What is Airsoft
Airsoft is a sport that was developed by the Japanese in the late eighties to replace the popular sport of paintball, Which had long been criticised for a lack of realism, high cost of the associated equipment (especially ammunition). Airsoft proposed an answer to all of these concerns:
Airsoft replica weapons are legally described as ‘toys' and have been manufactured to closely resemble their real counterparts, both in size and weight, with most of them working on both singe shot and fully automatic using a simple piston driven by a battery powered electric motor.
Due to the forces required to fire a 1' diameter paintball, bruising is hence a common side affect from the impact. Airsoft BB's are only 0.20G in weight and therefore the force required to fire them is considerably less, hence the effects of impact are negligible (an Airsoft BB will not break human skin even at close range).
Is it Dangerous

When airsoft replica weapons are used properly under supervised conditions the answer is quite simply no. Of course common sense precautions such as eye protection are highly recommended and indeed most well run sites enforce this vigorously. Of course airsoft guns are also classed as replica firearms and all care should  be taken in accordance with the current law.

Can an Airsoft Replica weapon be converted into a real weapon
Absolutely NOT!

Although they look the same as their original counterparts, they are produced as toys from either hard ABS plastics or mild metal alloys. Internal parts are not even remotely the same as real weapons and original gun parts cannot be made to fit.