You must be in possession of all types of weapons in the game either in a saved file or in a current game. The achievement will pop when you take the final one.
All play-throughs after your first should be started using "Once Again". If for some reason you beat the game but are not seeing "Once Again," load a cleared game file and it will appear again. However, people are claiming the game records progress across multiple files. For simplicity's sake, I suggest a single file but no need to panic and redo it if you have two files either.
If you have any specific questions about where these can be obtained check this great map out. It is a true marvel for any RE fan. Otherwise, most of these should come pretty easy.
http://www.evilresource.com/resident-evil-remake/maps/mansio...You must save your game at some point after finding them. This is true except for the last one you find. Check progress at:
https://www.residentevil.net/. For example, if you find the .44 as Jill but die afterwords, and then you come back and do not pick it up again to complete the game, it will not count.
1.1 Shotgun- Found downstairs mansion east wing.
1.2 Broken shotgun- This is most likely not needed for the achievement but as Chris it is definitely needed to get the Shotgun. It is mentioned here only to distinguish between the two different items.
2. Grenade Launcher - Jill only. Found upstairs in the mansion. Combine all three types; grenade acid, flame, grenade. The acid and flame rounds are given to you by Barry or found throughout the campaign.
3. Richard's Combat Shotgun
4. .357 Magnum
5. 22 Derringer (Self-defense/suicide pistol ) SUPER POWERFUL: worth 5 magnum rounds but only one shot. Found in the residence area inside the room 001 with the hanging man.
6.1 Flame Thrower - Chris only. This is found in the courtyard basement before fighting a certain 8 legged, large boss. Check the walls of the previous cave for one. You can only use it in this area because removing it from the wall also locks the door behind you.
6.2 Broken flamethrower- this is story related and probably not needed for this achievement but it is mentioned only to distinguish the two as different items.
7. .44 Magnum
8. Mark 1 Rocket Launcher
*** Spoiler - click to reveal ***You must save Barry as Jill or save Rebecca as Chris for it to appear. For Jill/Barry, give him his gun back and keep him alive in the Lisa fight.
For Rebecca/Chris you must save her from the hunter. This will happen after you are done with the plant and back in the mansion. When Chris enters a cut-scene and hears her scream, run fast upstairs to the room where you found the dog whistle. The room you are going for is a small library.
The rocket launcher will spawn in the middle of the final boss fight.
BONUS WEAPONS - needed
1. Samurai Edge (Infinite Ammo) Beat the game in under 5 hours on normal or hard
2. Rocket Launcher (Infinite Ammo) Beat the game in under 3 hours on normal or hard (both weapons will unlock)
SELF DEFENSE WEAPONS- not needed
1. Flash Bang Grenade (Chris only)
2. Dagger
3. Stun Gun (Jill only)
You can check the progress of this by syncing your account with residentevil.net. The weird thing about their count is that the Samurai pistol does not count. I guess technically it is only a modification of your pistol. In any case, both of the rocket launchers are listed. The broken items are not listed nor are the defense items. I listed those only for the purpose of completion. They are easy to find and are probably not needed for the achievement but if for some reason it does not unlock, make sure you have the broken shotgun and have collected the defense weapons.