Patocazador,
I don't know anything about blackhorn powder...so I just did a quick search on it. Sounds like it's intended for inlines w/high velocity copper jacketed bullets. They were talking about speeds over 2,000 fps. At those speeds, pure lead becomes like mercury.
You'd have to slow it down w/pure lead, or ram fmj ball ammo in the ROA. :-D
Sounds like interesting stuff. I would think the only way to find out if it'll work would be to try some.
Don't know if 12 - 28 threads will take blackhorn pressure. Those inliners have robust breech plugs in 'em made for that stuff. I'd start w/a very lite load, a card wad, & oat bran compressed on top under the bullet. Hold yer ears 'n' have someone else shoot it! :-D
Also, as you prolly know, small pistol primers & small rifle primers do not have the same physical dimensions. Small rifle primers are ~ .004'' taller. Same O.D.