If you're going to dislike, please leave feedback as to what is wrong and I will change it.
Thanks.
Good News: Unlike the "Faster, Baby!" DLC, the tracker was spot on and unlocked at 50 for me. No glitches this time.
Bad News: Unless picked up during the DLC mission "In Country" (possible other areas TBC), the gun you need is unavailable for purchase to you until you complete all 9 districts with Vito alive and/or Vito's favours (not sure which criteria it is yet). Either way, that's hours of your time.
However, don't fear for if you have not yet done "In Country" (first mission out of New Bordeaux) you can still pick up the Hartmann easily.
So, for this achievement, you will need to do 2 things:
1) To complete the Stones Unturned DLC for the Party Animal outfit whilst also picking up the Hartman. Easy.
2) To pickup the Hartmann 7.62 gun during "In Country" (or do the long, painful route of completingall 9 districts with Vito alive and/or Vito's favours (not sure which criteria it is yet)). Close to the start is a battle site, before you interrogate a militant. In one of the buildings in this first main battle is the weapon in a easy to find open crate.
Once you have the Hartmann via pickup, KEEP THIS ON YOU AT ALL TIMES throughout the DLC, having a good pistol to back you up. Complete the DLC, and when you end up back in New Bordeaux you'll still have the Hartmann equipped. Then do the following.
1) Change into your new Party Animal outfit.
2) Go to the police station in French Ward and keep killing 15-20 cops per go.
3) Die on purpose when you run out of ammo / health. Dying actually helps as it triggers an autosave which keeps track of your kills.
4) Restock on ammo, then rinse & repeat. If, like me, refilling ammo is not working then shoot a few rounds out of your secondary weapon so both guns need refilling, this worked every time for me.