I think I would first try to see if the barrel is really full of lead, and if it is lead and not steel weld. If it is lead, I would try to melt it out as described. If it is weld, it may be shallow and a counterbore could remove the weld, leaving a usable rifle. Each of those guns really has to be considered as its own case, since there is no uniformity.
Filling the barrel with lead or welding over the muzzle were common ways vets used to keep a souvenir yet render it harmless to their children or, in some cases, remove a registration requirement. (BATFE does not consider such deactivation as removing an NFA item from the NFA registration requirement.) I have seen a couple of guns in which the muzzle was welded shut but the chamber and the rest of the gun untouched, so if the gun were fired the barrel would have burst.
Jim