They are only found in the levels "Burn 'em Out" and "Breaking Point". Picking one up from mortar crates results in the player's character striking the tip of the mortar shell on a hard surface, effectively priming it. The mortar round can then be thrown, resulting in an incredible explosion. They are used as a means of destroying the multiple Japanese outposts. They are extremely powerful and have a large blast radius - probably the reason they were not included in multiplayer. They are found in mortar pits. Mortar rounds can also be used to blow up buildings and can become a great advantage if the player is caught with no grenades because they are deadlier and more explosive than grenades, and are extremely effective. Some people take them with them when leaving a pit to enter the next area. They detonate when they come in contact with anything so use them carefully.
From the file zombies.str it can be ascertained that at one point, mortar rounds were available for purchase in Nacht der Untoten; where it would've cost 2000 points. It also seems to have been planned for multiplayer, as there is a file named "mortarround_mp".
The player can melee with the Mortar Round. To make it work, the player must have the Mortar in their hand and get attacked by a Banzai Charger and survive, when the player melees the crosshair disappears and it takes 3+ hits to kill.
The Painkiller Death Card works with mortar rounds.
Mortar rounds have English lettering on the side despite the fact that they are Japanese.
This is a reference to the movie Saving Private Ryan where Capt. Miller and Pvt. Ryan uses mortar rounds against a German counter attack when they run out of ammo.