I reload 45 Colt, and moderately heavy, but only in my Ruger Blackhawks. I don't have a Judge or Governor to feed, but everything I heard about them was to keep things to fairly normal 45 Colt levels. I'm not aware of 300 gn bullet loads that would keep low enough pressures and provide useful muzzle velocity, but most of my loads are in the 250-280gn range.