Just got myself a co2 tank designed to fill small paintball tanks.

I plan to use this mostly for combat robotics and possibly
for a MIG set up when I get MIG.

Did you get one for paintball filling or for welding? the paintball unit has
a tube inside the tank so liquid comes out when its upright, can't be used
to weld. A weld tank has to be upside down to dispense liquid.

a small tube and valve kit is sold to hook the paintball tank up to the big
tank. Comes with directions. basically you vent a little gas out of the
paintball can and the pressure difference forces liquid across. Biggest
issue with filling is freezing a line up - go slow.

DON'T OVERFILL. You need 20% headspace. Use a scale. "The kid" filled
paintball tanks for all his friends when he was home. He overfilled one and,
not knowing of a problem, put it in the car and started driving. It popped
the relief valve with a bang while they were driving. They almost had an
accident trying to jump out of the car cause they thought a bomb had went
off.

Karl