Well that answer was not useful or to the point.
Regardless.. as I have said a zillion times before, it is harder to find a powder than won't work well in a 223, than it is to find one that is better than all the rest... and I imagine a Montana has one hell of an accurate barrel as it stands..
JB gave a good universal answer, H 335.. I don't care for it because I have had several times there has been some temp insensitivity issues with it...
Lately I have been loading for a young man who just graduated high school, got his Eagle Scout Rank, and is watiing to ship out in January for his Navy Basic Training at Great Lakes.. We got him a Ruger American in 223, and per JBs recommendation, we have been loading 25 grains of TAC in it.. and he's been nailing little varmints on their farm out to 350 yds with it...
I've been running 25 grains of H 322 in my Predator American in 223 myself...with the full power loads...its also been a TAC driver....
but anything with a burn rate of RL 15 or faster is going to be a tack driver if the rifle is up to it...
That answer suit ya a little better? Glad to hear it..
