ORIGINAL: Semisane
I've never done it, but here's what I would do.
Clean the barrel well. Pusha sabot (no bullet, no powder) down the bore with your ram rod with no jag on it. Leave your ramrod sitting in the sabot. Hold the gun straight up (vertical). Pop a cap and see how far the ram rod moves.
Repeat the same process with another cap.
make it every 1/2 in.
could also take a pale with dry sand and see how much it displaces. (maybe a dumb idea)
camera is required for this one.