SMG War Relics

Signature Weapons are special versions of existing weapons in with a unique name, upped specs, and even custom paint jobs:

• AMR — Z93 with extended magazine, high powered scope, explosive rounds and custom paint. Available after finding 20 Relics. (Sniper Rifle)
• Bull — M133 with Reflex Sight, extended magazine and custom paint. Available after finding 10 Relics. (Shotgun)
• Bushman — P416 with marksman scope, extended magazine, suppressor and custom paint. Available after deactivating all 18 Radio Towers. (Assault Rifle)
• Cannon — .44 Magnum with extended barrel, optical sight, 6 round cylinder and custom paint. Unlocked via Uplay. (Pistol — Revolver)
• Japanese Tanto — Custom Machete. Available after finding 6 Letters of the Lost.
• Ripper — MKG with optical scope, extended magazine, and custom paint. Available after 6 Trials of the Rakyat. (LMG)
• Shadow — 1911 with RDS, extended magazine, suppressor and custom paint. Available after liberating 17 Outposts. (Pistol — Semi-automatic)
• Shredder — Vector .45 ACP with optical scope, suppressor, extended magazine and custom paint. Available after finding 10 Memory Cards. (SMG)

There are no Signature Weapons for the Launcher and special categories.

Far Cry 3 - All Signature Weapons

Acquisition

Signature weapons are usually unlocked by completing certain objectives. After unlocking the weapon, they may be purchased from the store or any self-service weapon locker. Liberating Radio Towers cannot earn you free Signature Weapons, but liberating all 17 Radio Towers will unlock the Bushman for purchase. Check the individual weapon pages for instructions on acquiring each weapon.

Far Cry 4

Signature Weapons page as seen in Far Cry 4

• 87 — 1887 with sawed-off barrel and reflex sight. Available after collecting 4 Mohan Ghale's journal entries. This is a "must have" early game signature weapon and can even kill Rhinos with 3-4 shots. Its short range is offset by a reasonably good scatter pattern allowing multiple kills on attacking packs of wildlife once a user becomes familiar with it. (The 87 is also a sidearm so it can be used whilst driving .etc.)
• AMR — Z93 with extended magazine and advanced scope. Available after liberating 23 of the 24 outposts. As its name implies, the anti-materiel rifle is capable of punching through thick volumes of metals and will easily neutralize enemies with a single round - including heavily-armored soldiers. However this power comes at a cost of having no suppressor.
• Bull — M133 shotgun with extended magazine, reflex sight and sound suppressor. Available after completing 2 Bomb Defusing quests. Second only to the Elephant Gun in terms of performance. Where it lacks in range and accuracy it makes up for in better DPS output and larger magazine size.
• Bushman — P416 with marksman sight, extended magazine and sound suppressor. Available after reaching rank 10 in the Shanath Arena. One of the best assault class weapons in the game, plus its ammo pool is very easy to replenish, considering that the majority of human enemies in Kyrat use the same cartridge type. It can be acquired in a much less time-consuming way by using the Far Cry 4 Arena mobile app.
• Buzzsaw — MG42 with an 400-round magazine and optical sight. Available after liberating 17 bell towers. Sadly this weapon is a game breaker, as it is over-powered and is capable of killing even the heaviest foe with a burst it makes the last challenges in the game fairly mundane. Lots of fun, but best avoided if playing for the story element.
• Cannon — .44 Magnum with extended barrel and optical sight. Available after completing 5 Hostage Rescue quests.
• Stinger — Skorpion with extended magazine, optical scope and sound suppressor. Available after destroying 2 Pagan's Wrath convoys. Easily the best stealth "spray and pray" weapon in the early game.
• Rebel — A99 with extended magazine and red dot sight. Available after liberating 10 outposts. Uses pistol rounds so ammo drains quickly, even with extended magazine.
• Ripper — MKG with 200 round magazine and marksman sight. Available after conquering all 4 fortresses.
• Shredder — Vector with extended magazine, optical sight and sound suppressor. Available after reaching Karma level 6. Powerful but comparable to a fully customized BZ19 in terms of stats.
• Stormer — MP34 with extended magazine and red dot sight. Available after removing 1 propaganda poster.42:06

  Far Cry 4 - All Signature Weapons - An In-Depth Analysis (ish)

• Trooper — MS16 with extended magazine, marksman sight and sound suppressor. Available after removing 20 Masks of Yalung. Surprisingly, it has no stat boosts, and is only remarkable because it has a marksman scope, which the normal MS16 cannot equip. Available after collecting 20 masks of Yalung.
• Warrior — AK-47 with an extended magazine, red dot sight and sound suppressor. Available after hijacking 2 Royal cargo trucks. A good weapon for AK fans but generally inferior to other assault rifles in game.
• Harpoon gun Also called the Impaler, it's a joke-like but powerful special weapon that shoots harpoons, which kill enemies in one hit. Available with "Hurk's Redemption" DLC.