20# might be a bit thick. 2 wraps is proper and 2 1/2 thou is usually a good thickness for an 8 to .010" increase.

We've shot paper patched bullets for years in our Sharps and Rolling Blocks. I'll take a picture of the paper patching 'board' if you'd like. Makes it easy to roll the paper onto the bullet.

The Remington Scalloped Jacket RN might be a bullet that could be relyed upon to expand well at lower speeds.

Lightly rolling the jacketed bullet with a sharp-toothed rasp to raise little sharp pics (roughness), can help also, with very smooth, hard bullets.