I know that with a 12g CO2 cannister in a Kythera/UGS build, i can get about 128(ish) shots @ approx 1.73J using .30 before it starts ro fall off.
Usually it works out, before completely dead, to an ares mag at 140 rounds to one cannister.
I have now sourced access to a high pressure compressor, so may be makin an explorarion ro a tank/bottle setup .
Cheers,
BroHay