Replica guns like airsoft guns are dangerous primarily because of how similar they look to real guns, say lawyers.
They do not even have to be in working condition to be considered controlled items under the law, said criminal lawyer Rajan Supramaniam.
"It is very difficult to make a distinction between them and a real gun, especially to vulnerable people such as the elderly and children. It looks very realistic, and can cause alarm and instil fear," he added.
Mr Rajan, managing director of Hilborne Law, cited the example of a case in July, when a Bangladeshi national attempted to rob a pawnshop outside Boon Lay MRT station with a plastic replica gun.
For airsoft guns, there is also the additional danger of its firepower, he added.
Mr Ashwin Ganapathy, a lawyer at IRB Law, said such guns are regulated mainly for the potential harm they can cause.
"The act is geared towards preventing the commission of offences using this kind of items, such as robberies," he added. "Furthermore, such guns were sold off the shelf everywhere in the past. But the authorities probably realised its potential for harm. If it lands in the hands of a child, it can cause hurt to himself or someone else."
Mr Rajan said his clients have brought back replica pistols, mainly from South-east Asian countries such as Thailand and Indonesia, and have been stopped during the security screening process at the airport.
Those stopped will be questioned for a couple of hours on where they got the items and will have to surrender these guns, he added.
He said his clients have received warnings for attempting to bring in such illegal items as it was their first offence.
As for the Water BB (WBB) guns, Mr Rajan said they are illegal due to their ability to cause public alarm. He added: "The firepower is actually secondary in this case."
Every gun or replica gun has to be declared to the authorities, and owners must obtain licences for them, said Mr Rajan. Paintball guns, for example, are registered to operators and kept in the premises.
Mr Ashwin said a WBB gun can be very dangerous too, if it is modified to shoot faster and with more force.
If it hits a vulnerable part of the human body, such as the eye, one can only imagine the harm it can cause, he added.