The easiest way to get a stuck bullet out of a barrel is to pour mercury on top of it, and stand it in jar overnight. In the morning, the bullet and the mercury will be in the jar.
Except for the fact that any scratches in the barrel can allow the mercury to infiltrate the crystalline structure of the steel, and substantially weaken the barrel. (Aside from the health risks.)
The process is not as pronounced as it is with aluminum, but the end result can be the same: Mercury attacks Aluminum