The gun must shoot under 400 fps on the first shots of the canister. This way there's no question whether or not someone bled x number of shots out of it to get under 400 fps.
If I'm running chrono, I have the person insert a new canister AT the chrono, and if it's under 400, then it's good. If it's over, it's a no go. You can't prove once it's on the field the guy isn't putting a new canister in at whatever time, and proving he's doing a sufficient number of bleed shots to get it under 400.
I futz with V2s, V3s and V6s. I could be wrong... but probably, most likely not, as far as I know.