Hi,
As far as I know, these guns are illegal in Singapore. There was
once where quite a few shop sold these guns (China made) a few
years ago & these owners was charged in court.
For BB guns, they come in 3 categories, namely the air gun, gas gun
& electric gun. The most powerful will be the gas series.
Bro, better not play play with these guns as the present situation
whereby terrorist are lurking everywhere, security will be the
prime concern to the men in blue.
Last heard there was this place in Marina Square provide these guns
to member to shoot at the shooting range. But I think it had since
closed down.
Cheers, 
