This weekend I printed a Picatinny rail sight mount for my Reprap'd Milestag gun. It came out very well thanks to the engineering drawings on Wikipedia.
Here it is hot off the printer:
Again I cleaned the ABS with MGR before spraying. When I printed the gun main body, I placed four holes to align with the holes in the rail mounting plate, but the holes are covered by about 3 layers of ABS. This allows me to have an unblemished gun body if I decide not to fit this rail.
To fit it, I simply align the plate and screw #4 self tap screws through the plate into the gun body, they pierce the first 3 'blanking' layers then engage in the holes.
Here is the rail fitted to the gun with a red dot optical sight: