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Likely would not harm the barrel--maybe some leading, but your patterns would be ####ty. The rifling damages the shot pellets and puts a bit of a spin on the shot charge--neither of which contributes to good patterns.
I know that Dominion used to make cardboard bullets filled with Shot loaded in Snider brass for this purpose. Upon leaving the bore, the cardboard would clover-leaf and fall away from the protected undamaged shot which continued on to target. Printed like 20 gauge rounds.
I suspect that a properly motivated Hawken shooter could devise something similar with a little experimentation. Just don't use a plastic wad as it will melt and coat your bore with burnt plastic.
I'd be tempted to try a sabot as the "over powder" wad. Follow that with an ounce of 7 1/2 shot and something (easy to blow out) as the "over shot" wad. Maybe a tallow-soaked punched felt wad ? It may be worth experimenting with. Perhaps a wrapped paper cylinder to contain the shot (to protect against the rifling)?
Just remember that you don't want the load shifting off the powder and you don't want to create a bore obstruction.
Check out what kinds of products the black powder shotgunners are using.
No matter how you do it,it will make a donut pattern. Not a biggy for close up snakes and such,but a 20 yd partridge will probably fly off wondering what the fuss was all about