My .600 Gillon project is complete. Took a little while to arrive at a good load that was satisfying to me and "Thumper". I started with Elephant ffg. Full case(43 gr) only gave 450 fps velocity. Not fast enough. Changed to Goex fffg. Worked up to full case(43 gr). Gave 680 fps. Wow! By George, now we are cooking. Shot 15 rounds. Cylinder rotation got too stiff from fouling to rotate without help. Also latch screw is starting to bend, loosening frame lockup slightly.
After cleaning revolver and straightening latch screw, I ran out and bought a can of Pinnacle fffg to try. I compressed 45 gr in cartridge. Shot 20 rounds for average of 600 fps. Still has a pretty hefty recoil with lots of fire and smoke. A BIG NASTY for it's day. Latch loosened ever so slightly. I have settled on this loading, and will make a new latch screw from 4140. Should take care of that problem.
A word on my experience with Pinnacle. It is a very coarse powder. fffg looks like ffg black. Doesn't have the power of fffg black, which in my case is what I wanted. It doesn't foul like black. After 20 rounds, the cylinder turned just as free as before I started. I could look through the bore and see the rifling. It leaves a smooth waxy coating in the bore. Manufacturer claims no black powder lube is required. I believe them. Clean up is very easy using Hoppe's #9+. Don't need to worry about what soot blows into the action. I sprayed a little WD-40 into it. Blew it out with air, and lubed with Ballistol. One word on cases. I made the mistake of putting the cases in the tumbler without prior cleaning. The tumbling media stuck to the waxy substance and made a mess. Had to wash the cases in hot soapy water, then solvent, before putting them back in tumbler. Carry a jar of detergent water with you when you shoot, and drop the fired cases into it immediately. When you get home just rinse, dry, and tumble.
My eyes have been giving me trouble, and my new spectacles aren't in yet, so I shot this target at 30 feet, two handed, double action. 20 rounds fired. I am satisfied that it would do the job it was designed for. Long live the oldies. P.S. I did have to install a higher front sight. With the original, Thumper shot 16" high at 30'. Now, on to the next project.