I don't own an M40A1, but I do own a Steyr SSG.69 with a USO MST-100 mounted on it. I have to say, of all the scopes and glass I have looked at over many years, this is by far the clearest, sharpest, and best riflescope I have ever mounted to a rifle of my own. Fixed 10x magnification with Mil-Dot reticle; bullet drop compensation elevation turret. Built like a tank. My average 3-shot group size at 100 yard has been between .501", with two groups measuring .343" and .224", respectively.
Like so many others, I originally planned to build an M40A1 clone, but after mounting the MST-100 on the SSG.69 and shooting that rifle, I would not change it out for a winning lottery ticket. I was lucky enough to get one of the limited productions scopes USO put out last year, and it has been the best investment I ever made with regard to shooting glass.
Now I need something to put on my Lee-Enfield L.39 clone. . . .