60's comic book about shooting invading aliens inside their force fields

I’m trying to find a comic book I read in the 60’s, which probably contained more than one story, but one was about invading humanoid-type aliens, each of which was surrounded by an impenetrable force field, from inside of which they shot their raygun-style weapons. Someone (a farmer or rancher?) eventually realizes that there’s an opening in the force field at the moment the alien shoots, and by shooting his rifle at that spot the same moment, he can kill the alien.