Much simpler than it sounds;
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, head over to the next mission (
the Refinery), there is a group of 3 enemies sat around a fire as you enter. First off, save your game, so if you mess up the shot, you can simply reload.
Ease towards the group via the hallway on the right hand side, get out the Nailgun, load up your Railgun slugs and align two of the enemies heads through a wall.
(NOTE: ShootZombies rightly pointed out that it
MUST be a WALL, it won't work through any other bits of cover)
It took me 2 tries as you need to make sure the alignment is pretty accurate, as soon as you get them both with the shot through a wall, the achievement will pop!
If you'd like a more visual walkthrough, McoreBE made an excellent video guide below;