Re: Too much pressure in the 300 RUM?
I would consider it a max load, or slightly over. It does depend on the firing pin to bore fit and you may be alright if it's just a sloppy fit, or conversely, you could be really hot if it's a tight clearance.
I would pay real close attention to ALL of the pressure indicators, like heavy bolt lift, difficult extraction, prominant ejector marks, primer pocket life.
I get to about 100gr Retumbo with a 178 A-Max and consider this a top load "in my rifle".