Mafia III

The Hartmann 7.62 becomes available as a favor from Vito during the final district takeover. If you don't receive it by then due to complications with Vito, it'll be up for sale via the Arms Dealer after the final district is under your control. All in all, it's a late-game unlockable.

The Party Animal outfit will only be available to wear after completing the narrative of the DLC. In fact, after completing the final story mission, you will be loaded back into the game world inside of Marshall's office and there will be a wardrobe closet nearby. Interact with the closet, scroll over to the "Special" tab, and equip the Party Animal Outfit.

Now exit the building and call the Arms Dealer. When he arrives, browse his inventory; you will find the Hartmann 7.62mm under the "Special" tab. With the outfit and weapon equip, all that's left is to kill fifty enemies. The fastest way to do this is to deliberately catch heat by killing a pedestrian. Once the cops show up, go on a rampage and kill as many policemen possible before you die or run out of ammo. The achievement will unlock once fifty enemies have been killed with the Hartmann, provided the Party Animal outfit was worn the entire time.