It is a subsection in the firearms law. It basically states that any projectile firing weapon must have a muzzle energy of 1 joule or less of energy to be legal. Above that and it is an illegal firearm. Airsoft replicas don't have unique serial numbers or anything to be registered for a license so it can't be upgraded That translates to 100m/s for a 0.2g BB. About 89m/s for a 0.25g one and so on. The formula used is: (1/2)x(mass [in kg])x(velocity squared [in m/s]) So our airsoft rifles are referred to as RIFs (Realistic Imitation Firearms) key word being imitation. We don't have limits on colours or stuff so we can keep ours looking cool but they must be under 1 Joule, plain and simple. The 1 Joule law used to be more of a problem a few years ago as a lot of manufacturers made their AEGs over our limit so there was the difficulty of getting it downgraded to within legal limits before it reached the countries. Retailers had to take this cost on top of shipping and tax so their was a difference between our prices and other countries. Nowadays a lot more manufacturers have begun to make 1 Joule AEGs because a lot of countries have adopted 1 Joule as their airsoft limit so it makes trading easier for them. For example, a company called G&G used to be redistributed in europe by a wholesaler. The wholesaler started asking them to downgrade their AEGs in the factory for their customers and after a few years of doing this, G&G stopped using the middleman and made 1 Joule AEGs and sold them directly to retailers as there was more money in it for both parties and they had gotten a reputation as a good and reliable manufacturer from their years of selling through the wholesaler. 1 Joule is popular because it is safe. It can hurt and may damage an eye if it hits directly but won't rupture it. But everyone playing at sites wear eye protection. It is not legally required but everyone does it because it makes sense. The only other laws regarding airsoft are proper storage and transport like real firearms. So you must carry it in a bag in public and can't show it as you can be charged with brandishing a firearm. So it is a full firearms offence.