Chance Glasgo, a first-person animator at Infinity Ward, did a Reddit Ask Me Anything thread last weekend, taking on questions about the complicated weapon animations seen in games like Call of Duty and Battlefield. You should read through the whole thread if you're interested, but PC Gamer has kindly grabbed some of the most fascinating insights.
Glasgo says that "most serial numbers on guns in CoD have significance" of some kind, whether it's just a reference to his workout gym of choice, or a hint to something in the game's larger mythology.
He also explains that while gun aficionados will often tell him he's made some sort of technical mistake in rendering realistic weapons, the choice to do things differently is simply creative. For example, you're never supposed to point a real gun anywhere but forward even while reloading it, but following that rule would make some of these just so ... boring.