Remington attaches their sight bases with a really tough, high-temp brass-based brazing compound.
To get the bases off you either have to heat the barrel to a red heat of over 1200 degrees, or you can cut them off with a saw and files.
If you remove them with heat, you have to heat the barrel enough to melt the braze, so you want to use a big torch and work FAST to limit heat spread.
A propane torch will NOT do it.
Heat the sight base until it and the immediate barrel around it are at a red heat, gently tapping the base with a small hammer until the braze melts and the hammer taps knock the base off.
DO NOT hammer the base. When it's at a red heat you can actually distort the barrel if you hammer on the base. Just tap gently until it pops off.
Use files and sand cloth wrapped around files to remove the braze left on the barrel.
Once you get the bases off, you'll have to refinish the barrel since the heat destroys the finish, and you should re-polish the bore since it'll be discolored.
To cut the bases off, use a fine tooth hacksaw or, better yet, a jeweler's "wire" saw.
Cut as close to the barrel as you can without actually cutting into it, then grind and file the rest of the way.
This will leave two bare spots on the barrel so, again, you'll need to refinish it.
