Army of Two: The 40th Day

Once you've unlocked all weapons and collected the necessary parts from your two playthroughs, you may still need to grind out some cash to buy them all.

Load up a new campaign starting at the Chapter 4 section titled "Don't Die In A Hospital", on Casual difficulty. From the start of the section save the hostages, follow the GPS downstairs and turn on the generator, and you should be at a checkpoint. Now quit out and repeat. This method should net you $30,000 a pop.

This is the simplest method I've come across so far courtesy of my buddy. If I find a better method I will update this solution.

Also if you buy the two guns you can purchase from playing the first AO2 (which aren't necessary for this achievement) early on in the game, you'll have to grind out a few hundred thousand more dollars.

!Here is a video for this achievement! keep in mind that this video only shows the hidden weapon parts (thx to 'Fundy1842' for that information).. Make sure to get to a checkpoint before quiting out and going to the next mission (thx to 'Salvator 2' for that information)!
('I do not own nor take credit for this video')
(I give full credit to 'Roosterteeth' for making this video)

Army of Two-The 40th Day
Weapons & Parts

You must play through the game twice,once all positive and once all negative(morale choices,and note that you must buy every item not(excluding the two guns from army of two save data,and the camo's)

MISSION 1
G14 Thunderstrike Barrel in the tutorial strongbox.
Slanted G5 Grip on the square catwalk above the first mounted gun turret in the construction yard.
Custom Shield past gate control.its practically next to radio 3 in this mission

MISSION 2
HM Shotgun and M416-this is from the morale choice
P90 from first group of hostages.
Sodacan Silencer on the yellow seats right after the first hole-before co-op mock surrender tutorial

MISSION 3(kill the tiger for some weapon parts as well)
DE .44 Handgun from rescuing the second set of hostages.
FR Elite Model 3 Sniper Buttstock, V17 Bayonet, for morale choice
Chepznaz Redstar Part in the tram control room-after hippo pit
PCG Delta EX (6X) at the enemy strongbox where chopper crash site is.
Pipe Bomb launcher on top of the cage across from the zoo-worker hostage. climb the ladder and grab this one.
MISSION 4
7337 Defender shotgun barrel after the first group of hostages but before activating the comms array on the roof. From the comm-room door, go down the stairs and head straight to a tenament with two windows, a lantern, and a crate on a table.
G18 will be unlocked at the hospital weapon room only if you made all "good" morality choices..
M636 Scope (2X) Rifle scope from the first hostage at the hospital..
HL Reflex Core Stock is found in the U-shaped hallway before meeting the kid-this is after starting the generator.check all blue doors.its in a closet.
Barett .50 Cal Sniper-let the kid try and get the gun for you.
D56 Suppressor Part from saving the hostages in the lecture hall-after meeting the kid.
T6-400 Shield Part at end of the hallway after the two Flamethrower Super-Heavies.

MISSION 5
TruShot Mk.1 barrel in the APC alley (where you start). Clear the zone of enemies-Flamer heavy & shotgun heavy-as well as enemies. Locate a blue truck cab at the end of the alley.A large trash bin near this blue truck cab has the item next to it.
Grenade Launcher-from the hostage at the construction yard.
Kitchen Knife is found in comm array tower. When you're about to place the bombs you can get this part. Face the central tower-the bombs behind you-and circle the floor to your right. There is a small boutique with the weapon behind the shop counter.

MISSION 6
Red Dot Sight-3X Scope in the first pier. There is a circular plaza (with columns) at the first pier. This is just before the checkpoint mid-way through. You want to check the tunnel under the plaza-the placard is in the middle of the hallway.

MISSION 7
J37 Defender Mk4 Stock Shotgun Part can be found in the little room after opening the first gate at the outer wall. The storage room has a green sign with Chinese calligraphy above it.

Also missing is from Mission 2
+ E2 Muzzle Enhancer on other side of the skyscraper fight (where Rios and Salem fight downstairs while separated after saving or killing two hostages).On the third level down (the floor above the gun turret) check the dark corners behind a counter to find this part (it'll be by two elevators).

Be aware regarding the G18C pistol which has to be unlocked too.

Chapter 4:

G18 will be unlocked at the hospital weapon room, which the doctor will open, only if you made all "good" morality choices in the first three stages.

In other words:

Chapter 1....spare JB

Chapter 2....return the guns

Chapter 3...spare the tiger

This will trigger the G18C to become available in Chapter 4.

I have read elsewhere that you only need to make 2 out of 3 good moral choices but I would personally do all 3 good just incase the 2/3 doesn't work.

I can confirm that doing the 3 good moral choices unlocks the G18C pistol as just unlocked the achievement. On Chapter 4 Mission 'Don't die in the hospital' after the Doc asks for your help and you drop down to another floor below, the weapon room will be open right across from you. You also get a message on-screen and the doc will also mentioned it.

Mission 002: In Case of Fire:Z1 Muzzle Reducer on a desk in the room with the second PMC strongbox (this
is after descending the skyscraper on separate routes but before the moral
choice at the consulate's weapons locker).

Mission 003: The Zoo:Chepznaz Redstar Part (Sniper rifle barrel) in the tram control room (the
one after the hippo pit).

Mission 004: Hospital:7337 Defender shotgun barrel after the first group of hostages but before
activating the comms array on the roof. From the comm-room door, go down the
stairs and head straight to a tenament with two windows, a lantern, and a crate
on a table. The placard is to the the table's left.

Mission 007: Chinese Temple:J37 Defender Mk4 Stock Shotgun Part can be found in the little room after
opening the first gate at the outer wall. The storage room has a green sign w/
Chinese calligraphy above it.

these are the missing weapon parts in 40th day if you dont believe me you try it out if you do not have these ones cause they are not in the guide

Do Not buy:
1. Primary - AS-KRI
2. Special - S-8Z "Grand Pinger"