I'm using a pair of Ruger Vaqueros in 44 mag loaded with black powder. My load is 32 grains of Goex fffg under a 200 grain Mav Dutchman big lube bullet. Lube is 50/50 crisco and beeswax, with a little canola oil. Cases are Starline, primers are Winchester LP. They average 832 fps out of my 4 5/8" barrels. I haven't chronographed them out of my 24" barrel EMF/Rossi Hartford '92 in 44 mag. Recovery time is not good with this load in pistols, but the fireworks are great!
For faster pistol shooting, I sometimes use .44 Russians in the pistols (they don't cycle through my rifle). Using the same bullet, I can get about 20 grains of Goex in a modern Russian case. I haven't chronographed it, since I'm sure it's slow enough to be SASS legal, but recoil is mild. Some folks use reduced loads in 44 mag cases, then use a filler to take up the airspace between the black powder and the base of the bullet (always make sure there is NO airspace in a black powder load).
If I was going to shoot real black powder in a .357, I'd get a Snakebite Greasewagon big lube bullet mould (158 grain). If you ask the guys on the SASS Wire, you might be able to find someone willing to sell you pre-cast big lube bullets for your guns.
A lot of the fast shooters do gravitate to .32s and .357s. Up to now, many have also downloaded quite a bit, even in the black powder classes. That should be ending soon, with the 15 grain equivalent smoke rule coming into effect. That's pretty much a case full for the guys shooting black powder out of .38 Special or .357 mag. Recoil is very mild, to me, with the smaller cartridges, even when stoked full of black powder.
If the .44s you have suit you better, you'll probably shoot just fine with them. Unless the ribbons they hand out at your SASS matches come attached to new Cadillacs, we're just there to have fun, right?
Seriously, reliable guns+practice=good scores. Any gun+black powder =fun. Caliber choice comes much further down the list of important things, as long as you don't take it to extremes, like a .45 Colt loaded with 255 grain bullets over 36 grains of fffg.
