There is not 100% safe air unless it is 100% empty. Nitrogen i would think would be more safer due to the gauges, and the carbon fibers spun into most (depending on the tank). Though, keep in mind, nitrogen is $100+ more expensive then CO2
No, paintball co2 is much higher pressure.
No paintball gun will work without a CO2 tank or other propellant tank. without Co2 there is nothing to push the ball out of the barrel.
Where the air tank or co2 tank normally attaches
There is no such thing as a "CA" tank.
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CO2, stored in the tank is highly pressurized. The paintball being shot is caused by it being pushed out by the rapidly expanding gas. CO2 also cycles and reloads the next paintball.
Most paintball markers can hold a N2/Nitrogen/HPA/Air tank so I couldn't possibly list them all, if you submit specific markers I could tell you if they fired using Nitrogen.
No I went 4 hours straight on just a 9oz. tank
No, any tank will work.
unscrew the tank, this will vent any co2 that is in the marker out the asa
You cannot unless you have a gauge installed, or know the exact weight of the tank.
you need Co2 to fire any paintball gun, it is what physically moves the ball out of the barrel.