Yes it is a 300 ultra mag. Like i said always work up your loads one grain at a time. I ended up yousing 92 gr of H-1000 i could of yoused more powder but my best ES where at 92gr. Retumbo didn,t get good ES in my gun. I shot retumbo in my 338 AX 1/2 moa past 1400 yds. Rember the vel in the reloading book is so they don;t get suide if something goes rong. Just go slow let your rifle tail you when you reached the max then back off two grs. I will usley back off three or for grs. then load three shot at 1/2 gr incress to i find the best ES . Like 90gr then 90 1/2 then 91 then 92 that is where i found my 2964 2965 2968. Now i could of went further but that was my best load. My 300 ultra mite be a faster barrel i don,t know. So start at 86 and work up. My rem has a 25 1/2'' rockcreek barrel. I just order a Basix trigger set at one pound from compene. That sould help get my groups under 1/2 moa. See i do not care if i get to three THOUSAND that would not be hard but low ES IS WAY MORE INPORTENT TO ME. I hope this helped. Joe