One youtube video tested Clays, Trail Boss and TightGroup, all Hoghdon pistol powders as canidates. I like the Trail Boss because it is sold in 9 oz containers because that is all they can get in a traditional 1 pound container. I'm nervous about 3-4 grain charges in a 223/5.56 size case and would like to get something to sorta fill the case.
Stuff it full of kapok.