Depending on whether you do the campaign or the DLC missions first, you should unlock this pretty early on. You can get it during the Sparta mission in the campaign by picking 3 different ammo type guns at the equipment vendor (shotgun, automatic rifle, and pistol works fine). However, if you start out with the DLC missions first you can easily get this on the Pavel level as there are numerous types of weapons that the enemies have.
I've included a video below showing the routes I took through 7 of the DLC missions. The Pavel mission starts at 44:32