I did this immediately after achieving the 'Wall Hack' achievement, it's nice and easy;
The Wall Hack achievement in RAGE worth 42 pointsKill 2 enemies with one Railgun Slug shot through a wall When you finish the first Scorchers mission (
the Hagar Caves), you are given the Nailgun, which will get you both the Railgun and Rebar ammo achievements.
After you've left the caves, head over to shop in the Hagar Settlement (down the central ramp towards the exit of the town), here you can buy Rebar and Railgun ammunition, get both.
Now, when you've entered the next area (
the Refinery), there will be a group of 3 enemies around a fire, simply load up your Rebar shot and shoot one of them, this should pin the enemy to either the wall or the floor, popping the achievement.
Thanks to McoreBE for the video guide;