This Game is meant for Boy Scouts.

tennis ball or other soft ball

You may need to have penalties for unnecessary roughness amongst the fish or for a shooter trying to injure a scout by throwing too hard or aiming for the head.

Mark out a large circular area.
All scouts except one are inside the circle. The scout outside has the ball.

On 'GO' signal, the scout outside the circle begins shooting the fish in the barrel by throwing the ball at them.
When a scout is hit with the ball, he leaves the circle and becomes a shooter also.
The last scout left in the barrel wins.

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An alternative is to have one patrol inside the circle and everyone else outside. A timer is used to track how long it takes to eliminate all members of the patrol in the barrel. When a scout is hit, he leaves the circle but does not join in the shooting.
Each patrol takes a turn in the barrel to determine which one lasts the longest.