When you have got the sighting perfect, work carefully with your file (taking great care not to spoil the edge of the "U" nearest to the eye when aiming), and give a chamfered or bevelled edge to the other side of the "U," so that it has a knife-edge. This is to make the "U" look clear and yet allow the back sight to be strong. On this principle, you can let the hind sight be strong and over a quarter of an inch thick, and yet have a nice, clear "U." Do not have the "U" deeper than a semicircle. If this "U" is too deep, it hampers your view of the object aimed at. In fact, it should not be quite a real "U," but a semicircle. You can also file all round the front sight, giving it a taper toward the muzzle, but keeping unaltered the silhouette that you see when aiming, so that the outline shall then stand clear to the eye.
A gunmaker's vise (padded, so as not to bruise the revolver) is a useful thing, as it leaves both your hands free to use the files.
I cannot tell you how much you may undercut the front sight, assuming you intend to use it in competition, as the rules alter so from year to year. I have an undercut bead-sight which some years was allowed at Bisley as "Military," and in other years not. The best plan, if you are in any doubt as to its passing, is to send your revolver to be passed by the committee before competing.
When you have finished, and have had a final shoot to see if this finishing has not spoilt your elevation, etc., you can send your pistol to the maker, and ask him to make your sights precisely like your model ones, and to fix them permanently on the pistol _without screws, if for Bisley use_, so as to comply with the rules. When you get the pistol with these sights, if the work has been properly done, a very little more filing will put the matter right.
Should you not be shooting at Bisley, or at any of those clubs which shoot under Bisley rules, you can, of course, get a pistol with Smith & Wesson's "Ira Paine" adjustable sights. Carry a miniature folding gilt screw-driver and sight-case on your watch-chain, as I do, and you will then be able to shoot in any light, at any range, or in any style of shooting, by merely giving a slight turn to the adjusting screws to alter your elevation or direction; or take out a sight from your little case of sights, if a sight breaks or you want a different size or shape. Public opinion has not yet been educated to the point of considering this "a practical military sight," but this will come--in time.