The mortar had an early form of Krupp horizontal sliding-block breech and it fired separate-loading, bagged charges and projectiles.
[1] In the coastal defense role, the M1877's theory of operation was that a low-velocity mortar firing a large shell at a high-angle was more likely to destroy an enemy ship by penetrating its thin deck armor than a high-velocity low-angle naval gun attempting to penetrate its thicker belt armor.
The downside was that high-angle indirect fire was harder to aim correctly so more mortars would be needed to defend an area from attack.
However, if the area was constrained by geography like a port at the mouth of a river the navigation channels could be measured ahead of time and firing ranges calculated.
The mount consisted of a rectangular steel firing platform with a pivot at the front and two wheels at the rear to give 360° of traverse.