Let's put this into perspective. The only FX airgun which can be ordered (OEM) with two tanks is the Pantera. All the rest have but one.
The 580 cc tank (which I use) isn't the largest one available, but is about as long as you can use with the standard 600 mm barrel (liner actually) and small suppressor. Depending on the caliber and tune, the shot count would be from about 40, to as high as 120, perhaps more. My .25 caliber, Mk III Impact, has a 700 mm tensioned barrel assembly, and is tuned to ≈63 FPE. I shoot the NSA 26.8 grain slugs, at 1,011 FPS ±2 FPS. It gets (depending on the ambient conditions), 65 shots before the first regulator drops below 150 BAR. The second regulator is set for 122 BAR, meaning I can get an additional 25 shots or so before a refill is necessary.
If yours isn't getting this number of shots, then you need to visit Ernest Rowe's site, and follow his recommendations of how to tune an Impact. Remember too, local conditions change the tune! I'm at ≈3850 AoS, and ≈90° during the summer months. Your settings may be different. If you're using an Impact Mk III with a stock 600 mm barrel, the the settings are about 3.5 on the micro adjustment, and the macro at 16. The hammer stop should be adjusted (VERY! slowly) to get the FPS as constant as you can. As noted, ±2 is easily obtainable, especially when using slugs vs. pellets. If you do shot slug, buy the faster twist liner meant for slugs.
By the way, I'm a new member here. I'm 83+ years old, grew up in a sporting goods store in Kansas City, MO in the late 40s through the 50s. I've owned about every gun most can think of, in more calibers than I can remember. The same goes for airguns, especially of late. I think my blood lead level is off the chart!!
