Why not just buy RST 10GA x 2-7/8" loaded ammo and bang them off in your Damascus barrel gun (the way I do)? The hulls RST use appear to be sourced from Federal, are a tad pricey, don't need modification for 2-7/8" chambers, and I would expect you'd get a goodly number of moderate smokeless powder reloads out of them.
Offered FWIW.