Necromunda: Hired Gun

For this, you simply need to kill an enemy with each weapon available in the game. More weapon types become available as loot and in the shop as you progress through the game and rarity/mods are irrelevant. The full list of weapons includes:

Pistols:
IRONFIST (stubgun)
ESCHER'S KISS (autopistol)
EPITAPH (sawn off shotgun)
SCARS MACHINA (bolt pistol)
FIREBIRD (plasma pistol)

Basic:
STRIKER (autogun)
VPANTHERS (autogun)
M'MA (stub cannon)
DEATH BRINGER (bolter)
DOGHAMMER (combat shotgun)
PIOUS LEGACY (shotgun)

Special:
HASHIN'S EYE (long las)
BURNINGSUN (plasma gun)
WRATH (gravgun)
FUNERAL BALL (grenade launcher)

Heavy:
IRON RAIN (heavy stubber)
TEMPEST (heavy stubber)
SCATTER (heavy bolter)