The Overseer's Guardian is sold as either a short or long-barreled combat rifle. The type of barrel is apparently randomly generated. This weapon comes guaranteed with the Two shot effect, which shoots an additional projectile with each shot. Furthermore, these additional projectiles do not affect the player character’s ammunition count.
This gun can be acquired very early in the game and does not have any requirements other than its price. Its double-shot mechanic and high base damage makes it one of the most powerful guns during the entirety of the game, with its ammo being relatively common (.45 round) and its consumption being relatively small in comparison.
If combined with Rifleman and the .308 Receiver, the Overseer's Guardian excels in medium ranged engagements, almost certainly crippling weaker enemies' limbs in one hit.
When used as a fully automatic rifle, the weapon is best used as a close-quarters last resort gun, as it can still deal a relatively high amount of damage up close while becoming somewhat outgunned by automatic energy weapons later on.
In V.A.T.S. or when triggering a critical, two distinct bullets will be seen with a very small amount of spread, like Charon's shotgun in Fallout 3.
Fully upgraded it can be as heavy as an unmodified minigun.