If... If...
WHO has tried those powders in the 7-08?
You ?
Jerry
Apparently Alliant has tried RL15, as they have some load data for both cartridges using that powder on their site.
www.alliantpowder.com If you look, you'll see that the velocity of their single bullet weight to bullet weight comparison in the 7mm08 and 284 has a MV difference of approximately 150 fps in favor of the larger 284.
No, I don't reload for the 7mm08, I'm happy shooting 139 grain Hornady American Whitetail factory ammo, and I prefer to shoot my 7x57s and 7mm SAUM in place of my 284s which are all heavily greased and boxed for storage.
