as said, anti-syphon tube will do the job but CO2 will perish the o-rings alot quicker than air i believe (or so i've been told

) so putting alot of paint through would be no good at all.
however, 50 paintballs should be absolutely fine. after i'd stripped mine down, cleaned and re-assembled, i put about 100 through using CO2 with controlled firing (it was a cold night anyway, not useful!) and just made sure it was fine on semi and ramping. air is preferable tho, better dry firing your nuts off using air then using co2 and freezing the important bits
mew