You should get tired from aiming a rifle too long. A basic M4 weighs somewhere between 5 and 7 kg's loaded. Doesn't seem like much but try holding it up for an hour or two and see how tired you get. That's why polymer frames are so popular cause you can shoot longer without fatiguing.
Shooting itself is also tiring cause you have to keep the right grip, stance, holster, upholster, reload, charge or rack the weapons. Keep doing all that while shooting for an hour and you'll be really tired added in with all the running around plus carrying a pack full of gear