fenders:
Dixie Gunworks catalog mentions that the Lyman Great Plains Pistol has a .500 diameter bore with .516 diameter grooves in it.
I mention this because of several things.
While there is no doubt that a .490 diameter ball with a .018 thick patch is an excellent choice for a .50 caliber rifle it may be a bit hard to load in a pistol.
With a rifle you've got a great big mass of weight and lots of places to get ahold of it while starting the patched ball.
With a pistol it is fairly light weight and the short barrel is the only thing that is easy to get a grip on.
Depending on the direction of the grips wood grain, resting the pistol on a hard surface can shatter the stock if too much power is needed to start the patched ball.
In any case, you will have to use a short starter to get the patched ball started into the muzzle.
After it is started, pushing it the rest of the way down with the ramrod is pretty easy.
I mention this because when using a short starter it is hit with the palm of the hand to drive the patched ball into the bore. That imparts a lot of impact load to the grip if it is resting on a large hard surface.
What all of this is boiling down to is I recommend that you take a piece of carpet or several rags folded one, over the other to rest the pistols grip on while you short start the ball. This will help protect the stock.
Also, with .516 diameter grooves in the barrel a .495 diameter ball patched with a .010-.012 patch will be easier to start and it should seal the bore fine with pistol loads.
I agree with gettogun that 25-30 grains of powder will work very nicely for target work.
Have fun.