Designed for a shotgun with a picatinny rail and no iron sights. The 24mm holes were picked to show the potential pellet spread after some experimentation with shells an cardboard at distances I've needed a shotgun (20-100 ft) using “turkey” loads.

These are not friction fit: they need a 35mm M3 screw and nut to handle the recoil.   I've printed in PLA and PETG-CF with a smooth fit and good adjustment. Alignment holes for some paint. 

Freecad file is included, and has an attached spreadsheet for some of the parameters.