300RUM, can be tough. Problem is probably the cases. I had such hard time with remington brass. Always had a thick side. I ended up buying several hundred pieces of federal brass. And they performed much better, but after 3-4 firings, wouldn't hold primer well. I don't load for the RUM anymore, so I sold all the brass. Here is where you can get some more. I tried everything and ended up culling the rem brass based off of neck thickness variance.

http://www.dlsports.com/fed_300_rum_brass.html