The Kimber Warrior appears in-game as the "1911". It incorrectly holds 10 rounds in its magazine; its real-world counterpart only holds 8 rounds in the magazine.
SIG-Sauer P250 Compact
The SIG-Sauer P250 Compact appears in-game as the "P-9mm". Just like in the first game, it is incorrectly described as being Austrian, and holds 17 rounds in its magazine. It is erroneously the standard-issue duty pistol of the San Francisco Police Department and the Oakland Police Department, the former issues 40.S&W SIG-Sauer P226 and P229 while the former uses the Glock 17.
The PP-2000 appears in-game as the "R-2000". Its stock has been completely removed, rather than folded to the side.
Shotguns
AA-12
The AA-12 appears in-game as the "ATSG-12". It is used by both the SFPD and the Oakland PD.
Mossberg 500 Cruiser
The Mossberg 500 Cruiser appears in-game as the "SG-90". It erroneously holds 8 rounds in its 5-round tube.
Saiga 12K
The Saiga 12K appears in-game as the "SGR-12". It has a custom buttstock, pistol grip, and railed handguard. It is used by SFPD SWAT, which isn't actually outside the realm of possibility as some American law enforcement agencies do have Saiga and VEPR shotguns in their inventories.
Assault Rifles
AK-103
The custom AK-103 from Far Cry 3 & Far Cry 4, as well as the previous game, returns as the "AK-47".
Galil ACE 23
The Galil ACE 23 appears in-game as the "AC-AR". It is equipped with a scope and is classified as a sniper rifle.
"OCP-11"
The fictional "OCP-11" returns from the original Watch Dogs, once again as the cheapest assault rifle in-game.
Patriot Ordnance Factory P416
The Patriot Ordnance Factory P416 appears in-game as the "Goblin". It uses the same model from the previous game, which itself originates from Far Cry 3, despite its name referring to its depiction in Ghost Recon: Future Soldier. It is used as standard-issue rifle of the SFPD SWAT, a somewhat odd choice outside of the world of Ubisoft-isms.
Machine Guns
Ultimax 100 Mark 3
The Ultimax 100 Mark 3 appears in-game as the "U100". The Ultimax is also used by SWAT, a rather questionable choice.
Sniper Rifles
SVD Dragunov
The same SVD Dragunov from the first game reappears as the "SVD". A full description of its attributes can be found on the Watch_Dogs page.
Fictional 3D-printed guns
In addition to normal firearms, Watch_Dogs 2 features 3D-printed guns. While this is possible in real life, real home-made 3D-printed guns are quite fragile; thus, they are restricted to very simple designs, or require incredibly expensive equipment such as laser-sintered metal deposition machines that cost a minimum of half a million dollars, vastly more than the traditional tools needed to stamp or machine firearm parts. In essence this game uses 3D printers as if they are Star Trek replicators, much as "nanotechnology" was used in older near-future sci-fi (and before that "robots" and then the more traditional "magic"). The 3D-printed guns in-game are more or less just normal firearms.
Handguns
"4N00bs Pistol"
The "4N00bs Pistol" is based off of the Kimber Warrior, and is in fact just a slightly modified model of the game's "1911".
"Core Dump Pistol"
The "Core Dump Pistol" is nearly identical to the "4N00bs Pistol", but with an added suppressor and scope.
"Help Desk Denial Pistol"
The "Help Desk Denial Pistol" once again is a modified version of the game's "1911" model, albeit with a larger slide and extended magazine.
Assault Rifles
"DOT_EXE Rifle"
The "DOT_EXE Rifle" is heavily based off of the UMP45.
"HHOS Rifle"
"DOT_FILE Rifle"
"Zero Day Rifle"
Shotguns
All of the 3D Printed shotguns in Watch_Dogs 2 are based primarily off of the Benelli M4 Super 90.
"Bullet Hell Shotgun"
The "Bullet Hell Shotgun" is based off the the Benelli M4 Super 90, with a skeletonized stock and pistol grip, and with a handguard resembling that of a typical pump-action shotgun's pump.
"DDoS Shotgun"
The "DDoS Shotgun is also based off the Benelli M4 Super 90, with what appears to be a skeletonized collapsible stock and pistol grip, railed handguard, and is fitted with a box magazine.
"Goodbye, World Shotgun"
The "Goodbye, World Shotgun" is once again based off of the Benelli M4 Super 90, this time with a pistol-grip only stock, and using a drum magazine. The handguard appears to be based off of the heatshield on the Franchi SPAS-12
Sniper Rifles
As an interesting note, all 3D-Printed sniper rifles in Watch Dogs 2 are covered in spikes, most notably on the cheekrest. This would obviously prevent the cheekrest from being used safely--the recoil from firing would potentially cause severe trauma. However, the spikes do not have any impact on gameplay, and were likely added to make the 3D-printed snipers more visually distinct.