The Mk 2 Mod 1 81-mm mortar was an unconventional weapon for an unconventional war. The unusual system combined two different weapons, both well known to the U.S. Marine Corps and the Army: the 81-mm mortar and the ubiquitous Browning M2 .50-caliber heavy-barrel (HB) machine gun. The Navy and Coast Guard also used the M2, but not the mortar—except for some on PT boats during World War II—yet these two services became the primary users of the combined system. What eventually became a very effective and widely used waterborne weapon had its genesis in a non-combat role: Rescue at sea.
In the early 1960s, the Bureau of Ordnance at Naval Weapons Station, Crane, Indiana, looked to the M1 81-mm mortar, used by the Army and Marine Corps, as a starting point for a weapon to provide not only high-angle but also direct fire for smaller vessels. The resulting Mk 2 Mod 0 81-mm mortar is unique among mortars in that it is flexible—it can freely move in azimuth and elevation. The muzzle-loaded weapon also has the option of traditional mortar drop fire—a round is dropped into the muzzle and is fired by a fixed pin at the weapon’s base—or trigger fire, similar to an overly large handgun. For drop fire, the elevation must be set between 35 and 71.5 degrees. In trigger-fire mode, it must be loaded at an angle between 30 and 35 degrees.
This sea-going mortar also is distinct from its land-based cousin in having a counter-recoil mechanism, a large cylinder above the tube of nearly the same size. The mount is a substantial fixed tripod fitted to a reinforcing ring base attached to a boat’s or ship’s deck. The mount uses clamps to control traverse and elevation. Unless mechanical “no fire zone” stops are used, the weapon can swing horizontally through 360 degrees and between -30 and +71.5 degrees of elevation and is easily controlled by one person. A basket construction at the breech protects the gunner from the recoil. The mortar’s rate of fire is 18 and 10 rounds per minute in drop-fire and trigger-fire modes, respectively.
As the Navy was developing the Mk 2, the Coast Guard in 1962 was looking for a better way to provide emergency illumination than firing star shells from 3- and 5-inch guns. The Navy’s mortar offered a solution: The Coast Guard received two mounts for testing, which proved very successful. The illumination rounds were brighter, lasted longer, could be fired quicker, and did not adversely affect the weapon’s bore. Because the mount was relatively lightweight at 580 pounds and served a dual purpose as a weapon, the service considered it as a replacement on board its small craft—82- and 95-foot patrol boats—for their aging 20-mm Oerlikon machine guns. Those were fine for law enforcement work, but not much help in search-and-rescue missions, which had become the service’s bread and butter. The Coast Guard sought to improve on the mortar’s offensive capabilities.
The project was handed to Chief Warrant Officer (Guns) Elmer L. Hicks, the Coast Guard’s only ordnance man at headquarters. In mid-1964, he suggested mounting an M2 HB machine gun on top of the mortar’s recoil cylinder. A prototype was completed in just eight hours at the Coast Guard Yard, Curtis Bay, Maryland. But there was a slight snag: The Navy owned (and still owns) all Coast Guard weapons of .50-caliber and above, so they needed permission from the Bureau of Naval Weapons. Once that had been granted, the first Mk 2 Mod 1 81-mm mortar mount was finished in two days.
Late that year, the combination was mounted on the back of a flatbed truck and tested successfully at U.S. Naval Weapons Laboratory, Dahlgren, Virginia. Two mounts then were installed on board a 95-foot cutter at Norfolk, Virginia, for further tests. A surprising and beneficial side effect of the combination was that the added weight of the mortar provided a highly stable mount that increased the M2’s accuracy in the face of the quick pitch and roll common to small craft. During the Vietnam War, it was found that underway craft could be hit easily at more than 1,000 yards.
A simple modification was needed to reconfigure the machine gun for right-hand feed to account for the mortar’s open-yoke-type sight and counterweight spring on the left side. A 100-round .50-caliber ammunition box was fitted on the right, but in Vietnam a 400-round box often was used. The Coast Guard also added an extended handle to aid traverse and removed the bottom of the recoil protection basket to allow expended .50-caliber cases to fall through so they would not impede the mortar’s recoil.
The two-in-one mounting of dissimilar weapons provided great flexibility to small craft with limited deck space, weight capability, and manning. The system required only a single weapon station and one crewman, although usually two were employed. While both weapons could not be fired simultaneously, gunners often opened with a high-explosive (HE) round, following up immediately with .50-caliber fire.
The mortar primarily used three types of ammunition. HE and white phosphorus (WP) rounds were used for both direct and indirect fire against watercraft and shore targets, and WP also could be used to lay smoke screens. Additionally, there were illumination rounds—the system’s original purpose. Fuzes were point-detonating, proximity, or mechanical time. The M2 machine gun typically used incendiary, armor-piercing incendiary, and armor-piercing incendiary tracer ammunition.
In 1969, the Navy introduced a new mortar round specifically designed for this weapon, the Mk 120 Mod 0 antipersonnel round. Resembling a World Wars I– and II–era German Stielhandgranate “potato masher” hand grenade with fins, this was a vicious projectile. The blunt-nosed warhead “can” had no fuze or bursting charge, but contained 1,300 1.25-inch-long flechettes—fin-stabilized darts—that burst from the muzzle in shotgun-like fashion. The short-range round was effective out to about 600 feet, but the flechettes would have dropped only about six feet from the line of sight at that range. Designed primarily as an antipersonnel round, it was equally effective at stripping camouflage and foliage from riverbanks.
Hicks’s piggyback idea spawned a number of imitations, but none were sanctioned. One Coast Guardsman mounted a Mk 19 40-mm grenade launcher in the cradle atop the M2. But the most unusual was a 2.75-inch rocket pod supposedly mounted upside down on a homemade bracket attached to Hicks’s cradle.
The Mk 2 Mod 1 81-mm mortar is an example of a quickly fielded innovation that became a widely used and important weapon. It saw extensive use on a variety of coastal patrol and riverine craft, including PCF Swift boats, Nasty- and Osprey-class PTF patrol boats, Point- and Cape-class WPB Coast Guard cutters, heavy SEAL support craft, command and control boats, and river monitors throughout the Vietnam War.