Resident Evil

Based on the link below, these are all the guns required for the achievement. 

https://www.residentevil.net/one/en/...0.html?ref=one...

1. Handgun
2. Shotgun
3. Assault Shotgun
4. Grenade Launcher
   • Explosive Rounds (loaded into GL)
   • Acid Rounds (loaded into GL)
   • Incendiary Rounds (loaded into GL)
5. Self-Defense Gun
6. Magnum Revolver
7. Barry's 44. Magnum
8. Flamethrower
9. Rocket Launcher (Endgame)
10. Rocket Launcher (Infinite Ammo, 3-hour run Normal+)
11. Samurai Edge (Infinite Ammo, 5-hour run Normal+)

Here are detailed descriptions of each weapon's location and how to obtain them.

Handgun - Immediately in Jill's Inventory, picked up in Main Hall by Chris after returning from the dining room.

Shotgun - Located on the East Wing of the first floor in the Mansion. On the north-east area, there is a corridor that leads to a bathroom, the Ceiling-Trap Room (and Shotgun room) and entrances to the lobby outside Spencer's office as well as the Cerberus Corridor (and a door leading outside to pick up the Chemical). 
In order to obtain the Shotgun, you need to have the Broken Shotgun which is located on the West Wing of the first floor near the save room in this area. In order to get into that room, you simply need to unlock it with your lockpick (Jill) or use a Small Key (Chris), with one being easily found in the bathroom on the East Wing. 
Swap the broken shotgun for the working one to obtain the weapon.

Note: For Jill, you can avoid going for the broken shotgun if you access this room BEFORE you encounter Barry in the main hall for the Acid Rounds. To do this easiest, obtain the sword key and immediately head back to the main hall, going through the door on the right of the first floor bringing you into the gallery. Run to the door leading to the Cerberus Corridor and make your way to the shotgun room (because you entered from this direction, the Cerberus' will NOT break through the windows at all this run through). After you grab the shotgun, head back into the Ceiling Trap Room and try the door leading out to the hallway. Turn around and try the door back to the shotgun room. If both are confirmed locked, the scene will start. If you head back inside, then you cannot get the shotgun this way (due to either having accessed the Acid Rounds scene or something else). You can still get the acid round scene, exiting back to the Main Hall from either the first or second floor on this wing of the mansion. Warning: You will not be able to return to the shotgun room at all using this method, so if you want the Ink Ribbons and Dagger make sure to grab them while you're there.

Grenade Launcher (JILL ONLY!) - After obtaining the armor Key, return to the Main Hall's second floor. The door on the bottom right of the map leads outside to a balcony where you will discover the body of Forest Speyer, with the Grenade Launcher perched next to him. It is possible (I forget the exact requirements) that Barry will be here, handing you the grenade launcher anyways. The only benefit to this other than an optional scene is that if you head off to pick up the green herbs on the far right walkway, Forest will not get up to attack you. Anyways, you can just grab the grenade launcher and leave without any fight if you ignore the right walkway.

Grenade Launcher Ammo Types (all must be loaded into the launcher):

Explosive Rounds (Grenade Launcher) - Already inside of the grenade launcher. You'll likely come across more, so whether or not loading more rounds into the thing is "required" you'll probably be changing ammo later anyways.

Acid Rounds (Grenade Launcher) - Found in a few locations throughout the game, lesser on higher difficulties. Always guaranteed from Barry if you get the "Can of Fizz" scene in the Main Hall. All this requires is grabbing the sword key, going down to where the Cerberus Hallway is and unlocking the door. You can then just turn around and head back out for the scene. Alternatively, you can still get the "Jill Sandwich" (technically, "Fit nicely into a sandwich") scene for the shotgun and exit back into the main hall. The scene will also occur on the second floor, though slightly different if you exit from up there (he'll point his gun at you and ease back).

Incendiary Rounds (Grenade Launcher) - Earliest place I can think of is a "drop" by Barry in one of the save rooms as you progress through the Mansion. The save room in question is on the East Wing, next to the door with the weakened handle. You'll know when Barry's made the drop as a piece of paper is outside stuck to the wall. Some other goodies are included, depending on the difficulty, you will discover more of this ammo later too.

Assault Shotgun - REQUIRES THAT YOU SAVE RICHARD WITH THE SERUM.

• Jill - After saving Richard, he will come to aide you in the first fight with Yawn. You need to help him damage the snake enough that it fakes a death...then the shotgun will be available after the scene (pick it up on the ground).
• Chris - After arriving in the Aqua Ring below the Residence, you will find Richard again. After you drain the water for the tank, the shotgun is located down where the Mother Neptune is situated (you'll spot it on the ground near one of the smaller Neptunes). It's right in your main path.

Magnum Revolver - In order to obtain this weapon, you need to enter a room and solve a puzzle on the 2nd Floor East Wing for the Mansion. The room in question is located on the north-east corridor above the save room here, with a couple zombies hanging about depending on the difficulty. You need the armor key to unlock the door. After entering, one of the doors leads to a room featuring a lot of different trophies and a fish tank. Examine the area to find a Fish Hook, Golden Bee and an actual Bee itself. Combine the fish-hook with the Golden Bee and place it with the other hooks, then place the actual Bee on the Insect Collection. Hit the button and the collection will slide up, revealing the Windcrest. The bee will come alive but is easily ignored, grab the medal and exit the room.

Now, you will need this medal when you are heading towards Lisa Trevor's cabin and you pass through a cemetery. On your right is a path leading to two large headstones, one of which has a slot for the Windcrest. Place it, then the thing will rotate revealing three more crests (all these items were used instead of the Death Masks in the original RE on PS1). Examine the backs of each and place each in the other headstone to reveal the Magnum. 

Note: This looks exactly like Barry's 44. magnum, insure that you examine the name later on if you have both guns as one is infinitely more powerful than the other.

Self-Defense Gun - A small Derringer pistol that fires .22 Magnum Bullets. However, does not take ammo that's used for the .347 (or magnum revolver, if you want to go by game-name). It's located in the Residence, in the room with the hanging body (001, first room on the left upon entering). You'll need a key from room 002's bathroom in order to enter it. It's located on the desk in 001, accompanied by a suicide note.

This gun is relatively strong, it's wise to save it for fighting the Tyrant later. Though with just one shot it will take up some space.

Flamethrower (CHRIS ONLY!) - Found in the caves on a latch before fighting the giant spider Black Tiger. You can use the weapon against the spiders and even the webs, but afterwards you need to put it on another slot to open the next door. Can't keep it and can't reload. 

Note: A broken one can be found by Jill but does not count as a weapon, only a key item.

Barry's .44 Magnum (JILL ONLY!) - Best gun in the game. This gun unfortunately requires you to not give it back to Barry during the final Lisa fight. Note that if you don't give it back, you won't get the "Trust Him" and "I'm a Member of S.T.A.R.S" achievement so if you have them already great if not, either load a previous save or get it next playthrough. 

The gun will NOT be in your inventory it will be laying on the ground. I forgot about this myself during my speed-run and ran all the way to the labs before realizing it wasn't in my inventory had to run back (thankfully this isn't an issue like it was on the Gamecube, as there's no disc swap). 

Rocket Launcher (Endgame) - Found in endings where the Self-Destruct System is active, you'll fire a signal flare for Brad to pick you up but the Tyrant will emerge from the roof. You need to damage him enough until Brad throws down the launcher, which is located near the center of the helipad. Pick it up and fire at the Tyrant, sometimes he can destroy a rocket or two but he will die by the last rocket.

Rocket Launcher (Infinite Ammo) - This requires you to complete the game on Normal or Higher in under three hours. Check out my guide below for tips in what to do. I'm no expert speed-runner and I managed it, you can too!

Samurai Edge - I don't see it on ResidentEvil.net, unless it's the "handgun" image they have but if all else fails you'll be unlocking this with the Infinite Rocket Launcher anyways. As it requires you to complete the game on Normal or higher in five hours or less. So, two birds with one stone!