So what's the most powerful factory load out there for the .357 Magnum? This gun would be a 4" Ruger GP100. I'm thinking Underwood, but I'm open to other loads. By "powerful", let's use the highest velocity number, followed by the most ft-lbs of energy. I have a 50-round box of Underwood 125gr. GDHP that I've been dying to shoot, but I just haven't had the time to get to the range. Muzzle blast and fireball is okay in this case. Wanna kill these ads? We can help!