I once tried to shoot regular lead shot in a fully rifled shotgun barrel (made for sabot slugs). It was ugly. Great big donut shaped pattern with alarge void in the middle.

With the same reamers used for back boring a shotgun, a gunsmith could probably ream this barrel out to 28 guage - or even possibly 20 guage - and install the choke tubes. The cost might be approaching that of a regular shotgun barrel, though.

I am also interested in how much lapping was required to "screw up" a barrel. Did you use valve grinding compound?