Its really the barrel radius you mean, right?
On a tapered barrel wrap one layer of wet/dry sandpaper around the barrel a bit ahead (smaller dia.) than where you want the sight to sit, set the sight base on it and work back & forth with even pressure. The base will conform to the dia. but not be too loose when in the right position. What you want to avoid is having the middle of the base hit the barrel and the edges be too wide so it can rock side to side. A little 'high' in the middle with good edge contact is pretty good.
On the breech end of uniform dia. it gets touchy for the above reason. I always go through my stuff to find a dia. a bit smaller, or turn a spud on the lathe.
If, by chance, you get it too wide, you will have to bed it. This is so easily prevented by doing it right.