Chapter 11

An exterior view of the machine gun is shown in figure 11-4, and a closeup of the rear sight is shown in figure 11-5. The sight is a leaf and blade type. It is graduated in both yards and mils for range — from 100 to 2,600 yards and 0 to 62 mils. A windage screw allows for a wind correction of 5 mils either to the right or the left.

Fast patrol boats with hull numbers 17–26 (fig. 11-6) have the following installed ordnance: one 81 mm mortar, a single 40 mm mount, 2–20 mm single mounts, and one .50 caliber machine gun mounted over the mortar. The 81 mm mortar with its piggyback .50 caliber machine gun (fig. 11-7) is located in the bow section of the boat. One of the 20 mm guns is mounted on the starboard side and the other is mounted on the port side.

PTFs with hull numbers 3–16 have 2–40 mm guns — one forward and one aft — and 2–20 mm guns — one to starboard and the other to port.

The 81 mm mortar shown in figure 11-7 is a Mk 2 Mod 1 design; the one in figure 11-8 is a Mk 2 Mod 0. The major design difference between the two is that the Mod 1 has the piggyback machine gun.

The mortar is capable of firing high explosive shells, illuminating shells, and white phosphorous (WP) smoke shells. This capability makes it an effective weapon in bombardment, laying smoke screens, and providing nighttime target illumination. Fuzes for the mortar’s ammunition are of two types: point detonating and time. However, there are three types of fuze actions because point detonating fuzes can be either superquick (explode on impact) or delay (activate on impact but explode a fraction of a second later).

The mortar is simple in design. It can be operated by one man, though two usually perform this duty. It is designed for free swinging in both train and elevation for quick change in direction, or it can be fired from a fixed position, by either drop-fire or controlled trigger fire. (In drop-fire, the projectile is fired as soon as it slides down the barrel and hits the firing pin; trigger fire is similar to firing a handgun.)

The 81 mm mortar has an open yoke-type sight mounted at the left side of the slide (fig. 11-9). It is a manually-adjusted arrangement with offset limits of 14° 20′ deflection, right and left, and 75° sight angle. The mortar has an elevation scale that is graduated in 5° increments from −30° to +75° and a training scale graduated in 1° increments from 0° to 360°.

Additional information on the 81 mm mortar, which is also mounted on Hydrofoil Gunboats (PGHs) and Fast Patrol Craft (PCFs), can be found in OP 1743.

The 20 mm gun on the PTF (and other small craft) was originally designed in the early 1930s for use on aircraft. The first known successful application of this weapon for use on surface vessels was on the Navy’s small boats in Vietnam — boats such as PCFs, PBRs, and ASPBs. The 20 mm gun may be mounted in an open mount arrangement, as on the PTF, or in an enclosed mount (fig. 11-10).

For discussion purposes, the 20 mm aircraft gun may be considered to be divided into seven major components (fig. 10-22): gun barrel, receiver assembly, recoil mechanism assembly, gas mechanism assembly, breechblock assembly, buffer assembly, and charger assembly. These assemblies include all the elements necessary for chambering a round of ammunition, closing and opening the breech, extracting an empty case, and controlling the recoil and counter-recoil actions.

In addition to being useful against aircraft, the 20 mm gun can be used against small craft and personnel. Its primary uses on the Navy’s small boats in Vietnam are against personnel and small craft. A detailed description of the 20 mm aircraft gun can be found in Aviation Ordnanceman 3 & 2, NavPers 10345 series, or OP 3476.

The fast patrol craft, “swift” type (fig. 11-11), is generally equipped with an 81 mm mortar with a piggyback machine gun mount, aft, and a twin .50 caliber machine gun mount atop the pilot house. Also, they may be equipped with a Mk 19, 40 mm machine gun mounted forward of the pilot house and M60 (7.62) machine guns. PCFs, as well as other small boats, are equipped with hand-carried ordnance such as M79 grenade launchers and handguns, as the need arises.

Presently, there are two hydrofoil gunboats in service. Both are armed with one 40 mm gun forward, four .50 caliber (two twin mounts) machine guns atop the pilot house, and one 81 mm mortar aft. Hydrofoil Gunboat Tucumcari (PGH-2) is shown in figure 11-12. Its high speed capability (above 40 knots), ease in maneuvering, and variety of weapons makes it very useful for coastal operations.

The river patrol boat shown in figure 11-13 is armed with three .50 caliber machine guns and a 40 mm (M79) grenade launcher. The twin machine gun mount is located in the bow section of the boat. The 40 mm grenade launcher is hand carried. The single machine gun is located near the boat’s stern.

The armament of different PBRs varies. Some PBRs have a 40 mm machine gun mounted over the single .50 caliber machine gun. Others have 60 mm mortars and 7.62 mm machine guns in addition to their .50 caliber machine guns and 40 mm machine gun.

The M79 grenade launcher is a single-shot, shoulder fire weapon. It is breech loading and chambered for a 40 mm metallic cartridge case with internal primer. It is trigger-fired the same as a shotgun — and even resembles one somewhat (fig. 11-14). Because of its versatility, the grenade launcher is issued to most of the riverine craft, and its primary use is the launching of antipersonnel projectiles against enemy ground forces. The operation and maintenance of the M79 grenade launcher is described in Army Technical Manual TM 9-1010-205-12.

Like the 81 mm mortar, the 60 mm mortar (fig. 11-15) can be used for bombardment, laying smoke screens, and for providing nighttime target illumination. It too has a recoil-counter-recoil mechanism to reduce brake load and a trigger-firing mechanism.

The main characteristics of the 60 mm mortar are as follows:

Another weapon that is commonly used on the PBR and other riverine craft (as well as on some coastal patrol craft) is the 7.62 mm machine gun. This is the Army’s machine gun mounted for use on Navy small boats, and it is capable of engaging distant targets (up to 3,200 meters) with a heavy volume of controlled and accurate fire.

The M60 machine gun (fig. 11-16) is an air-cooled, belt-fed, gas-operated automatic weapon. The ammunition for this weapon, which fires from the open-bolt position, is fed into the gun by a disintegrating metallic split-link belt.

Armament of ASPBs may vary, but generally it consists of two Mk 26 gun mounts (with either .50 caliber machine guns or Mk 20, 40 mm machine guns), two 20 mm machine guns, and two Mk 19 high-velocity grenade machine guns — plus small arms and night vision devices. The ASPB (fig. 11-17), with its assortment of armament, is used mainly for patrolling inland waterways and for supporting troop landings and movements.

The Mk 26 mount is a universal tripod, lightweight unit developed for the .50 caliber heavy barrel machine gun and adaptable for mounting other weapons. The mount is quite versatile with respect to location on a variety of small boats. It weighs 205 pounds and is 59 inches high. It can elevate through a 100° arc — from −15° to 85°. Figure 11-18 shows the Mk 26 with a .50 caliber machine gun mounted. The Mk 20 machine gun requires an adapter for its installation on the Mk 26 mount. On other small craft, the Mk 26 may have other ordnance mounted — such as the 7.62 mm machine gun or the Mk 20 machine gun.

The Mk 20 machine gun (fig. 11-19) is a fully automatic, low-velocity grenade machine gun used as an antipersonnel weapon. It fires the same round as the M79 grenade launcher, and it utilizes a metallic link fed from a 24-round ammunition can. The gun was designed to be pedestal mounted; however, it is recoilless and lends itself to multiple mounting systems such as tripods or bipods or, if necessary, it may be fired from a hand-held position. To fire the weapon, the first cartridge is loaded manually into firing position; after that shot is fired, the remaining operation is automatic or semiautomatic depending upon the mode of fire.

The Mk 48 gun mount is a medium-armored, small-caliber, flexible multiple-gun mount for use on small craft employed in patrol, escort, and support duties. The mount provides fire power effective against other small craft as well as personnel and shore emplacements. It has unobstructed peripheral vision for easy target recognition from all angles.

The Mk 48 mount comes in several mods. The Mod 0 mount has one Mk 19 machine gun and one 20 mm machine gun; the Mod 1 has one Mk 19 machine gun and two 7.62 mm machine guns; and the Mod 2 has one Mk 19 machine gun and two .50 caliber machine guns. The mount is 39 inches above the deck. It is manually operated through 360° in train and can be elevated from −15° to 65°. It has a periscope-type sight for daytime use and a light amplification type for use at night.

The Mk 19 machine gun (fig. 11-20) is a mechanically-fed, blowback-operated weapon designed to fire 40 mm high velocity grenades. It can be fired by hand or remotely by using a solenoid.

The monitor, a converted landing craft (LCM), provides fire support for riverine operations. It has two basic hull configurations — the Howitzer and the Flame System. The Howitzer configuration (fig. 11-21) has a 105 mm howitzer mount forward, two Mk 48 Mod 0 gun mounts aft (one to port and the other to starboard), and two 7.62 machine guns installed on Mk 26 gun mounts. The flame system configuration has two flame throwers instead of the howitzer and a Mk 20 machine gun in place of one of the 7.62 mm machine guns. Both configurations have small arms and night vision devices.

The heavily armored monitor is popularly referred to as the “battleship” of the riverine fleet. Its 105 mm howitzer or flame thrower gives it added punch in routing the enemy.

3. Command and Control Boat (CCB)

The CCB (fig. 11-23) serves as an afloat command post, which provides command and communications facilities for ground force and boat group commanders. They are heavily armored and are armed with two 20 mm machine guns, two .30 caliber (or 7.62 mm) machine guns, and two Mk 19 machine guns.

4. River Minesweepers (MSR)

River minesweepers are converted landing craft. They are heavily armored craft that are used for clearing mines from the rivers. MSRs are armed with two Mk 48 Mod 0 mounts amidships — one to starboard and one to port — and a .50 caliber machine gun.

5. Coastal Minesweeper (MSC)

The MSC is constructed throughout of wood and other materials with a low magnetic attraction. The Bluebird Class (fig. 11-24) has one twin 20 mm mount forward. The Albatross Class (fig. 11-25) are classified as MSCOs and have a single 40 mm mount forward.

These are but a sampling of the various small boats used in coastal and riverine operations. Again, bear in mind that the armament may vary considerably from boat to boat. For example, the armament on one PBR may be quite different from the armament on another PBR.