Back in the early 80s I had a Green River Forge Northwest Trade gun,30" barrel, 20 gauge. I couldn't get 60+ cal. balls all the time, so I shot .570 balls double patched with cotton canvas that measured.020 thick. Two patches =.040 thick surrounding the ball which gave .080 of material + a .570 ball gave a total of .650 to go down a .620 bore. the material compressed nicely and gave a snug fit but not overly tight.
That load shot very well out to 75 yrds. which was plenty fine for a smooth bore.
I don't know if it would work for a rifle gun, but I'd give it a try, might end up with an interesting result ?
Mark