Re: US869 powder and hang fire
+1 on possible bad primers. Try the Federal gold medal match 215 magnum primers.
Also, are you sure it is a hang fire and not excessive pressure from other circumstances? If youre using a tight chamber and especialy if it includes a tight neck chamber, you can easily experiance VERY high pressures quick.
Long range shooting is a process that ends with a result. Once you start to focus on the result (how bad your last shot was, how big the group is going to be, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.