I've had increasing charges of black under ball in bore rifles spread the groups -- I'd at least try more powder before ditching the idea, unless more velocity isn't what you want.

I've also run through Goex 2f, 3f, Swiss 1-1/2f, 2f etc in search of the right load - eventually finding success.
You can also work with your patch material and ball diameter.
What have you tried so far (ball dia, patch thickness, lube etc)?

You'll get something that works, can't wait to see how you do.

Cheers
Tinker