RE: Would this be illegal?
Matt- why do that when you can just mine the field with small explosives. Then when the geese land to feed and walk around they will set off the explosives. Or you could place explosives that send shrapnel in all diffrent directions and will kill everything in a 30 foot radius of each device. This will give you a higher percentage of kills by far than shooting a firecreacker with a slingshot.