head for gas guns.
As for powders, normally I would say buy Varget, but I never used bullets less than 68 grains. I do know IMR 3031 and H4895 work great with light bullets in the 223, and H4895 works great with the heavier. Load mid range for multiple guns. Different chambers, different barrels will blow primers. I had to cut my loads for my Krieger barrels by 1.5 to 2 grains from the Wilson and Douglas barrel loads.
Seat the bullet to the cannelure. If there is no cannelure, don't seat the bullet out longer than magazine length!. With an AR15, there is not a lot of flexibility in seating the bullet long. I think I set my bullets to 2.25 plus or minus, and forgot about it. Never ever try to seat to the lands. If you do, someday, for some reason, you will have to extract an unfired round, and the bullet will stay in the throat, and you will have a lot of powder in the action.There is no need to crimp a bullet, case neck tension ought to suffice in keeping the bullet in place.
Works for me.