Inside a disused fort on the top of Portsdown Hill, black figures wearing helmets and military webbing stack up against a wall preparing to raid a room.
Armed with assault rifles and pistols with torches attached, they press their bodies close to the red-orange brickwork of Fort Widley, waiting to enter.
It would be easy to mistake the men as some sort of army assault team – their uniforms and equipment look almost identical.
But these men are not in the military, and nor are they armed with real guns; they are in fact using toy BB guns – albeit almost indistinguishable from their real counterparts – which fire plastic pellets.
The sport the men are playing is airsoft and it’s one that is gathering a growing fanbase in Portsmouth, with hundreds of people taking part each weekend.
The city has several sites on its outskirts, ranging from sweeping woodland near Rowlands Castle and Wickham to former military bases in Lee-on-the-Solent.
But the hobby has come under pressure in the light of recent terror attacks and the jailing of a man armed with a BB gun in Southsea.
Now, the city’s airsoft community has come out to defend their sport, which is ‘on a knife-edge’.
Stephen Banks is the owner of Combat South, a company he started in 1998. He runs two airsoft sites in the Portsmouth area as well as a retail store at Fairways Business Centre, in Airport Service Road, Copnor.
‘We totally condemn that kind of behaviour,’ he said.
‘I think it’s highly irresponsible to be in a public place with one of those replica firearms and people will likely find themselves in big trouble, with armed response knocking on their door.’
He said despite the activity’s growing popularity it had faced the constant threat of closure for the best part of a decade.
‘It’s been on a knife edge since 2007,’ he said.
‘I believe it doesn’t need an act of parliament to change the laws, it just needs a signature from the Home Office to ban the sport totally.
‘For us, that would be absolutely devastating.’
He claimed there had in the past been a problem with criminals using BB guns to commit crime.
However, Hampshire police has stressed there is no such trend in Portsmouth.
Mr Banks explained that airsoft enthusiasts had taken steps to clamp down on the sale of realistic-looking guns nationally.
Retailers and sites run a register system which makes it tougher for people to purchase the airsoft weapons.
‘The problem is people wanting a gun quickly for a crime,’ he added.
‘There are systems in place where people now have to jump through a few hoops to get a gun.
‘They have to play for a certain amount of time at a regular site before they can buy a gun and they need to be over 18.’
The ‘in-house’ rules seem to please the government, Mr Banks claimed.
‘We have had no backlash from the Home Office about it; they seem quite happy with it,’ he said.
He added: ‘We’re not a terrorist training camp.
‘It’s a good fun day out, especially for the youngsters. It gets them out and away from their keyboards and doing something active.’
The sport attracts people of all ages and abilities.
It sees them taking part in military-themed games, from rescuing a hostage to defending a building against attack. Some 500 people are estimated to take part in the sport from the city, with thousands more joining across the UK.
The University of Portsmouth even has its own society dedicated to the activity, with 70 students being part of it.
Thomas Minsaas is the president of the university’s Airsoft Society.
The 20-year-old Norwegian said reckless incidents of people carrying BB guns often weren’t down to the airsoft community.
He branded the people who did such a thing as ‘immature’.
‘It really hurts the community and it tars everyone with the same brush,’ he said.
He explained that stopping people from buying replica firearms and BB guns would be difficult.
‘You really can’t stop people from getting their hands on the kit,’ he said.
‘It’s hard to clamp down on it without doing something like a whole psychological evaluation.’
He added that only a minority of people could spoil it for everyone else.
‘I am worried that it could get shut down and people would clamp down on the sport.’
In September a man was jailed after he painted his bright orange BB gun black.
Anthony Bird, 38, of no fixed address was jailed for six months after he was spotted with the BB gun sticking out of his waistband in Southsea.
A Hampshire police spokeswoman said officers had no grounds to seize the gun when it was bright orange as it was clearly a toy but this changed when he painted it black.