It doesn't work like that. There are loads of different guns in the game and they can all be found in different states of disrepair. If you pick up a gun covered in rust and dirt then it might jam (I saw this happen three times in an entire days playing and even then only once to the player), but the guns themselves don't degrade over time. If you pick up a low quality gun then you can swap it if want to avoid jamming and change it for a higher quality, cleaner gun. Even when the guns do jam you just hammer E a few times and you'll quickly fix the gun. It takes 3-5 seconds. Different guns have different chances of jamming too. The AK-47 practically never jams, ever. The M16 on the other hand is really quite fragile, though slightly more accurate. It is a case of balancing.