>   Hello I'm new at the sport and I know it's probably a dumb question, but
>   what is constant air?  Does that  just mean that instead of putting a 
>   little co2 bottle you use a big bottle.  Are the big bottles disposable
>   or can you refil them.

CA (Constant Air) is a bit deceptively named.  Let me explain....

All CO2 tanks used for paintball contain compressed CO2.  When you
compress any gas far enough, it will change state into a liquid.
When you lower the pressure in the tank by releasing some of the gas
that is still in it (at the top) some of the liquid "boils" and changes
state to a gas.  This continues until the pressure in the tank returns
to the pressure required to turn that gas into a liquid.  Every time
you take gas out, the liquid will expand into the gas and return the
pressure in the tank back to this "constant" pressure, until of
course you run out of liquid.

A constant pressure is required for most paintball guns because a change
in pressure will cause a change in velocity, which we do not want!
The term Constant Air referrs to the tank's ability to maintain a
constant air (specifically CO2) pressure.

The little disposable 12 gram cylniders you buy at the hobby shop are
constant air tanks, but are not reusable or refillable.  The larger
cylinders most players use are more commonly referred to as CA tanks.
The most common sizes are 7, 9, 12, and 20 ounces, where ounces refers
to how many ounces of liquid CO2 the tank can safely hold under normal
conditions of external pressure and temperature.  3.5 and 16 ounce
tanks also exist, but are less common.  Almost all tanks are made
of spun aluminum, except most 9 oz, which are steel. (and HEAVY)

These tanks can be filled using a large (bulk) tank, say 50 lbs.
The cylinder to be filled is first chilled a bit, and is then connected
to the bulk tank and the airway opened.  The bulk tank will typically
have a siphon tube on it to draw the liquid from the bottom of the tank
instead of the gas from the top.  Because the temperature of the cylinder
is less than of the tank, the CO2 in the cylinder will stay a liquid at
a slightly lower pressure, which for boring physics reasons causes the
cylinder to fill up with liquid.  The cylinder is usually on a scale
while being filled, and the airway is closed when the recommended
safe weight of CO2 has moved into the cylinder.  The airway is then closed
and the cylinder removed.

Because temperature changes will greatly effect the volume of the liquid
in the cylinder, a cylinder filled to its design maximum will have a
fairly large "air space" inside where there is only gas CO2, instead
of liquid.  Heating a cylinder much above 90 degrees will expand the
liquid, which will increase the pressure inside the cylinder, which will
cause the gas in the air space to change to liquid, which drops the
cylinder's pressure, holding equillibrium.  When all gas has changed
to liquid, the tank's pressure rises unchecked, and eventually the
"burst disk" (a safety device on the side of the valve) will rupture
and somewhat harmlessly empty the tank. (it gets VERY cold when it
vents)  The disk can be replaced, but it is not recommended to trigger
this safety device more often than can be avoided.

A 7 ounce tank holds 20 * 7 = 140 grams of liquid CO2.  That's
140 / 12 = 11.8 (or so) times the amount a 12 gram cartirage can
hold, so you get a dozen times the air.  Fields typically charge
$1 to fill a 7 oz tank, and you can get 12g cartirages 5 for $3,
so you can see that a CA tank is a very ecconomical thing to have.
(as long as you can get it filled when you need to...)