This requires that one of your Rangers has Weaponsmithing, a weapon mod, and a weapon that the mod can be applied to. Each mod has a required level of Weaponsmithing, but it doesn't matter who holds the weapon (or the mod, for that matter), so long as someone can apply it. Realistically, you're likely going to get this one as you play through the game.
The achievement does count the same mod being repeatedly, but doesn't appear to count if you restore an old save game (for me, anyway). Saving, applying a mod, restoring, then applying again doesn't appear to count as 2.
What worked quickest and easiest for me, as I'm impatient and didn't want to wait, was a pair of Sturdy Mags and an assault rifle. Opening up the details of the assault rifle, I then hit Y for the mods, found the magazine slot, and used my Weaponsmithing 1 to put in the mod. Then I did it again with the other mod, replacing the first one. Since the magazine has a 100% recovery chance, it never failed and I could just do the flip until the achievement popped.
(With just one mod, you'd have to manually pull it out, then put it back in again - I'm lazy, so used two. With a barrel mod, there was a 25% chance it would be destroyed, so that's just a matter of time until disappointment or reloading is necessary, but should still work.)