the dove tail.
the first thing is the location, make that decision.
i use my digital calipers to determine the thickness and length of the sight or underlug. then i mark on the barrel the shortest/upper part of the dovetail LESS about 1/16" (room to work into the correct length). you can carefully cut little sections within those marks with a hacksaw then file them OR remove the all that metal with a file. If you use the hacksaw be very careful it is easy to go to deep. and its easy to go off of level. go slow, this first part of filing the dovetail is really important. it must be level with the plane of the breachplug and it must be the same depth front to rear. i lay a tiny straight edge across the bottom of the dovetail as im working on it and sight it down the barrel to insure it is level. i use the digital calipers to measure the depth front to rear, don't file deeper than the sight's base's thickness.
you now need a small safe three cornered file, (a three corner file with one side smooth). you can buy one or you can grind the teeth of of one side but don't try filing the corners until you have the correct file.
so at this point you should have a square slot filed in your barrel that is shorter than you need and just about the correct depth.
with you three corner file safe side down start filing the ends of the dovetail slot. work slowly removing metal from both ends until you are close. NOW. check that sight, in the slot, from both sides because for me i know i can't file exactly straight and you don't want to tap the sight in from one side and it fall out the other.
once you have the sight fitted so it will tap in about 3/8 of the way you are probably done, it should tap in tightly the rest of the way with a brass drift, if not just the tiniest amount of file strokes until it can be driven in
ou
tom