The heavier the round the more "kinetic energy" it has, meaning it hits harder. Velocity also comes into the equation. On 90% of airsoft, electric, spring, and most gas guns have a relatively consistent drop in FPS when introducing a heavier weight BB, keeping the kinetic energy, or joule limit, roughly the same. For instance, if your FPS limit was 420 with a 0.20g BB, your energy limit would be 1.64 joules of energy, and so one.
There are a few guns, mostly gas systems, that do not suffer from this FPS drop in the same way. So at most modern airsoft events, these guns are subject to chronographing with the weight of the BB they'll be using to ensure they are within the joule limit, and then cannot switch BB weights without re-chronographing.
"In a nutshell"
What generally happens with a gun that's over is this; there will be a field limit such as 400 FPS, which is the gold standard FPS for that fields performance. Then they'll have a safety window of 20 FPS in case of a fun that is "slightly" over, or a gas platform that may fluctuate in temperature change, creating a "hard limit" at 420 FPS. So in essence if you're a little over like 410, no big deal, but keep an eye on your gun. If you shoot 421 FPS however, your gun is not allowed. So for the sake of argument, if your gun shot 417 FPS, I'd seriously consider downgrading it, because a hot day could push you over the hard cap, which is the gun owners responsibility.
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Last edited by Ricochet; May 14th, 2014 at 16:17..