52 has a barrel sleeve that could be removed to shed some weight, old T01 units have a metal foresight assembly as well, not sure about the newer ones. All those items will have been designed with FAC power in mind and placed to prevent muzzle flip, with our power levels and a sleeve down to 22mm I seriously doubt they are in any way necessary though they will have a dampening effect on recoil.

Another contender, albeit one that's not particularly efficient, would be the Sterling HR81 with a compression tube and trigger modified to be shot "backwards" (would need customised sears which would be difficult to say the least as they are not small order) with a breech and barrel off the likes of a Crosman facing back down the way. Not easy by any means but do-able with the right tools.