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.
The SVD-M Dragunov appears in-game simply as the "SVD". As in the first game, the model has the same shortened barrel as the SVD from Far Cry 3, but it has the correct stock for an SVD-M and is now correctly modeled as a right-handed weapon. Presumably to use the same hand positions as other rifles, the SVD's receiver is shortened to the point the trigger guard is connected to the magazine release as on an FPK / PSL Sniper Rifle; it appears the receiver is a hybrid of the ones from the Far Cry 3 SVD and the Ghost Recon: Future Soldier PSL-54C.
The scope reticle is from a Belarusian Zenit 8x42 POSP scope, with civilian 1.5m and 0.5m stadiametric rangefinder lines intended for hunting elk and deer instead of the single 1.7m human line of the PSO-1 and military POSP.
Fictional weapons, printed on a 3D Printer.
This game is notable for being the first to feature 3D-printed guns. While this is possible in real life, real 3D-printed guns are quite fragile; thus, they are restricted to very simple designs. The 3D-printed guns in-game are notably more advanced, although they are obviously modeled after real 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.