Re: Powder for .223 77gr
With lighter bullets (like the 55s) Benchmark is an outstanding choice; even better than RL-15 or Varget. These two are better suited to the heavier bullets, like the 77s we mentioned earlier. The lighter ones, like the 55s, will need a faster burning powder. Benchmark is ideal, and used to be one of my standard accuracy powders for testing 224" bullets in the 223 Rem. Meters well, give good velocity and outstanding accuracy.
This goes along with the comments I made earleir from your initial post about 4831. Again, great powder, but just not suited to this role. Put it in a 300 Win Mag and it runs great. In a small capacity case, light bullet type load like this, it's just not the right choice. Gotta match up all the componenets for things to run right.