Now This would be Cool (I think :-))

making refillable co2 cylanders not hard,done heaps them,good way to fill bulk co2 cylanders is to used syhon type co2 fire extingishers,you can but bulk filling stations for extingishers i got mine off trade me and wasnt expenisive.

I would bulk fill it, I did this to a crossman 1077 a while back and the consistency was awesome. I used a paintball bottle then tried HPA with a 850psi regulator (co2 runs at around 750-800psi) I probably crossed the boundary of legality as it turned it into a pcp but the consistency per shot was so much better even over the bulk c02

I made a dummy co2 canister out of solid aluminum that had a 1/8 bsp thread in the side for a HPA quick release to thread into. You would need to drill into the stock and the canister tube for the quick release nipple to come out. A paint ball remote line then connects between the bottle and quick release.

PM me and I will send you a photo

I still like the idea the guy in the YouTube video has, just not sure of the pros & cons of it.

Andy, I've just looked on Aliexpress and you can buy refillable Co2 canisters off there. They look like they have a threaded join half way up. You could buy a few of those and see if Mike can turn up up a longer cartridge to extend it

have never been a fan of hanging large heavy co2 bottles off co2 guns,and dont like eposed co2 lines either just to fussy i guess bout weight and looks eh,have made up quite a few custom made bulk co3 bottles for different guns,allways done in such a way it didnt destroy the oridional gun,so cuold be put back to origional simply be removing bulk fill bottle,had made several refillable recycled 88gram co2 bottles out of empty ones,i bore the necks out /thread them and fit two way valves in i make myself for guns that use these bottles this a good way to keep origional looks but get the cost advantage of cheaper bulk fill etc,its not hard to make special bulk co2 bottles for most co2 guns evan the pistols that use the little 12gran cylanders can be bulk filled with specially made small co2 cylanders if you got the time and equipment to doso,nothing is impossible but miricals do take little longer.