Yeah...there's usually 5 or so shots taken. If the majority of the shots (i.e. 4/5) are under you're good to go.
Note: For gas pistols and rifles...the power stuff goes out of whack. With a heavier BB in a gas gun, the expanding gas behind it has more time to do it's thing and you'll end up with a higher Joule for a heavy BB than a faster lighter BB out of the same gun.
Theoretically AEG systems are constant...GBB can be variable. With a NPAS system the shift to redirect gas backwards only happens when the BB is out of the barrel...the longer it's in the barrel, the more gas is dumped down towards the muzzle. We're talking slivers of time here...but it does make a FPS/Joule difference.
Stick to the simple rule:
- use a real chrony...no lame pop-can tests
- use 0.20g BBs...if you're just using them to chrony with...a bag will go a long, long way and there's no question as to the result.
- at games/events where chrony-ing is strict...simply have a loader ready with 0.20's...have the player step up with a mag, fill with 8-12 0.20 BBs (that's about 2-3 plunges from the average loader) and then chrony.