Available after finding 10 Relics and costs $3010 to buy it. Bull - M133 with reflex sight, extended magazine and custom ( Dry Lands) paint.The bullets themselves have zero splash damage, but instead it just does a massive amount of damage, enough to set most vehicles on fire or outright blow them up in a single shot. The description states it fires explosive rounds, which is a bit misleading. Available after finding 20 Relics and costs $4800 to buy it.
They usually cost more than their base versions. Signature Weapons are special versions of existing weapons in Far Cry 3 with a unique name, upgraded specs, and even custom paint jobs.