You need a 50 caliber rifle to kill enemies through walls, even seemingly weak stuff like crappy rotted wood. I could never get a 7.62 or .338 rifle to penetrate and all I succeeded in doing was alerting enemies.
Knowing that, you can shoot through literally anything with the 50 cal to nab this achievement. Make sure to get the rifle out for this occasion and don't take it with you on a mission if you're the stealthy type, because no 50 cal has a silencer and they have the report of a naval cannon.