When I was a kid, Combat was one of my favorite TV shows to watch. I have always wondered what type of rifles were used to make that distinct sound. Here's a clip and the shooting that starts at 11:16. The gun shot audio sounds familiar in every episode, so quite possibly an audio track was edited.
https://www.youtube.com/watch?v=jpOO4XhR60Y
https://www.youtube.com/watch?v=jpOO4XhR60Y