Because we do not know how much, if any, freebore there is in the throat area nor the other dimensions we need to be more precise I would start with 75.0Gr of 7828SSC and go up from there one grain at a time. I would suspect that you will end up in the low to mid 80's for a final charge but no way to know without starting at 75 and chrono'ing as you go up. Again, once you hit 3200fps, STOP. That case size will not be able to produce higher numbers than that without running way high pressure EVEN IF you do not see any evidence of high pressure such as flattened primers, Shiny spots on the cartridge case head or sticky extraction. IF you see any of these signs or if the velocities are not going up in a smooth non parabolic curve then stop right there and back off at least one full grain of powder. Even if you need to stop at 3100, that is faster than than a 300 Win MAg and a DEADLY combo.
Lastly, there is a co out there that makes red recoil pads to fit your rifle that say Winchester on them. This would be a nice added touch on this truly fantastic rifle of yours. LONG LIVE THE 300 COULTER MAGNUM!!
