I've alway's enjoyed pass shooting: getting under a flight line and shooting birds as they travel by. Although it's less fashionable these days, you don't freeze picking up a bunch of decoys at the end of a pass shoot. You take your birds and go home. There's something to be said for that on a cold day after sunset.
When done correctly, pass shooting requires scouting, concealment and shooting skill. It's also an ethical way to hunt as long as it's done responsibly. The universally reviled activity of skybusting, on the other hand, is unethical pass shooting.
So what's the difference? I'd say it's skybusting if:
- you are shooting beyond the clean killing range of your equipment*
To elaborate on the last point: if you're hunting in a crowded marsh where lots of people have decoys out, and your pass shooting is discouraging birds from flying low and working, you're skybusting even if you can make the shots. Shooting birds that are swinging someone else's decoys is skybusting, too.
Here's another example:
Years ago one of my neighbors told me how he used to go down to Swan Lake National Wildlife Refuge in Missouri and pass shoot Canadas, back when pass shooting around refuges was the main way to hunt geese. Geese would come out to feed, often quite high up, and hunters lining the refuge boundaries would shoot at them. My neighbor's group shot buckshot until steel shot came along. Early steel loads were terrible and they didn't come in big shot sizes. My neighbor and his friends found a particular size ball-bearing that approximated a 1 buck pellet and handloaded them. They still weren't very effective.