So why don't you do it like the old boys did. Cut your dovetail a bit undersize and then put a cold chisle in the corner and raise a hump on the outside of the barrel, slide your underlug into place, then set the hump back down to pin the underlug into place. It is very fast easy to do, requires no special expensive files, and is more secure than a filed dovetail. This also works very well for setting rear sights, just don't punch down the hump until the gun is sighted in. Do not use this method for front sights as the blade of a front sight has to set tight down on the barrel. K,I,S.S.
Ron