Helium is only used on extreme power airguns, it’s not at all practical for two reasons, it’s expensive and it is very good at finding leaks.

Nitrogen is ok but still not as cheap as air

Argon would work but it’s more dense than air so would reduce velocity.

Your best alternative is a compressor

Bb
 
Thanks.
Yes, air is free until you must compress it. I do have a 7L 300 bar SCBA cylinder but the only dive shop compressor close to me is only filling to 230 bar, used to be 250 bar but he said it is putting to much strain on his compressor. Also he became expensive. So I am looking for an alternative.

Nitrogen seems to be pressured to 200 bar only.
Booster would work.
 
Thanks.
Yes, air is free until you must compress it. I do have a 7L 300 bar SCBA cylinder but the only dive shop compressor close to me is only filling to 230 bar, used to be 250 bar but he said it is putting to much strain on his compressor. Also he became expensive. So I am looking for an alternative.

Nitrogen seems to be pressured to 200 bar only.
you can run Nitrogen thru a Altros box , i checked to make sure .
 
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