What calibre? I would think 50gr. perfect for a .50 and a bit light for a heavily loaded .54 cal. The twist must needs be slow enough to give good results. The faster twist 'pistol' barrels usually shooot best with very light charges, although i haven't yet tested my .45 with 18" twist to see how heavily I can load it.
I am shooting 57gr. 3F in my .54 and it's a perfect load - accurate in the slow 66" rifle barrel twist, 9" long.
How BP actually burns inside the barrel depends a lot on the resistance it meets by the patched ball, thickness of patch which tightens the patches ball inside the bore as well as how much or little the compression of the powder upon loading. In a flint or percussion pistol barrel, I would think a .45 cal could use up to 55gr. just fine in your 15" barrel. The Walker Colts usual charge was 55gr. It had a 9" barrel, not including the cylinder. I will also add that the cylinders originally were cast iron and many of them blew up. This was solved by going to steel and reducing the capacity to about 50gr. in the Dragoon models. The Walkers were preferred for running buffalo - shooting piont blank, a couple shots per buff and worked well, apparently. Some buff runner's still preffered the Martial .54's or English, French or German smoothbore dueling pistols of larger calibre as only one shot per buff would work. These pistols were held in 'holsters' on each side fo the pomel. 2 per horse and of course, other's on the person. The reason for stating all this, is to bring to light, the power of these pistols when used close in.
If your pistol is .50 or .55 cal, it may shoot more powder yet. 15" is a long tube and in either, you could easily break 1,400fps, I think - perhaps, maybe. It could be more effective close in than a .44 mag long barreled revolver - if loaded appropriately.
spell check isn't working - hope this is readable.
