RE: 300.wsm vs. 300 rem ultra mag.
The point is factory loads are close with different offerings and what one puts into a round and what one gets out. Most people don't handload. I don't believe the RUM is an efficient round when you look at how much more goes in and what come out.
Try some Hornady and Federal premium rounds if you don't believe you can't get factory ammo to shoot like handloads. I've put various boxes thru a crony and got a max of 20fps deviation.
And to those that like to throw around insults , I was shooting while some were in diapers and worked at a commercial range for almost 15 years. I've shot just about every thing imaginable from 22 shorts to 50 bmg's . You shouldn't use your mouth for breaking wind son.