IIRC, Caspian was/is making aftermarket slides for Glocks. Part of the sales pitch I first heard for them was that the stock Glock slides are too hard to be milled for aftermarket sights, etc. How true that is, I do not know, but I do know that I can't cut my Glock slide with a mill file. Damn, that's pretty hard!
Now that I think of it, doesn't Caspian sell a Glock slide that is already cut for the Bomar?