High Pressure Air is not available in 12g capsules. CO2 is but then we are back to the issues of a power source that has to boil off even before it is regualted. HPA is just that, air under pressure. You will not be able to carry as much gas but what ever bottle you do decide to buy it will most likly be more than enough for a bolt gun.
Using external air is IMO a performance upgrade mainly because you can regulate the PSI. Running
[email protected] in must pistols result in damaged parts. My speculation on this is that the damage is caused by the PSI of the gas. Once a valve is opened, it purges gas into the system. Replicas using CO2 12g capsules are operating at unregulated CO2 pressures, well over 850PSI, which is absurd for airsoft. Instead of actually using regulated gas these replicas just get a very short burst of high PSI gas. If you use external air, you can afford to increase the volume of gas and keep your PSI much ower, more in the 90-140PSI range, to operate your replica. Not only is this better for your gun but the FPS is more consistent, there is no more cook-off, FPS and in some cases ROF is adjustable and the report signicure is easier to control.
That may have been more than you wanted to know. I use CO2 or HPA in all my replicas so fooling around with a 12g when I have 1 or 2 20oz tanks on me is silly when I could just use a Y union and another reg to control a sidearm. I use my sidearm frequently but I hardly ever need to reload mid fire-fight. Having a hose attaching me to my pistol, like a lanyard, seems to be a reasonable compromise to use a pistol at 350FPS year-round with all the perfromance prks mentioned above.