There is a small difference in 12g cartridges, specifically in the height and design of the puncture nozzle. I have seen different 12g cartridges with as much as an 1/8" difference in overall height.
Not really a problem with airsoft guns as the magazine wells for the CO2 cartridges are screw in to tighten, but thought I'd point it out.
The Tier One Tactical Operator
Tippmann Arms M4A1
KWA USP .45, KJW P226, KJW hi-cappa