The Remington New Model Army is available from the mystery box in the Zombies map Buried. It is functionally similar to the Python, but with the ability to reload all six rounds at once, and having higher chest and head multipliers when Pack-a-Punched.
When stored in the Weapon Locker in Buried, it becomes a Python (or a Cobra, if it is Pack-a-Punched) in TranZit and Die Rise. If a Python is placed in the Weapon Locker and taken out in Buried, the reverse occurs.
When Pack-a-Punched, it becomes the Sassafras and is effective until rounds in the 40s. The cylinder size is doubled to twelve rounds and reserve ammunition is increased.
The Remington New Model Army is also seen in the hands of Tank Dempsey during the introduction scene on the map Origins, despite not being available to players on that map.
Remington New Model Army vs Sassafras
Remington New Model Army
Rate of fire
Double magazine capacity, more ammo, higher damage at long range
Tank Dempsey using the Remington New Model Army in the intro scene of Origins
Call of Duty: Black Ops III
The Remington New Model Army is used by Dempsey in the introductory cutscene of the remastered map Origins, from the Zombies Chronicles DLC. However, the weapon isn't available in the game at all.
Call of Duty: Black Ops 4
A promotional artwork shows Scarlett holding a Remington New Model Army, along with another revolver. A Remington New Model Army can also be seen on the wall in the multiplayer maps Hacienda (and Hacienda Twilight).
Scarlett with a Remington New Model Army in her left hand
Call of Duty: Black Ops II
"Sassafras" refers to a family of trees native to eastern North America and eastern Asia which was used to make root beer, a drink associated with the Old West.
The reload animation is actually the player field stripping the weapon and replacing the empty cylinder.