probably an HPA tap/line to run high pressure air.
HPA doesn't cool down like propane/greengas on decompression so it's far more consistent. FPS remains the same and you can run it indoors.
It's not a great idea for pistols since you have to tap the magazine to do it, but for rifles you run the air line through the pistol grip, it's a lot more mobile since you run it on a remote line to a tank on the belt. I suppose you could do it with a pistol as well, but it seems more clumsy to me.
I futz with V2s, V3s and V6s. I could be wrong... but probably, most likely not, as far as I know.