For various reasons my other projects are on hold, so I've started on something hopefully a bit easier that I've wanted to do for a while.
A stock that can have a standard break barrel action dropped into it without any modifications. It will probably look similar to the 'mechano' style target rifles, with adjustable everything and bolt on cheek/grip set. But most importantly it will have a built in sledge system.
(It could be built into a wooden stock but I don't have the tools or know-how for that).

I started transfering my rough sketches into CAD models last night and it's looking feasible.
It will be based on a sliding pin system like the FWB300 or Diana Airking. No need to reinvent the wheel! The difference is it will be be more powerful than the FWB, and lighter than the Diana.

With aluminium body and plastic grip/cheek/buttpad, target weight is 3kg / 6.6 lbs

I'm going to do the 99 first (lightweight full UK power recoiless)
Then either an HW30 (fully adjustable bell target version) or a 57 (lightweight recoiless underlever)