Why do you have to ream the flash holes? I would have thought that the primer firing would create the same pressure in the primer pocket regardless of powder charge, and with no powder, your overall chamber pressure would be a tiny fraction of a full load. I don't understand! 
During firing of a full load the firing pin strikes the primer, primer ignites, backs out of the primer pocket slightly, ignites the powder charge, mass of the bullet moves forward, at the same time the pressure is building, expnds the brass case, seals the chamber, recoils rearward, re-seats the primer flat into the primer pocket.
With the wax, or rubber bullet, there is not enough mass or pressure to drive the primer back into the primer pocket flush. The protruding primer then blocks up the revolvers cylinder from turning, thus hanging up the gun.
The larger flash hole lessens the pressure inside of the pocket, allowing the primer to stay where it needs to be.
After reaming out the flash hole, the case cannot be used to load regular full power round. It is dedicated for wax bullet reloading only.