What is the alloy you're casting the balls with?
Have you thought of simply going with a harder ball?

Daryl's noted denim, and rather thick denim at that.
There's a thread here somewhere here where he discusses his experiences with patching materials in his fourteen bore single barrel rifle.
He's getting considerably higher velocities then 1500fps in that gun with no patch problems.

I'm sure he'll chime in.

--Tinker