im having a 338 whisper built. its a 7mm br case necked up to 338. i only want to push the bullet to 1040 fps so i stay subsonic. im trying to find some load ideas. i have a few ruff loads to play with. i would like to use a pistol powder so it is all be used up in the barrel. i know with a slow burning powder to little can be very bad. is it the same with a fast burn powder. i have a few diff powders to play with but i am a little worried about messing around with my own loads.