This is one of the rounds that got our company going.
Initially, as a garage handloader, I worked with up to 25 grains of Varget in Military brass. Around 24 grains was good for 5.56 pressures. Anything more, and primers started to pop occasionally.
As as a commercial loader, we've had success with Varget, H4895, and IMR 8208 XBR. The latter we switched to early, and have had great results with.
For MK262 type performance, i recommend starting around 23.2, and working up to 23.8 grains. We seat these at 2.250 max COAL, in military brass, with a CCI military or #450 primer. This is what we load our match ammo with, and it is outstanding. Our sponsored shooter used it along with our 308 to win the Summer Mammoth Sniper Challenge in 2011.
This is for 223 Wylde or 5.56 chambers only