Hi, looking for some advice. My father had an air rifle which he and I use to shoot feral pigeons in the barn. I took it out the other day and it was shooting way off. I zeroed it and found it was shooting 18 inches high but still dead centre. I've used the gun many times before and it's always held its zero. Anyway when I tried to zero it I found that at the lowest scope adjustment it still shot 5 inches high. The only thing I can think of is the scope has been knocked but was wondering if there's anyway to get it back or is it useless now. It's a Tasco world class .22 with 1/4inch clicks at 100 yards.