I have noticed that in most of the load data I see for the 300WSM , they have it listed out of a 26" barrel and all the data for the 300RSUM is listed out of a 24" tube While looking through a free Hodgdon manual I noticed that both of the short mag were tested out of a 24" tube. here is some of the listed data , highest velocities per bullet listed 300 WSM , win brass and primer 150gr bullet H414 66.0gr 3204fps 63,300 psi 180gr bullet H414 62.5gr 2959fps 63,000 psi 200gr bullet H414 61.0gr 2803fps 63,200 psi 300 RSUM , Rem brass and primer 150gr bullet H414 65.5gr 3201fps 62,800 psi 180gr bullet H414 60.5gr 2935fps 63,400 psi 200gr bullet H414 59.5gr 2788fps 64,100 psi NOTE : out of all the powders listed by Hodgdon H414 had the highest Velocities for each case. It seems to me that the 300RSUM isin't far enough behind the WSM to realy matter. What if you Ackley improved the RSUM , you would get a slight more powder capacity probably more effecient and it would still be a TRUE short action case , maybe some money saved on smithing. I know , I know ,Remington brass is crap but why coulden't you just resize some 300WSM brass Just a thought , I'm a big fan of the 300WSM but I have always wondered why the RSUM never made a hit ??