Every sane person. Even for a soldier the adrenaline rush is always subdued. Nobody screams while they shoot like in 80s films. Neither they make funny open mouth face too.
Come on, watch the movies. Want to watch war footage, go through that too. Even though it'll be pointless to compare with a reel version.
Lastly, they've one of the most macho superstar on screen & they won't be making a mistake of making him shout or make faces. He can take the pain then he can handle a MG42 like a man.