G'day McGrath,

Mi braincell is working overtime, trying to recall the 'Shooting Ballery' in the cicus sideshow, when I was a kid. You got a pea-rifle to shoot at fixed targets, or rows of moving tagets that flipped over when you hit them. One I recall also had a tub of turbulent water, with numbered ducks bobbing about on the surface. That was thetricki one.

Maybe the expression refers to an early variant, with a barrel?

Cheers - Sa,