5 grams isn't a lot of weight.... 22.4 L of air weighs 29 grams, and 22.4 L of CO2 (gas) weighs 44 grams (at 1 bar).... At 100 bar, 29 grams of air would occupy 0.224L = 224cc.... A Hatsan AT44 (short) tank is 180cc, so if filled with HPA to 200 bar, the air would weigh 29 x 180 x 200 / 22400 = 47 grams.... The 5 gram weight loss for 50 shots could be air or CO2, or a combination of both.... you would need a very accurate pressure gauge, and exactly the same temperature before and after shooting, to tell....

Bob