I do the ATI mounts on the MN receiver similar to how Steve Acker does the Leupold mount on a Mauser. A jig like the Forster won't work due to non-standard screw spacing.
Calipers to center the base on the receiver, flat-to-flat; small amount of epoxy on the bottom of the base to hold it for d&t after cure. Receiver bottomed out in the mill vise, small lathe-turned 60 degree center in the drill chuck, indicate the hole, center drill, then drill with the correct size fractional drill. Tap by extending tap through the handle, and let it gently be held by the chuck jaws- so that it is free to rotate in them but still held concentric with the hole as the tapping is done. Each hole center drilled, drilled, and tapped before moving to the next one.
Calipers to center the base on the receiver, flat-to-flat; small amount of epoxy on the bottom of the base to hold it for d&t after cure. Receiver bottomed out in the mill vise, small lathe-turned 60 degree center in the drill chuck, indicate the hole, center drill, then drill with the correct size fractional drill. Tap by extending tap through the handle, and let it gently be held by the chuck jaws- so that it is free to rotate in them but still held concentric with the hole as the tapping is done. Each hole center drilled, drilled, and tapped before moving to the next one.