Use a .22 cal pellet gun with flat point ammo. Get a pump, so you can adjust the actual hitting power. Many pump mfg's will spec how many ft/sec per pump. I'd say get a pump capable of 800-1000 fp/sec and try to maintain 300-400 fp/sec for your shots.
BTW, geese are hard to kill. I don't know if these hits will send them away.
Agreed. I hunt geese with a 12 gage, using #2 and "B" 3" heavi-shot. They are tougher than they look. I doubt a pellet gun will do it. Try a border collie to chase them, or fig dressing to eat them.
Thanks. I don't know if it will work, but my yellow lab came back from his morning out with goose dung all over his back (rolling in it of course). The black one may have it too -- you just don't notice it on him until you get downwind. So, I clearly need to try something. I'll report on the results.