I shouldn't stick my nose in to a Black Powder (or substitute) thread but there's an alarm bell in the opening post and I don't think it's a bad idea to add some thoughts.
Coming *ONLY* from my smokeless powder perspective, if you are looking at published cowboy action load data (which, in my experience, is only EVER going to list cast or soft swaged lead bullets) and you attempt to simply insert a jacketed bullet in place of the lead one, you are asking for trouble.
The published (smokeless!) load data that is specifically served up as "Cowboy Action" loads are very, VERY low in velocity and assumes a far-easier to launch lead bullet. Using this data with a jacketed slug will put you at great risk of sticking a jacketed bullet in the bore of your firearm.
Sticking a jacketed slug in a bore really sucks.
But it could be MUCH worse: if you pull the trigger on the next one when you've already got one stuck. Then you have major, major problems and at the very least...most likely a ruined firearm.
In my opinion (again, I don't know anything about non-smokeless), your best bet in this situation is to find someone local and swap him this powder rather than looking for ways to "make it work." Someone wants the stuff and can likely trade you something that you will need.