First off you can only earn progress towards this achievement in public matches and not private. Public matches do include the Versus AI playlists (any difficulty).
There are a total of 18 weapons in Gears of War 4 (not including grenades). Unfortunately heavy weapons do count so these will probably end up being the last weapons you will need to get 50 kills with.
Figuring out which weapons you have left to complete can be a bit difficult however you can initially use the war journal to get a partial idea under the ranked/social versus tab based on rifle kills, shotgun kills, pistol kills, etc. Unfortunately the Weapons tab mixes Campaign, Versus, and Horde weapon kills.
To get a better idea of the weapons you need you can use the achievement tracker. Use the overlay during gameplay and get 1-3 kills with a particular weapon. This should show partial progress (not always +1%) if you haven’t already got 50 kills with that weapon (you should see the tracker bar highlight slightly).
To start you can get the 5 loadout weapons complete easily by jumping in any game and focusing on them. To follow you can either use the Arms Race gametype or view the map list below to determine where best to find the other weapons you choose to go for.
Loadout Weapons:
Lancer, Gnasher, Snub, Hammerburst, EnforcerUPDATE - Hammerburst is no longer a loadout weapon. Use this link to find the maps it will now be placed on:
https://gearsofwar.com/en-us/community/whatsup-mar1-2018Secondary Weapons:
BoltokAvalanche, Blood Drive, Canals, Checkout, Clocktower, Dam, Dawn, Drydock, Forge, Forge Blitz, Fuel Depot, Glory, Gridlock, Harbor, Hotel, Impact Dark, Lift, Lift Apex, Mercy, Old Town, Reclaimed, Reclaimed Windflare, Security, The Slab,
Markza Mk. 1Dawn, Fallout, Forge Blitz, Harbor Haze, Old Town, Relic, Security,
Retro LancerCheckout, Dam, Diner, Forge, Fuel Depot, Reclaimed, Reclaimed Windflare, Security,
OverkillAvalanche, Forge, Impact Dark, Lift, Old Town, Security,
Power Weapons:
BoomshotClocktower, Dam, Forge, Forge Blitz, Foundation, Fuel Depot, Gridlock, Glory, Harbor Haze (after fog), Hotel, Lift, Lift Apex, Old Town, The Slab,
DropshotAvalanche (after avalanche), Clocktower, Dam, Dawn, Fallout, Forge Blitz, Foundation, Glory, Harbor, Hotel, Impact Dark, Lift, Lift Apex, Old Town, The Slab,
EMBARCheckout, Dam, Diner, Fallout, Harbor, Hotel, Lift Apex, Reclaimed, Reclaimed Windflare, Relic,
LongshotAvalanche, Blood Drive, Canals, Checkout, Clocktower, Dam, Diner, Drydock, Forge Blitz, Fuel Depot, Gridlock, Harbor, Impact, Lift, Lift Apex, Old Town, Reclaimed, Reclaimed Windflare, Speyer, War Machine,
Torque BowAvalanche (after avalanche), Blood Drive, Canals, Dawn, Drydock, Fallout, Forge, Forge Blitz, Foundation, Harbor Haze, Hotel, Impact, Mercy, Old Town, Relic, War Machine,
Heavy Weapons:
TrishotAvalanche, Drydock, Lift, Mercy, Reclaimed, Reclaimed Windflare, Rustlung, Security,
RL-4 SalvoRustlung (after approx. 5 minutes), Speyer,
BuzzkillAvalanche, Blood Drive, Impact, Reclaimed, Reclaimed Windflare, Relic, Security,
MulcherCheckout, Impact, Mercy, Relic,
Notes:
- Raven Down has no weapons towards this achievement.
- Some weapons cycle between rounds.