I recently got two barrels for my Handi. One was a .205 Ruger. That was boring. It shot under 1/2" at a hundred yds. with the first laod I tried.
The other was a 500 S&W and I wanted it as a plinking rifle. I don't like a lot of recoil. There was not a lot of info for low velocity loads for it. The only thing I found was Trail Boss, but that was 1000 fps at the top end. That seemed to wimpy, even for me. I had a can of Blue Dot. It is not a great powder, but from looking at a lot of loads for similar cartridges it looked workable.
I started at 10 gr. with a Ranier copper plated lead bullet. That did get the bullet out of the barrel. It smoked the shell - a sure sign of low pressure. If you try this, make sure that the bullet did leave the barrel.
I moved up a bit, and discovered that my chronograph showed a 200 fps difference between holding my rifle muzzle up before firing and holding it down.
I then added pillow stuffing, poked into the case. This solved the muzzle up/down problem. Fillers hold the powder near the primer.
I finally wound up with a load that duplicated the velocity of the 22LR. It shoots better than my ability to use open sights.
mike