I tend to try to get into matches going into maps that I know I can get some bullet penetration kills and move slowly, checking areas where I know there's a window or something with a thin wall around it that you can shoot the guy through. ie Derail can be good for sniping into that big building in the middle, especially right where those dark stairs come up. That big open map with the trench in the middle, well that thin wall in the middle of the little cave can be easily shot through and you can just see over the top to notice an enemy come down. Using Bling to have FMJ and ACOG you can get a lot of shots from the indoor balcony in the level with the cabin into those other buildings below, just watch for guys in the windows and such and shoot into the wall below the window to hit their body/legs. Firing blindly through thin walls where there might be enemies is good too, you can get some solid fluke kills like that. I also learned that shooting through a teammate (in HC anyways) counts as a bullet penetration kill
Personally I'm trying to max out everything but the headshots for all the assault rifles as I tend to have the best success getting kills with them. I was going to do snipers but getting penetration kills with those is moving way too slowly. Does anyone know if getting full mastery with a class of gun unlocks anything? what about full headshots?