The front sight base is a ring-sleeve that is soft-soldered on the muzzle-end of the barrel. You can remove it by first tapping the sight blade out of it's dovetail, then removing the set-screw that is usually under the blade. Then, heat the ring and surrounding barrel with a propane torch. A MAPP-gas torch will work more quickly. Once the solder is liquefied, use a mallet to tap the ring base forward off the barrel.
Reverse the procedure to re-install, sweating the ring back on like you would a copper-pipe plumbing fitting.
One more thing, before applying heat, insert about an 8" length of 1/4" wooden dowel-rod into the muzzle to where the end of the dowel is right at the end of the muzzle. The wood will smolder and smoke while you are heating it, which will create a buildup of carbon monoxide inside the bore. The CO will absorb most of the oxygen, thus helping to prevent oxidation of the steel inside the bore.