The 'Moss-12 is a standard shotgun featured in Uncharted: Drake's Fortune and Uncharted 2: Among Thieves. It also features in Uncharted: Golden Abyss. The Moss is lastly seen in Uncharted 4: A Thief's Ends opening credits and in the Epilogue photograph.
The term "Moss-12" appears to be a shortened version of the shotgun's full name, Mossberg Maverick. The 12 stands for "12-gauge".
The Moss-12 is extremely effective over short ranges, making it a good choice for close quarters combat. It is very powerful, and can kill most enemies with one shot. It is commonly found earlier on in Uncharted games as the first shotgun and is often wielded by larger enemies. Its accuracy when hip-firing or blind-firing is very high, at close range there is next to no chance of a miss.
The Moss-12 is replaced in Uncharted 3: Drake's Deception by the Sawed Off Shotgun, which now has at least two superior aspects to the SAS-12, rate of fire and accuracy.
In Uncharted 4: A Thief's End, the Moss-12 appears in the opening credits used by Nathan Drake during his adventure for the El Dorado statue. In the Epilogue, Cassie Drake finds her grandmother's book with a picture of Nate, Elena, and Sully with the El Dorado treasure. Nathan can be seen wielding the Moss-12 in the picture.