Single stroke pneumatic rifle? (are there any?)

[clarky]

Just wondering what the output would be if a fwb 600 or similar ssp 6ft .177 rifle was converted with a barrel change that had a longer barrel in .22, both being more efficient. Would the output bump up to near the 12ftb limit?

Its a nice idea but dont work in practice.
I found when trying such a mod on a Hw75 that you get an initial increase after 2 inches, then a hold for about 2 inches then a drop off that is very marked beyond that.
In otherwords barrel length only goes on giving us improvement for so long until an optimum is achieved and that comes quite quickly at these kind of power levels.
You actually need a cylinder up like 2ft long and a good 1.25 inches in diameter to get anything increasing in velocity beyond about 10 inches of barrel before tail off arrives.
Going upto to .22 proved very dismally disappointing when webley and daisy did it with the nemesis and 722!
About a .25 ftlb increase ....and terrible tragectory at about the 300 fps....much bigger gains come from the expansion of gas when doing bigger caliber mods in CO2 but air dont do a lot.
If anything its transfer port ratios design size that often improves going to .22 in springers more so than pneumatics.
An 18 inch rifle stocked 46m managed 600 fps apparently ....not too bad but 200 fps light of a humble hw50