I made this last year, 45 cal...just finished a tap loader conversion on a 30 cal barrel...
It’s big....21” with 10.5” barrel

I just re-read all the Giffard material at Trev's Cinedux site

http://cinedux.com/the-giffard-co2-rifle.php

and while there was a lot of nonsense hype about power/military applications around
when Giffard's invention became public, it seems that CO2 has persisted as a means
of propulsion among serious airgunners. Is it the relative safety/ low working pressures
and easy availability of the gas that appeals to you?

The allure of co2 in the guns I build is easy to explain...first, Oldair (Don) helped me with safety factor calcs on my reservoirs, and even though I test ‘em to 5k psi, the safe working pressure with co2 is well below max working pressure... plus, on co2, lots more shots were available...I’ve tried HPA, with mixed results...I’m not obsessed with max velocities, just good ol whacks for plinking...the pistol shown here gets 14-17 shots per fill, and that’s a casual fill, not cooling, bleeding etc...
Mike