i also seem to remember reading somewhere that cats cease to be personal possessions once they leave your property and are essentially the property of whoever owns the land they are on at the time, like game. in effect they become feral the moment they are allowed to roam out of the garden. tis may be bollocks, but its rattling round in the back of my mind somewhere......
Totally incorrect in the UK.
Cats belong to their owners regardless of where they happen to be, and to add insult to injury, their owners cannot be held liable for their actions.
I'm not sure how sensible the OP is in openly admitting intent to kill cats on the forum... the next time a cat goes missing in the vicinity he could find himself answering some difficult questions.
Given the fact that this is a commercial farm; I'd suggest the OP tells his employer to contact the NFU for advice and support. The police are duty bound to investigate any crime committed, and the killing of farm stock is a crime.
Whatever happens, I'd urge extreme caution when thinking about action with firearms against marauding dogs and cats.