Here's the weapons list from the old forum. This shows weapons found in multiple locations so you can choose however you wish to find them, I've also organized by mission instead of by gun title. If you know of any more to be found, just reply and
I'll try to add them.
Death of a Showman
Dragunov - Used for sniping exercise, drop in ICA crate to keep (see below).
Desert Eagle - From the man dressed in purple, found at the end of level.
A Vintage Year
Snub Nosed - Retrieve it from Don Fernando Delgado's body.
TMP - Retrieve it from Manuel Delgado's body.
SAF SMG - Carried by VIP guards, and can be picked up near sleeping guard, which is on the ground floor of the main building.
Shotgun - Can be picked up near sleeping guard, which is on the ground floor of the main building.
SLP .40 Pistol - From cops, very common.
MP5 - Carried by guards in black, and sitting guard who goes to the bathroom alot.
A New Life
SLP.40 Pistol - From the FBI agents you sedate in the van.
Bull.480 - Retrieve it from Vinnie Sinistra's body.
MP7 - Carried by FBI, and found in a crate.
Air Rifle - In the treehouse, which is in the garden to the right of the house with the person you are contracted to kill.
Murder of Crows
SG552 - Next to Mark Purayah II's desk.
Kazo TRG - Will be wherever Raymond Kulinsky (aka, Sniper Crow) spawns, propped up against a wall.
Desert Eagle - Retrieve from Mark Purayah II's body.
SLP .40 S - Retrieve from Angelina Mason's body.
MP9 - Carried by yellow birds, and found in a crate on the ground floor of Mark Purayah II's hideout.
Death on the Mississippi
Bull .480 - Carried by most of the gator gang.
FN-2000 - 2nd Deck Staff Quarters, in a room which should be marked on your map as a "Point Of Interest".
Till Death do us Part
Shotgun - Common, one by the starting point.
Six Shooter - Carried by most, retrieve it from the drunkard at the beginning of the level.
Elephant Rifle - Second floor, wall mounted.
House of Cards
SLP .40 S - Found on Henrick's body
TMP - Found in the back room behind the Sheik's location or on his bodyguards.
A Dance With The Devil
Dragunov - Next to the laptop you need to hack, it should be in a case on the right hand side of the desk.
FN-2000 - Found in the case the CIA target is carrying.
MP7 - Found on the table when you first enter the torture chamber.
MP5 - West Wing 1st Floor, the guards here seem to carry it.
M14 - Right at the beginning of the level, you should meet a lot of marines carrying this gun.
Common questions about weapon collecting:
Do I have to collect all the weapons on this list?
Not at all, just one of each type. This list shows where you can find them in multiple locations. If you find a certain weapon type, you can cross it off the list entirely. Just remember that you have to complete the mission for it to count.
How do I save weapons in "Death of a Showman"?
There are two ICA boxes located at the end of the level. You have to place weapons in the ICA box where you collect your mine and detonator. If you drop them in the first ICA box (below the stairs, with syringe in it), you don't get the weapons in your Hideout.
What weapons are not collectible?
All melee weapons will not be displayed in Hideout. In addition, the Custom 1911, the Nailgun, and the Real WWI Pistol from Curtains Down all do not display, and do not count towards the achievement. However, you may collect and use them in missions.
Credit: Banstyle, Tnn Ace03