if you shoot through catwalks, it takes no damage away but still counts as an fmj kill. also on terminal, in the back room by domination base a, there is a wall that you can see through and shoot through but it never breaks. you might have to camp a bit but you can easily get 10+ fmj kills a game doing this.
highrise, terminal, skidrow, all have windows that are good for this, highrise has alot and have good vanatage points expecially if ur playing demo. the plane in afgan the windows never breaks, so if your playing demo (A bomb) this could help alot. I also find it easy to just camp a halway from a spot were u can move behind a wall, and when u see someone just move behind the wall and wallbang them, work really well for assault rifles as well.
If you're going to try get FMJ kills with a assault rifle, the best way to do it is by aiming through door or windows. Then you aim at the person and more over so you just clip the window frame... The rest is history.