Bo-Mar sights are one of the more challenging sights to mill into your slide, but are a great sight and my favorite target sights. You will need a few tools in order to mill Bo-Mar sights in your handgun, first is a good milling machine and a 60deg by .359 carbide dovetail cutter. A 60 degree sight file, available from Brownells, is helpfull also. There are plenty of Bo-Mar copies, but I prefer to use the real thing. I usually use the Target BMCS sight, although the Combat model with dog ears is attractive also. First, chuck your slide in vise with paper as padding and ensure slide is level. Next, with a 3/8 4 flute carbide end mill, spinning at 1100RPM, mill to the depth of your stock dovetail and make your cut stop exacly 1 inch from the breechface. If done properly you should now look like (Fig 2). The next step is cutting the dovetail. Put your 60 degree x .359 carbide cutter in at this time. With bit spinning at 1100RPM, find the top of the flat and then move bit in .300 from end of new cut or (1.300 from breachface) and down .100, and make a pass. Always check your sight before making a cut, to ensure it is not out of spec. Once the pass with the dovetail cutter is complete you should look like (Fig 3). Without removing slide from vise, open up dovetail with file until you can drift the sight in dovetail (Fig 4). Next, scribe a line on the slide just like in (Fig 5). Remove sight from slide and with a 3/8 carbide cutter start to mill a pocket, .025 a pass, to a total depth of .130 and stop on your scribed line (Fig 6). Ensure the pocket you are milling does not get any wider than the width of your Bo-mar. Next, put a 3/16 4 flute carbide cutter in your mill. Start to enlarge the pocket until the width is proper for the bomar, and go approx. another .015 past where the scribed line was, and then go down another .010 for a total depth of .140. Now you should be able to put your sight back in, and it should look like (Fig 7). If you have done everything properly the sight should not come down all the way. You will notice in the pic in (Fig 7) that the rear sight blade is not clearanced yet. In other words one more cut must be made in order for the sight to hinge all the way down. I usually do the last cut with the Bo-mar still in the slide so I know when I have removed enough (Fig 8). At this time the site should hinge all the way down and the last step is to drill and tap the slide to accomadate the elevation screw. Get a #31 and #21 drill bit and a #6-48 tap. I use stub length cobalt bits ordered from a machinist catalogue, they are less likely to wander and are much shorter than regular drill bits. What you are going to do is start the hole with the #21 drill bit, do this right through the Bo-mar sight (Fig 9). Then replace with a #31 drill bit, and drill all the way to the firing pin tunnel. The #21 drill bit just starts the hole and keeps the #31 drill bit centered. All that is left to do now is to tap the hole with a #6-48 tap available from Brownells supply. If you want to get fancy, you could also mill a half circle to access the adjustment screw. I accomplish this by putting my slide in my Yavapi slide holder, and using a 3/16 mill bit as seen in (Fig 10). Well that about wraps it up. It takes a few more cuts to get this melted in look, but it looks much better and is the only way I will install a Bo-mar. One last note of concern, this install only applies to a series 70 slide. If your slide is a series 80 or it has a firing pin safety installed, you must stop your first cut 1.1 inches from the breach face instead of 1 inch and regulate all other cuts off that first one. Otherwise you will cut through the firing pin safety tunnel.
Good luck Blindhogg
The #6-48 tap is available from Brownells (641) 623-4000 the part #395-648-0002 and the cost is only $1.65. the 60 Deg by .359 carbide dovetail cutter is part # 080-621-060 and cost $47.91. The 60 Deg file is part # 080-648-160 for Medium and #080-648-060 for fine and cost $12.45