A shotgun slug can travel up to 8,921,823 fps and have a energy of 13,283,495 ft-lb.
For a rifled slug, you're only looking at a practical range of 100 metres/yards or so.
You will need to have it examined by a good gunsmith.
They are referred to as "gravity knives" and are prohibited.
Yes and no. The yes comes from simple fact, if the projectile can move down the barrel, it can be fired. I've seen .221 Fireballs fired out of a 7TCU barrel. The no is because you cannot do it well....