Powder Burn Rate directly affects Velocity
Joyce Hornady says, ". . . all the powder must burn within 95% of barrel length . . ." I use this as my guide related to burn rates. H4350 burns faster than RL-22. This knowledge assists me in component selection for maximum velocities concerning various bullet weights. Of course, I'm talking .243 Winchester here. Alliant RL-25 burns too slowly for maximum velocity in ANY .243 Winchester application. Powder burning out the end of the muzzle aides nothing velocitywise, but can create an extremely unstable, inaccurate bullet. .243 Winchester loads work well with many powders, but all within the proper burn rate range! My .223 Remington loads MUST use faster burning powders than any suited to .243 Winchester applications. I tried Hodgdon H4350 in .223 application and it's really slower in velocity then, say, Alliant RL-10x, yet a .243 Winnie light bullet screams forth at maximum velocity with the very same H4350! IMR 4350 actually burns slightly faster than Hodgdon H4350, and I prefer H4350 in my hot .243 loads. To each his or her own experiences, signed Cliffy.