I rebarreled a customers 300 RUM with a 26" #4 Rock Creek Sporter, 1:10, 5R, with a Harrel's Muzzle brake
. The rounds are being loaded with 180 Nosler Balistic Tips or 180 Swift Scirocco II. The throat has been opened to 3.315" Thus giving the bullets a .200" jump. I shot 5 shot groups with Reloader 22, H-1000, Retumbo, H-4831SC, IMR 7828. The brass was Remington, Primers were Federal 215 GMM. I full length sized, trimmed, chamfered, anealed, and cleaned primer pockets, between reloadings. My loads with H-4831SC were sub .5moa. and the loads with Retumbo were sub 1. moa. I shot velocities between 3,150 and 3,300fps. Quite a round. I shot over 100 rounds today.
One thing I noticed with the H-4831SC was I shoot 78.5grains with 180 Scirocco in my 300 Win Mag to get 3,1150 fps. With the 300RUM I was shooting 86. grains to get basically the same velocity.
With the 300 RUM it will be nice to be able to shoot 150 grain bullets at 3,400- 3500 fps.