Whilst it could be any number of things, the three main possibilities I see are:
i. cast bullet through a jacketed only brake. Last I heard S&W no longer supported the non brake replacement insert, however there may be some aftermarket manufacturers.
ii. Misalignment between the cylinder throat and the forcing cone.
iii. Cylinder throats oversize compared to projectile. Causes the high pressure gas column to strip the coating and lead from projectile before it makes the jump to the barrel.

I doubt whether amateur inspection would diagnose the actual cause. I would strongly suggest finding a specialist revolver smith and getting it checked out.
It may even be the reason why the previous owner sold it. But hopefully the cause can be identified and fixed.