I have two of the revolvers mentioned, an 1860 Army and a 1858 Remington, both Piettas. Have never shot the J&D bullets, but here is info for round balls (.454) and the Lee 200 grain conical. The ballistics are very similar, so I will not list both revolvers, which have 8" barrels. Volume loads using screw on spouts:
28 grains Goex Fffg RB 670 fps
35 grains Goex fffg RB 930 fps
28 grains Goex fffg Lee bullet 740 fps
35 grains Goex fffg Lee bullet 820 fps
777 (fffg) loads, which are much hotter!
28 gr. RB 910 fps
35 gr. RB 1080 fps
28 gr. Lee bullet 870 fps
35 gr. Lee bullet 1010 fps
I compress these loads pretty tightly, something many don't recommend with 777. The 28 gr. fffg load with RB doesn't compress at all, thus the lower velocity, in my opinion. 777 is consistently more accurate in all my revolvers, 13 of them,, than either Goex or Pyrodex P, which gets a little better velocity than Goex. I have gone to 777 almost exclusively, as it is readily available, and clearly better performing, velocity and accuracy wise, along with easier cleaning, than my other options. YMMV. And no, I don't work for Hodgdon. 777 is just better in my guns velocity and accuracy wise, without being what the old-timers used. Don't like the real stout loads in my Colts, just more pounding on the open frame design than I like.