It's not hard to work out.

I fill my FAC rapid to 210 bar, 400cc bottle, so 210 x 0.4 = 84ltrs of air
I get 55(ish) full power shots before they drop off at 120 bar, 120 x 0.4 = 48ltr
84 - 48 = 36ltrs used
55/36 = .654
so 654cc's per shot .25 at 42ftlb
or there abouts.

.177 is least economic with air, .25 most economic.