Found the following on the Durham County Council website faq section on public right of way- probably on others but this came up first on the google search. Although they do not cover airsofting they give an indication that should help. If you can use firearms over a public footpath then airsoft pellets should not be a problem.
Overall it probably comes down to what Ian was saying - have a written risk assessment which includes a player briefing where everyone is told to be aware of potential users of the footpaths. Make sue that the briefing tells then to call `cease fire' or similar as soon as walkers are spotted and then wait for a marshall to call `game on' before anyone starts firing again. If you also make it clear that anyone who fails to comply immediately with the `cease fire' call will be removed from the game/site then you should be OK (that is my view from reading this and not official legal advice).
I would guess that most sites have a `cease fire' rule for their insurance e.g. to protect a player who has his/her eye protection come off during a game.
Can firearms be used on a Public Right of Way?It is not an offence to shoot across a public right of way, although to do so may amount to a common law nuisance or intimidation, depending on the
circumstances. It is, however, an offence to discharge a firearm within 50 feet of the centre of a byway (or other route carrying public
vehicular rights) if it injures, interrupts or endangers any user of the route. (Highways Act 1980 section 16.1 )
It is also an offence to have a loaded air-weapon - or any other firearm whether loaded or not - together with ammunition in a public place, including any public right of way, unless the person has lawful authority or a reasonable excuse, such as a landowner or tenant shooting vermin on his own land. (Firearms Act 1968 section 19)
What if someone uses intimidation or threatening behaviour intended to deter use of a Public Right of Way?The use of intimidating behaviour with the intention of deterring the use of a Public Right of Way is possibly an offence and may amount to
obstruction of the path. In the first place, we will seek to address any underlying issues which have lead to the situation arising. We may then
issue a warning to the offender and involve the police as appropriate.
(Public Order Act 1986 section 4)
Who is responsible if I am injured because of dangers on a Right of Way?Owners and occupiers of land crossed by public rights of way can be liable for injuries caused to path users by their negligence. For example, if a
stile were to collapse under a walker, or if a path user were to be injured by an electric fence placed across a path, then the injured
party may pursue a claim against the occupier of the land. (Occupiers Liability Act 1957)