it originated in old time carnival attraction where you shot a twenty-two rifle at small moving metal targets that fell over when you hit them. One of the targets was a line of life sized ducklings

Sounds plausible Bill - in the UK you still get shooting galleries on piers and at fairs; often the targets used to be a moving line of ducks, though probably it's a more PC line of indeterminate beasties these days. It's definitely more difficult to knock the ducks down out of sequence than if you keep them bunched together.

I should add that the gun is never anything more powerful than an air rifle (BB-gun to USns, isn't it?) over here, but then the target is rarely any more than 15ft or so away (in a stall). A .22 rifle would blow a hole through anything at that range, and I'd imagine your carnival shooting range would have had to put the "ducks" quite a long way away - more than 40ft at a guess.