Full Arsenal Notes
Army of Two: The 40th Day Walkthrough and Guide
by CMBF 
Full Arsenal Notes
The Full Arsenal Achievement requires you to obtain all of the weapons in the game. There are a total of 20 weapon parts and 7 weapons to collect - and of course the weapons you need to purchase.
You accomplish this by doing ALL of the following:
(1) Play through the entire campaign making only positive morality choices.
(2) Play through the entire campaign making only negative morality choices.
(3) Save all of the hostages.
(4) Find all 17 weapon parts.
(5) Purchase all of the weapons.
(6) Purchase all of the weapon attachments (not including styles).
Note that you are going to need roughly 2.5 million dollars to make all of these purchases.
Note that there are two weapons that can only be unlocked if you have a save file on your system from the original Army of Two game... The good news is that the achievement Full Arsenal does not actually require those two weapons - but it is nice to have them anyway. If you are curious, the two weapons you get in this fashion are the AS-KRI and the S8-Z.
Also note that there is a known bug in the game that effects the two bonus weapons - no information is available on why this happens, but on some XBox 360 consoles when the game detects the save from the original Army of Two game, it unlocks the two bonus weapons, but when you quit the game and return to it, they are locked again and cannot be purchased. It is presumed that there will be a fix for this in the upcoming patch but there has been no official announcement to that effect.
Full Arsenal Trophy / Achievement
To unlock this you need to collect every single weapon (and weapon part). This includes all primary, secondary and special weapons.
Some of these will be found in enemy supply crates (one or two per chapter). The rest have to be purchased by you. The attachments that you need to get for this trophy are barrels, cartridges, suppressors, metal plates, and stocks.
You do NOT need to purchase the purely decorative stuff - meaning the camouflages or display styles - for this.
That said, the actual amount of money you need to purchase everything required in order to unlock this is the obscene amount of just over $1.8 million - a goal that most players found way WAY over the top - which is why most players do not have this one unlocked.
If you decide you have to have this, well, good on you mate! Good on you!
requirements for the full arsenal trophy are overboard , as it takes an age just to find and buy everything anyway
I've unlocked everything except the AS-KRI and S8-Z. I've gotten all 15 weapon blueprints and I've bought everything. How come the trophy hasn't popped?