Update 1/4/15: I added a 95mm rail, with 40mm screw spacing, which is the first and fourth (last) screw hole on the top of my Hatsan. I also added channels cross-wize, mid rail, so that zip ties can be used in place of the hardware. Zip ties could be used to secure the rail to the weapon for light stuff (like modding a Nerf gun), and I am also going to design a scope ring that uses just zip ties.

Many air rifles, especially foreign makes like my beastly Hatsan 125 Sniper, use a 12mm Dove Tail to mount optics. With the especially violent recoil of this spring piston rifle, the dovetail itself needs a scope stop to prevent scope rings from moving. However, even once mounted flush with a scope stop, I have seen the ring mounts move forward off the stop. By adapting the 12mm dove tail to a Picatinny rail, stability for optics will be improved.

This design is completely level, ie I have not angled the plane of the high rail down to account for barrel droop. If you have a dove tailed rifle with significant droop and need a design let me know.

I positioned the holes to line up with the back, and third from the back screw positions for the Hatsan 125 (31mm apart). Even with one screw, it seemed pretty solid. Let me know if you need a different screw pattern.

I am a little dubious about how well this will hold up. My rifle has already crushed two scopes because of the spring action. I will update if I see any issues.

I may also add more length. My little PrintrBro only does 100mm (until my new rods get herem hurry up McMasters!) - ideally this should be a 100-120mm long mount.

In this case, instead of mashing someone else's rail pattern, I looked up the drawing and rebuilt it myself in Solidworks.