Originally Posted by OM3GA
You can use co2 with all of the Madbull shells, pretty much any shells in fact unless stated otherwise.
Only the XM108HP, XM204HP and XMPB4 should be considered for CO2 use. The others will work but at much lower PSI. Not worth bothering above propane pressures.
Higher 'outdoor usable' PSI will put extra stress on the firing pin/mechanism on the launchers. So much so that they tend to break.