A proto slg can use CO2 or HPA. I contacted DYE tech support because their website says it can, but the manual says no. Their reply was "All SLG's are able to be used on CO2. Only being able to use High Pressure Air is a misprint in the manual. It might have been copy and pasted from our other manuals. Since all of our other markers cannot use CO2." So, yes, you can use CO2
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