An FAC air rifle is a very good tool, but it comes at a cost ! You need a variation, then over £1000 for a reasonable set up for a non springer. What about making a good hide in a good spot and luring them to a nut feeding station ? Depending on the range you could springer (cheaply) or 22 rim fire with a built in back stop. No one thing is perfect, or fits all situations. It all comes down to how much you wish to spend.
