Gears of War 4

Not a guide, but if you are looking to find Hammerburst...

Core Hammerburst Changes

Last week, we announced that the Hammerburst would be removed as a loadout weapon beginning with this Update. This change is being made to preserve its impactful role as a potent long-range rifle but removing the issue with its over-effectiveness when used by a team of players.

The Hammerburst will now be available as a secondary weapon pickup in Core, much like the Markza, Boltok, Overkill and Retro. Here’s the full list of the maps that will now include Hammerburst pickups along with any weapons they rotate with per round (ordered from first to second).

Avalanche – Boltok / Hammerburst
Blood Drive – Boltok / Hammerburst
Clocktower – Hammerburst replaces Smoke Grenade spawn up top
Diner – Shock Grenade / Hammerburst
Dry Dock – Hammerburst / Boltok
Glory – Shock Grenade / Hammerburst
Gridlock – Hammerburst / Boltok
Harbor – Hammerburst / Boltok
Hotel – Boltok / Hammerburst
Impact Dark – Boltok / Hammerburst
Lift – Hammerburst / Overkill
Mercy – Boltok / Hammerburst
Old Town – Hammerburst / Boltok
Relic – Markza / Hammerburst

https://gearsofwar.com/en-us/community/whatsup-mar1-2018

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How to prove your mastery with the hammerburst ?

"Get 10 kills or assists with the hammerburst".
In a single match ; 20 times..

The hardest part of this achievement is to find the hammerburst, that's why I have posted more information about localisation than how to do it :)

edit :

Tip for people looking to grind this out. You can only get this once per match, but that doesn't stop you leaving and rejoining to count it as another match. Best game mode is obviously king of the hill. You can quit as soon as the tracker counts towards the achievement, then when you rejoin you're free to get another 10 assists / kills. I resorted to this as it seemed to be so infrequent that I'd get a map with a hammerburst.
by Muzzily.

To speed up the achievement progress a bit, I recommend the following:
-Play with a partner or more "if you can put extra controls better, to fill the equipment"; in the King of the Hill game mode against Bots (preferable in the lowest difficulty).

-Now the ANFITRION for nothing in the world must LEAVE THE GAME while the other players act killing bots or whatever the achievement requires to increase their progress. Once the progress has increased, LEAVE THE GAME and have the host invite you again to return to the game (repeat this process each time you increase your achievement progress).

-I recommend doing it at night because there are usually few people and be agile since sometimes a different player can join.

A disadvantage is for the host because he has to stay in the game but between the members he can alternate the host.

¡REPEAT! is to accelerate a bit the progress of the achievement without having to finish the game and start a new one, I hope it helps. Good luck