In the second mission (The Island) there is a man under the second bridge with a scoped FG42 rifle which will get more ammunition whenever you kill someone with it. The rifle starts with 96 ammo remaining and you can kill about 150 people with it (not including melee). When that mission has been completed you will automatically keep the rifle for the third mission but from the fourth mission onwards you must save it if you want to keep it.
I found this out myself, believe it or not. I call it "motorcycle glitching". This glitch, allows you to go through walls, get under the level, and around the level, while being very hard to be seen. Okay, this glitch could take some practice to get used to, first get a motorcycle. We'll use Eder Dam for an example, as I know every glitch in that map with a motorcycle... First, go to the American spawn point. On the left side of the house, their'll be a stone wall with a hill. Drive up next to it, with the front of the cycle facing the stone wall. Make sure, that the side you get out of, is almost wedged into the wall. Now get out, you'll be behind the wall, no one can see you, except when yu go up the hill. Be careful, you can fall through the floor. That's one glitch spot. There are another 10 or so in Eder Dam. Good luck finding them.