The 300 Jarrett is an 8mm Rem Mag necked down to 30 cal. This gives you a case very similar in capacity to the 300 Wby only you don't have the long throat or freebore the Wby has. The short throat should provide a little better accuracy but it will also reach max pressures with a few grs less powder. I would suggest you reduce 300 Wby start loads by approx 4 grs and work up very carefully warching for signs of excessive pressure.I would suggest IMR 7828 and a Fed 215 mag primer would be a good place to start.
Thanks for the info, I have been working with RL25 with some good results. Not sure of what pressure signs to watch for but one load was very hard to open the bolt.
Whoa there big fella ! If you are already up into the "hard to open the bolt range" you are several grs OVER max.The first load that the bolt felt"sticky" to lift should have been an indication that you are over max and you should back off a grain or two. You should be watching for "slight cratering" on the primer,expansion of the case head after firing (measure with a micrometer) and things like loose primer pockets after just a couple firings. I would stongly suggest you read or re-read the safety section in your loading mannual while you still have the use of both eyes and all your fingers.
