"An offence to possess an airgun in a public place without lawful authority or a reasonable excuse" - agreed. I love the argument "because we all own public property" too.
Sadly, very few magistrates would consider "because I fancied shooting some tin cans on the school footie pitch" as a "reasonable excuse".
There are other areas of the Firearms Act 1968 which could also apply - section 21A would be a good example.
In short - don't shoot airguns on public land, you'll get arrested at the least and 6 months in pokey with a £5k fine at the worst.
It is one of those grey areas i admit, but in the word of the law it is possible, What i do if i want to shoot some groups which i cant do at the gunclub as they dont have a place to put up targets and such only the knockdowns. There is a wood i use, its public land. I go away from the path, set a range up, pin the targets to a backboard i made which pellets cannot pass through etc. Basically from my expierience it works like this. If you show responsibility and your prioritys are safety for others, and it will show with your set up then you are generally ok as people will see you are no idiot with a gun and if the police do show up they will also see it, they will see responsibility and treat the situation as such. I have sat and talked to the police and had a brew with them in such an instance and they were quite happy for me to be doing what i was doing.
Its all about the first impressions.
What is a reasonable excuse to carry an air rifle? There are none other than shooting with it or going to shoot with it. Its how you are shooting with it that makes the difference. Like with a knife it is how you are using the knife and in what context that matters not where or when. The law is the same and even worded almost the same.
Firearms laws only apply if an offence has been commited. Shooting on public land obeying the 50 yard rule but more importantly, shooting safely and responsibly is not an offence if you are shooting targets. Note the law states you need "reasonable excuse" to shoot the rifle while with a knife it states "good reason" so you need a more solid reason for carrying a knife than an air rifle
Did you know that the 50 yard rule applys everywhere and that if your back garden is within 17 metres of the centre of the road then you are breaking the law weather in your garden or not?
In short
It is not an offence to shoot on public land at all. Please dont state that you WILL get arrested as i can assure you that this is not the case.
You do realise if thats your opinion on the laws wording then that HAS to be you opinion on knives as it is the same. Do you use a knife in a public place? In the woods/campsites/work?