Unlike in the old version of the game the achievement is cumulative in this game meaning that you can just make a kill and it will save to your safe file. This means that after completing the game you can still go back through chapter select and make a kill with a previously missed gun.
I recommend playing through the game twice to obtain all the other achievements and go back via chapter select to find the guns you missed.
Here is a list with possible locations. Note that there are many more locations where most guns can be found:
- Revolver: Given to you during Hunter
- Bastard SMG: Given to you at the Exhibition armory
- Duplet DOUBLE BARREL SHOTGUN: Given to you during Chase
- Tikhar PNEUMATIC: Can be found in Lost Tunnels and Frontline
- Shambler SEMI AUTO SHOTGUN: Can be purchased at the vendor in Market
- Helsing PNEUMATIC: Can be found in Dead City and the vendor in Armory
- Hellbreath ELECTRIC GUN: Only found at the vendors in Armory and Church
- Kalash AK: Can be acquired during from Bourbon's backpack at the start of Dry
- VSV ASSAULT RIFLE: Can be purchased at the vendor in Armory and Defense, you also get one given to you during Black Station
- Bigun SHOTGUN: Can only be purchased at the vendors in Polis and Church
- Abzats AUTO SHOTGUN: Can only be acquired at Vladimir's armory in Church for free
- Flamethrower: Obtained at the start of Dark Star, carries over to D6 if not dropped
- Kalash 2012 AK: Can be found in the Nazi base from one of the enemy (campfire near the generator where commissar standing) and at the Polis vendor and in Church
- Trench Knife: Melee weapon, you start with this
- Fragmentation Grenades: Can be found commonly from various loot sources or any vendor in the game
- Sticky Grenades: Can be found from various loot sources or any vendor in the game
- Incendiary Grenades: Can be found from various loot sources or any vendor in the game
- Throwing Knives: Can be found from various loot sources such as target boards or any vendor in the game
Credit to the Metro Wiki for possible locations!