I had a friend who was ringing Twite up in the hills some years ago. He noticed some idiots shooting Canada Geese, so he called 999. An Armed Response Unit (ARU) arrived, along with a helicopter - I suspect the police used it as a training exercise. I don't know the result, but I'd guess the shooters needed a change of clothing
There have now been a few recent instances of birders encountering individuals armed with firearms in the county and in some cases discharging them, on or near public property or public rights of way, most recently including Elton Reservoir, Rumworth Lodge and Bickershaw Country Park. Whilst some of these have involved air rifles, the Police classify these as firearms and they are dealt with as such.
The law around firearms can be somewhat complicated, but birders concerned by such behaviour at public places in particular (giving some consideration to their use on or near farmland etc) should not hesitate to contact the Police on the emergency 999 number (note, if you ring the non-emergency number, they will advise you to ring 999 instead). The Police have attended swiftly and seriously to the recent incidents and our reporting of them is justified and correct. In any case, try to note any Police incident number for future refence and recording.
If you come across it yourself, please don't just ignore it and consider it someone else's job. We all have a responsibility and a duty, to at least try to make the locations where we go birding better places.