Of course, mileage will vary. CO2 typically gives less performance than what it's rated for simply because it is a highly volatile gas. As you shoot, both temperature and pressure fluctuate, and this can cause some guns (particularly autococking designs) to not work properly. By the time you're down to 1 ounce of CO2 in the tank, you're going to be shooting horribly, whereas with an HPA tank you will have solid performance right up until you hit your marker's cutoff point. HPA also won't freeze up, so you can shoot as fast as you can pull the trigger.
Now, I know these calcs aren't exact, by any means. However, they ARE a solid rule of thumb. A 16 ounce tank can give between about 800 and 1200 shots, by simple math using the existing numbers (multiply the already known 9 ounce values by 16/9). Go dead center on that, and it turns into about 1000, which match up nicely with my calcs at 25 shots per 12-gram, which are dead center between 20 and 30.
These numbers have passed a couple different validation means for comparison. They're overall very correct. You can use this to determine what HPA tank size compares to a CO2 tank. That way you can know how much gas you'll need to run your marker after you make the switch.
So, here's a list of common CO2 tank sizes and their approximate HPA tank size:
3000 PSI: 53.154 CI
4500 PSI: 35.44 CI
3000 PSI: 94.496 CI
4500 PSI: 62.997 CI
3000 PSI: 118.12
4500 PSI: 78.746
Note: mileage may vary depending on your marker and temperature. However, the proportions should remain largely accurate:
1 ounce of CO2 ~ 5.906 CI of HPA at 3000 PSI
1 ounce of CO2 ~ 3.9373 CI at 4500 PSI
This post has been edited by WardenWolf: 15 February 2008 - 12:05 AM