If its soft solder, you can heat it, as the temperature is below the heat treat temperature that the barrel was drawn to, and some silver based solder is also. However, don't get it any hotter than that, as if it gets hotter than what the barrel was heat treated to, its beyond repair, unless the whole thing is hardened, and re-drawn to the correct temperature. Then, it has to be re-straightened, and everything.
Mount it in a vise, to act as a heat sink, and even some wet rags wrapped around it close to the sight base will help. Then, apply heat to the sight base, not the barrel, until it breaks free, and pulls loose.
There's a trick to not overheating, too, in using a 390 F. Tempil Stick, for the temperature of just slightly above soft soldering (360 to 370 F.). Apply a mark on the barrel with the Tempil crayon, and if that mark melts while applying heat to the sight base, immediately take the heat away.
After the base is off, polish any remaining solder off before parkerizing.
http://www.tempil.com/