When a shotgun fires, the expanding gases in the barrel push the shell out. According to Newton's third law, the shell exerts as much force on the gases as the gases exert on the shell. Would it be correct to say that the shell does work on the gases?
In this scenario the shell would be doing negative work on the gases as the force the shell is exerting on the gases is in the opposite direction of the system's movement.
Would it be correct to say that the shell does work on the gases?
No. Work is defined as the product of the force exerted on an object times the distance the force actually moves the object. In this case the barrel does not move, and therefore it performs no work.