Option 1: Go on to Eden Editor, on any map, and spawn in one of the vehicles that has different weapons, such as the Humvees that can have different turrets, and certain tanks which have customisable secondary weapons (I cant name any names off the top of my head). Right click the vehicle and select "Customise Loadout". Change any of the customisable settings and click OK.
Option 2: This is for anyone whom the achievement doesn't activate for (the achievement is known to be broken). Download and install Steam Achievement Manager [I HAVE READ CONFLICTING ACCOUNTS OF PEOPLE BEING VAC BANNED WHEN USING THE PROGRAM WITH ANY GAME OPEN, USE AT YOUR OWN RISK], and select Arma3, and tick the checkbox next to the achievement. This can also be done for the Puppeteer and Dressing Doll achievements, which are also broken for some people, just don't select every achievement or it's obvious that you have 'cheated', as the timestamps will be identical.
Again, Option 2 is a little risky and controversial, but that's your call to make. It's useful for any game which has broken achievements, but some players consider it cheating as you may or may not have rightfully earned them.