328fps is 100m/s, and a 0.2g bb travelling at this speed possess the energy of exactly 1 Joule.
Energy (in joules) = 1/2 mass * velocity^2
Here is the link for the conversion chart. ☀http://www.airsoftmaster.com/fps-chart-for-airsoft-guns/
FPS is the most common way of measuring the velocity of an airsoft gun.
The FPS listed on websites selling airsoft guns is the FPS measured with a standard .2g bb. Heavier bbs will decrease the FPS, but increase the amount of kinetic energy the bb has in flight, thus increasing the hitting power and accuracy. There is no known sure formula for how FPS is effected by BB weight, so it must simply be tested in each gun.
FPS is measured by Chronographs, which are similar to a police officer's radar guns, in the way that they measure the speed of an object moving past it. These can be bought for as low a sixty dollars.
In spring pistols, FPS rarely exceeds 328 FPS, and can be as low as 100 FPS. This is with .12 gram BBs. With .20 gram BBs, the FPS will rarely exceed 270 FPS. Note that FPS
Spring rifles tend to have a much higher FPS compared to spring pistols. A spring rifle (or shotgun) is practically useless with low FPS, so many players upgrade their rifles to have 450 FPS and beyond.
The FPS range of AEGs encompasses that of spring pistols and goes far beyond. With .12 gram BBs, a range of 150-500 FPS may be seen (~100-430 with .20g). FPS over 400 is uncommon for use, since they may not be legal on most fields.
Green Gas and CO2 weapons vary in FPS depending on the exterior temperature. Green Gas guns tend to have less FPS than CO2 guns, although there are several Green Gas rifles with notably high FPS ratings (over 500). A Green Gas pistol's FPS usually does not exceed 350, while CO2 pistols generally do not go below 350. This is one reason why Green Gas weapons are favored in CQB.
FPS on spring rifles and AEGs can be adjusted mainly through the use of changing the spring. Spring power is measured in meters per second. Common springs are M100 and M120 (~330 FPS and ~400 FPS with .20s, respectively).
UK FPS Limits
In the UK the standard FPS limit for an AEG or GBB Rifle is 350fps with snipers coming in at 500 fps with a minimum engagement range of about 25 meters. These do vary from site to site but this is the general rule of thumb.
US FPS Limits
Depends on the field. However, most outdoor fields will have an FPS limit of 400 FPS, and indoors the limit will be 350 FPS. In outdoor play, this limit may vary for bolt action rifles. Note that FPS is measured with .20 grams. Using heavier BBs to convey a lower FPS is known as "cheating the chrono", and is highly disgraceful. Airsoft is a game of honesty. Be honest.