If you take a game like The Adventures of Dino Riki and move the timetable up a few million years, replace the generic cave-person with a generic cowboy person, and replace the throwable hammers with pistols (a blazin’!), you’ve got the makings of Gun.Smoke.
In this game you walk inexorably forward, shooting… desperados, I suppose, until you come across the boss, a super-powered hombre who is much more agile that you are and can take many more hits.
It goes on like this for several stages, with the enemies swarming more and the bosses getting more deadly until you either beat them all, run out of lives, or give up and play something else. Given that by the time I played this game that I had already put significant time into Dino Riki, I gave up pretty quickly.
I think I made it to stage three once.