All joking aside, I think the time it takes to properly clean and maintain an M-16 is a serious liability in combat. (Not like I would know from first hand). But it seems it takes you 15 minutes to clean the AK, at best I can clean the AR in 45.
That's a half hour per day that a serviceman could be preparing defenses, sleeping, writing home, ect. I have know idea how clean they are kept in combat, but I would imagine the are either being fired, cleaned, or immaculate at all times.