A gun is the weapon of the street. Street thugs don't care for their guns, they don't clean them, and they throw them away after using them to kill. A gun is the thing a child used to shoot cans, squirrels, or other game, game that can not harm you back.
A professional, a Marine or a Soldier, is trained that while they may have used 'guns' back at home to shoot squirrels or cans, a rifle is assigned to them, cared for by them, kept close by at all times, used to kill an enemy combatant, someone who can kill you, and is not to be carelessly handled, dropped, etc. A rifle becomes part of them. The rifle will save their lives and the lives of their platoon mates. The rifle is cleaned first, before the marine, even before eating or sleeping. A dirty rifle may misfire and a misfire can kill you, or worse, your fellow troops.
At this point, the only 'gun' left to the marine, is the one in their pants, to be used for pleasure only.
I, too, performed a drill much like the one shown, when in Army boot camp, in 1985.