I've heard alot of people ask this question, so I decided to reasearch this.. Here is a chart:

***Please keep in mind that these are just averages, the amount of shots you get from your tank may vary due to inline pressure, and overall gun efficiency.**
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CARBON DIOXIDE
HERE IS A GUIDE FOR CO2 SHOTS PER OZ. AT 80º F.

__65 TO 70 SHOTS PER OUNCE__
7oz TANK = 466 SHOTS

9oz TANK = 600 SHOTS

12oz TANK = 800 SHOTS

20oz TANK = 1333 SHOTS

COMPRESSED AIR
HERE IS A GUIDE FOR HPA SHOTS PER CUBIC INCH.

3000psi 4500psi

__10 SHOTS PER ci.__ __15 SHOTS PER ci.__
47ci = 470 SHOTS 68ci = 1020 SHOTS

68ci = 680 SHOTS 88ci = 1320 SHOTS

88ci = 880 SHOTS 114ci = 1720 SHOTS

114ci = 1140 SHOTS

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--ripped from:

http://www.geocities.com/divadcom/SHOT_CHART.html
thanks,

Albert