Here's a game my brother and my son play with each other - just informally at the shooting range.
It follows the idea behind the billiard game "cutthroat".
Each of us gets 5 reactive targets (pop cans filled w/ water or such) and we each set our own out in a group. We either group the targets according to whose they are or we assign a color to the cans. Basically you need to be able to identify whose targets are whose from a distance.
When we call "go" we start shooting everone one else's targets but your own. The person with the last standing can wins.
You can adjust for handy caps by letting the poorer shooters set their targets farther back.
It can also pay to shoot hefty loads. While the light loads improve the recovery time, a good hard "hit" can sometimes knock out more than one target by sending debris or a blast of water flying.
I know it's not official Cowboy anything - but I thought CAS shooters might find this game fun.
Try it - let me know what ya think. Or if you've got an interesting informal game you like post it here.