Hey Mate, there are basically 2 types of tanks. Well, off I go...
Basic bottles are made of steel. In an effort to make the bottle lighter but just as strong, there are bottles made of aluminum wrapped by carbon fiber. These wrapped bottles are lighter but more expensive than steel bottles.
Steel Tanks
Some Lighter Options
Lightest Cylinder Available
Now for the Shots per Fill...
A 47cu 3000psi tank = approx 470 shots per fill
A 68cu 3000psi tank = approx 680 shots per fill
A 88cu 3000psi tank = approx 880 shots per fill
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A 47cu 4500psi tank = approx 705 shots per fill
A 68cu 4500psi tank = approx 1020 shots per fill
A 88cu 4500psi tank = approx 1320 shots per fill
Well, hope that helps some
Oh by the way, what kind og gun do you have?