So something I would like to see incorporated into the game. Just another variable, not something that has to occur excessively. But my days on the Marine Corps rifle range, one thing your PMI teaches you is "tap, rack, bang". Basically hitting the side of your rifle, racking back the charging handle, then proceeding to put rounds down range.
Now when you are actually in a combat environment, shooting at the enemy a fix jam would be a great realistic variable to have in the game. I would think a good condition to have it happen in is the people who constantly reload their clips to full mags, most jams occur when you have your magazine filled tightly to the top or rolling around in the dirt and dirt gets into the weapon. And applying this to those that enjoy taking weapons underwater that arent meant to be underwater. Possible jams should definitely be included.