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
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?