I was watching a movie the other day and character A says of character B that he is a marksman. He can strike a man square in the chest at 50 meters. This was a modern piece with modern weaponry. There are different levels of writing what you know. The soul of some things can only be realized through experience. Others simply need research. In either case, you need to address a first-hand account of the subject and not just lend your interpretation to someone else's interpretation. That just becomes the operator/telephone game.
In this case, I immediately knew the writer had no idea what (s)he was talking about. 50 meters? Good god, not fifty meters! That's so hard!
...no, wait, no it's not. On a standard marksmanship target range, the SHORTEST target is placed at 50 meters. The longest is set at 300 meters.
Rather than paying attention to the movie, I began to wonder if the writer had even fired a gun before or just thought a dramatically delivered line would go unquestioned.
Now I'm not a hunter. I've never killed anything larger than an insect. ...except for a bird that done under my tire when I was a teenager. I was, at one time, a certified marksman (my preferred range being between 175 and 250--though I ALWAYS missed 300. I never got the arc right). If you're going to write a book that includes modern weaponry, I recommend finding a local sportsman's club and renting a pistol, a shotgun, a rifle, and if they have one, an M-16. Each of them has different uses, different feels, and different results. A little research will dramatically improve your weapon choice and description. You'll also avoid throwing your reader out of your story when they realize you have no idea what you're talking about.