Gears 5

Set-up
In Horde play as Jack, get 10K fabricator points then buy a forge from the fabricator.
How to
Hold to pick up a weapon while hovering over it.
Then when hovering over the forge and hold to melt the weapon.

Grenades and Pistols are not required and cannot be melted.
This can be completed accross multiple Horde games.

What you need to melt is:
-Boomshot - - - - (J.D.)
-Breaker Mace - - Only dropped by a Warden Boss.
-Buzzkill - - - - - - Only dropped by Scions
-Claw Light Machine Gun - - - Only dropped by Elite Drones
-Cryo Cannon - - - - - -Only dropped by Ice Scions
-Dropshot - - - - - - - - Only dropped by Scions
-Embar - - - - - - - - - - (P-Fahz)
-Enforcer - - - - - - - - -(Del)
-Gnasher Shotgun - - - (Marcus, JD, Kait)
-Hammerburst - - - - - -Dropped by Drones
-Lancer with chainsaw - Only dropped by allies. (Marcus)
-Lancer with Grenade Launcher - Only dropped by allies. (P-J.D.)
-Retro Lancer - - Only dropped by allies. (Kait, P-Marcus)
-Longshot - - - - (Fahz)
-Markza MK1 - - (Fahz)
-Mulcher - - - - -Only dropped by Scions
-Overkill - - - - - (Del, P-Kait)
-RL-4 Salvo - - - Dropped by Guardians and DR-1's
-Torque Bow - - -Only dropped by Hunter Elites
-Tri shot - - - - - -Dropped by a Stump mini-boss, Guardians and DR-1's

Notes:
"(Named Charater)" means they start with the weapon
A name with "P-" before it means they can purchase the weapon from the Fabricator

You cannot remove a weapon from a Guardian corpse as Jack so you would need your allies help if you were to go for their salvo/trishot.

Mini-Bosses only spawn in at waves 5,15,25,35,45.
Bosses only spawn in at waves 10,20,30,40,50.

Only dropped by allies:
The easiest way to get the Lancers done is to Play split screen or with a freind in a private game. Set the wave to 41 so you have enough scrap immediatly to make a forge before the wave starts and have your human drop a weapon, by holding his right or left D-Pad button for 7 seconds.

Struggling to find:
If you're struggling to find the boss you want, I recommend you start your own private game on wave 10 with in split screen or with a friend as a normal character and you as jack with the Ai turned off and keep getting killed until you get the boss you want then use the human player to slaughter the desired boss. (the same can be done on wave 5 if you want a miniboss weapon)

Waves 6 -10 (16-20 etc) tend to either be DR1's Gaurdians snipers and all other bot enemies with very few swarm enemies if any at all or Scions, Hunter Elites and only swarm allies, it tends to stay static for that 10 wave set unless you lose a round. if you're struggling to find specific swarm do the above solutions set up but be on wave 9 (19,29 etc) for the best results for specific swarm/bot weapons. it is random wether it changes over or not so this may require multiple round losts.

This achievement is much easier as of the Operation 4 update on July 14th, 2020 as all characters are now able able to purchase all weapons and the heavy weapons that were previously only dropped by bosses can now also be purchased.