Weapon Systems
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Weapons Systems on the PTFs - Page 2

[This section was contributed by Bob Stoner who served as a Gunners Mate on PTFs]

I was looking through the technical specifications on the PTF and noted some minor things: 

  1. Although the first PTFs carried 40mm guns fore and aft, this was changed to the 81mm mortar/.50 BMG combination gun mount forward and the 40mm gun aft. The probable reason was the 81mm could fire illumination rounds as well as high explosive and white phosphorous. Hence, it was more flexible. 
  2. All the 40mm guns I serviced on PTF-13, -17, -18, and -19 were Army M3 guns, and not Navy Mk3 guns. The difference is that the M3 gun does not have train and elevation power drives while the Mk3 does. The Mk3 has a lead compensating sight for the pointer; the M3 has fixed sights for both pointer and trainer. The Mk3 also has a bolt-on ready service rack for three 4-round clips of 40mm ammunition; the M3 does not. Maximum effective range of the 40mm gun was 4000 yards. 
  3. As far as I know, all PTFs in RVN used the Mk4 20mm Oerlikon gun, port and starboard on either side of the bridge. The Mk4 was a holdover from WW2. It is blowback operated, uses greased ammunition, and is fed from a 60-round snail drum on top of the gun. When we received PTF-17 through -19, the Mk4 guns had been replaced by the Mk16 Mod 5 20mm guns. The Mk16 Mod 5 gun was a reworked Mk3 or aircraft cannon using gas-assisted blowback operation. Lubricated ammunition was also required and provided by a pump that put a drop of semi-fluid lubricating oil (LSA) in the chamber each time the gun fired. Ready service ammunition was carried in either in two 200-round capacity boxes on the mount or one 385-round box on the mount. Mounts were either the Mk67 or Mk68. Ammunition was linked and fed through a feed chute to the delinking feeder on top of the gun. The reason why one mount carried two ready service ammo cans instead of one was it gave the gunner the opportunity to use two completely different types of ammunition; for example, one with AP and tracer and the other with HE and armor-piercing incendiary tracer (API-T). Ammunition for the Mk4 and Mk16 guns was not interchangeable. Maximum effective range of either 20mm guns was the same as that of the .50 Browning machine gun; 2000 yards.
  4. The 81mm mortar/.50 BMG was the Mk2 Mod 1. The mortar was capable of either drop (gravity) fire (ala a conventional high-angle mortar) or trigger fire (which allowed the mortar to be aimed and shot like a gun). Mortar rounds provided for the 81mm were high explosive (HE), illuminating (ILLUM), white phosphorous (WP), and antipersonnel (APERS). The APERS round looked like an old WW2 German "potato masher" hand grenade. The head contained 2,000 13-grain flechettes. When trigger fired, the mortar became a fearsome shotgun at short ranges. The .50 BMG was the standard AN-M2 Heavy Barrel gun so common on tanks and combat vehicles. The .50 fired at 450-550 rounds per minute or single-shot (semi-automatic). Ammunition was contained in a 100-round box attached to the side of the Mk2 Mod 1 gun mount (although some users fabricated ammunition boxes with a larger supply to avoid having to change the box in a firefight). The most common ammunition load was linked 2 incendiary (INC), 2 armor-piercing incendiary (API), and 1 API-T. (I called this the "fruit salad" load; lots of bad stuff for any occasion.) 
  5. Mounts were also provided for M60 7.62mm machine guns both lookout stations behind the bridge. This did not include either M14 7.62mm or M16 5.56mm rifles, M79 40mm grenade launchers, 12ga riot shotguns, and side arms for the crew.

Emailed questions from Chip Marshall
  1. The M60 mounts at the lookout stations (new information to me).
  2. More questions on the bow mounted 40mm on later boats.
  3. More questions on the 20mm weapons and their installation.
  4. Installation of launchers for Russian made 122mm rockets.
  5. Installation of 50 cal BMGs at the stern of the boats.
  6. Use of 57mm recoilless rifles on the boats.

Good to hear from you Chip.
  1. The M60 mounts were field modifications to the PTF-17, -18, and -19 boats.  They consisted of a bracket and a piece of pipe (stanchion).  All you had to do was clip a pintle for the M122 tripod to the M60 and drop it into the stanchion.  If you have a photo of PTF-17 at the Museum at Buffalo, NY, look very carefully along the top of the superstructure near the holes for the lookout cutouts.  If you find a bracket with a stanchion (it looks like some kind of mount for a radio whip antenna), that's the M60 mount.  As this was a field modification there was no way an ammo box could be installed (for example, the M142 tray for the M4 pedestal vehicular mount on the M151 jeep carries a 200-round can in an easily attached bracket).  The reason was that the pintle hole of the M122 tripod and that of the M4 pedestal mount are different sizes (the M142 pintle is about twice as big).  If I was designing this gizmo, I would have made it for the M142 tray.  That way I wouldn't have a loose belt flopping about!
  2. The bow of the PTF were designed to accept the recoil stresses of the 40mm Bofors gun (single mount M3).  The 81mm/.50 Mk 2 Mod 1 gun mount was essentially a drop in installation; it had neither the weight nor recoil stresses of the 40mm.  The 40mm Bofors is a prodigious ammunition hog; rate of fire is adjustable from 120 rounds/minute to 160 rounds/minute.  That is why you will find many more ready service boxes on the deck of PTF3 and PTF4 than you will on the mortar-equipped PTF13 (only one for the 81mm mortar rounds and also shared by the .50 100-round boxes).  PTF13 and the other PTFs I served on carried their 40mm ready service in a simple box made of steel angle and pipe.  There were four trays attached to the deck; two adjacent to the 40mm on either side.  Each box could carry four or five 40mm ammunition cans of 16 rounds in 4-round clips.  On the back of the 40mm mount was a rail for both the first loader and mount captain; the loader stood on the left, the mount captain on the right.  The railing had brackets for additional clips of 40mm ammunition. 

    The 40mm ammunition can was designed for shipment of the ammuntion.  The ammunition had to go into the can in a certain way.  The outside 4-round clips went in first with the clips facing each other.  The inside 4-round clips went in with the clips above the base of the outside rounds and the clips facing away from each other.  Each of the 4-round clips was secured by a spring steel retainer that had to be pushed out of the way to pull the clip out of the can.  To operate the 40mm efficiently, a crew of at least six was required:  pointer, to elevate and fire the gun; trainer to track the target with his hand wheel; first loader to load the 8-round magazine, safe the gun, and select the type of fire - full or semiautomatic; mount captain to run the mount, keep the crew safe, and assist the first loader; two second loaders to break out the ammunition and pass it to the first loader/mount captain.

  3. The 20mm guns were either the Mk4 Oerlikon (from WW2) or the Mk16 Mod 5 (which is the Korean War M3 aircraft gun).  The Mk4 was fed from a 60-round, snail-shaped magazine.  The ammunition was greased as it was loaded into the drum magazine and then the spring was wound to the correct tension to feed the ammunition.  Operation of the Mk4 was advanced primer ignition, direct blowback.  That is, the bolt is still traveling forward when the round is fired in the chamber.  The inertia of the bolt, coupled with the driving springs, keeps the breech closed until the pressure drops to safe levels after ignition.  The gun has no primary extraction.  Instead the grease on the case keeps the brass from sticking to the walls of the chamber during the violent extraction and ejection stroke.

    The Mk16 Mod 5 is a modification of the M3 aircraft machine gun.  On firing, this gun taps off a portion of the gas about 1/2 way down the barrel.  The gas expands against a gas piston in the gas cylinder.  The piston is attached to a yoke (forming a Y-shape) on the top of the gun.  As the yoke moves back under gas pressure, the ends of the Y strike a pair of spring loaded rods.  These rods transfer the energy to a pair of inertia blocks.  The inertia blocks are attached to the firing pin and bolt.  The blocks retract the firing pin and then get the bolt moving to rear.  At the end of the recoil stroke, the compressed driving springs push the bolt forward.  The bolt carrier strikes the end of the breech first.  The inertia blocks carry the bolt forward and extend the firing pin to fire the cartridge.  Because the Mk16 gun is primarily a blowback (with gas assist) weapon, it requires lubricated ammunition.  For this purpose, a hole has been drilled in the chamber wall and a pump is attached to the LH side of the barrel.  When the gun recoils and counterrecoils, the pump injects a drop of LSA lubricant into the chamber to lubricate the  case.  (A fine idea, but in practice the reservoir of the lubricating pump only holds enough LSA for 200 rounds.  The Mk67 or Mk68 gun mount carries either 385 or 400 rounds in the ready service ammunition boxes.  The usual practice was to put LSA on the rounds as they went into the boxes.  That way, if the pump runs dry, the lubricant on the rounds will keep you from having a ruptured case in the breech.)

    Because the ancestor of the Mk16 was an aircraft gun, the gun itself was designed so that every bit and piece had to be safety-wired!  This is the only gun I worked on that took two minutes to field strip and an hour to put back together.  In addition, the design used tab washers to keep certain parts from loosening under fire.  Guess what never was supplied to the gunner's mates?  Bingo!  Also, the Mk16 receiver was mounted in a cradle.  The gun recoiled and counterrecoiled in the cradle.  The barrel was supported by a trunnion block which was shimmed so that the gun moved forth and back in a straight line.  If you lost the shims or switched them, the gun would fire and then jam.  (In this instance, it had to be sent back to the depot for overhaul).  Another problem with the Mk16 was its time between overhauls --- 6,000 rounds.  At 6,000 rounds the gun had to be turned in to the depot (NAS Alameda) for overhaul!

    I was not a fan of the Mk16 gun.

  4. I don't know if Russian 122mm rockets were fitted to the boat, but it would have been a simple matter to rig some kind of rack for the rockets so that the rocket exhaust was directed into the wake of the boat and then aim them by turning the boat towards the target.  The VC made launchers for the 122mm rockets from bamboo poles and fired them with a car battery and a coupe of wires, so it would not have been an insurmountable problem to fire them from a PTF.
  5. It would have been very easy to mount a .50 on the stern of the PTF.  The corners of the deck which formed part of the transom were reinforced.  A pedestal mount could have easily been bolted to the deck without a problem, although I never saw this in practice.
  6. The 57mm recoilless rifle M18 and M18A1 was a very portable and useful tool.  It was in the SEAL armory and the Vietnamese had them also.  The 57mm RR probably was not fired from a mount.  I would have been far easier for the operator to shoulder it and fire it at whatever he intended to hit.  Also the 57mm had a folding bipod and adjustable monopod.  The operator could have fired it from the prone position off the deck of the PTF (being careful where the backblast went).

From Chip

The information on the website on the 40mm installation is something I got my hands on at the Naval Historical Center in Washington, DC. PTF3 and 4 and the aluminum PTs (809-812) all had the gee wiz Navy Mk3. I saw a picture of PTF3 at Quantico with the CNO on board sitting in the left seat of the 40mm mount looking through the scope.

The contraption you saw on the bow of the 17 boat in Buffalo is (I was told) a GE 7.62 electric minigun on a pedestal mount. Apparently this weapon is pretty rare. Be nice to have on a PTF (I would imagine), if you carry the ammo the thing eats up. Probably be better mounted aft of the deck house.

Did the Viet Nam boats have the M60s mounted?

One other thing, I am trying to get my hands on drawings for the 50cal/81mm mortar over/under setup. If you run across something like that, let me know. I will be haunting the halls of the Naval Historical Center something within the next month and will check their records.

Thanks for all of the information. You have actually created more questions than you have answered. But that is the way it works in the historical (or hysterical, depending on the level of beer and the individual's sanity at any given moment) business.


From Bob:   

If the contraption is indeed a 7.62mm Minigun, the museum would be better off putting it somewhere that is out of the weather instead of where it is. We did a field modification to both our MSSCs at SEA FLOAT to remove the after .50 gun and install a 7.62mm Minigun. It came as a kit. GMG2 Barta came over from BSU-1 to do the mod to all active MSSCs. The Minigun we used was the flexible gun used by the Jolly Green Giant HH-3 helicopters. The gun was suspended in a cradle and the cradle's pintle plugged into that of the .50 BMG. There was a semicircular brass and link catcher underneath the gun because the Mini dumps 8 to 12 live rounds as part of its clearing cycle when the trigger is released to prevent cookoffs. [NOTE 1: The M61A1 20mm gun does not dump live rounds. When the trigger is released, the feed solenoid disengages the feed pawl, the incoming ammo stops at the feeder, and any live rounds are fired as the barrels spin down. Once the round is fired, the bolt goes into the clearing path at the back of the gun (to allow air to circulate through the open breech. The six bolts remain in the clearing path at the back of the gun until the trigger is closed, where upon the begin their feed, chamber, lock, fire, extract, and eject cycle at 6,000 shots per minute.]

I don't know if any of the Viet Nam boats mounted M60s for the lookouts. However, the mounting/dismounting is so simple that it could be done by anyone at any time.

I will check, but I'm certain that I kept and retained copies of the Mk 2 Mod 1 (and Mod 0) 81/.50 mount as well as the Mk 16 Mod 4 and Mod 5 20mm guns. The OPs have beautiful line art that show all the pertinent characteristics of the ordnance. [NOTE 2: The difference between the Mk 16 Mod 4 and Mod 5 was how the sear was operated. The Mod 4 used an electric solenoid to release the sear; the Mod 5 used a mechanical release; that is, a braded steel cable attached to the trigger lever on the mount handgrip. This arrangement is similar to the M63 AA mount used by the Army and Marines for the .50 BMG.

04-26-01 From Chip

Just got home and your fax was in the machine.  Thanks!

The deck layout I sent you is one I drew from official US Navy PTF9-16 drawings.  This was the first series construction of Nasty class boats built specifically for the US Navy to specification.  PTF3-8 were (I am told) started as Norwegian Navy Tjeld Klasse boats with Norwegian spec deck fittings, radar and below deck layout.  The first two boats, 3 & 4 were brought to the US and fitted out with weapons and electronics (except radar) at the Philadelphia Naval Shipyard.  I understand they are also the only boats to come from the "factory" fitted with bow mounted 40mm cannon.  The next six boats (and possibly some of the other Norwegian built boats) were brought directly from Norway to the Philippines by LSD.  The Ship Repair Facility at Subic Bay ripped out the chief's quarters, removed the bow mounted 40's (if fitted) and installed six additional 600 fuel cells to increase the range of the boats.  The Trumpy built boats were actually fabricated for the most part in Norway and assembled (for political purposes) in Annapolis.  With few exceptions, they were supposed to be copies of PTF9-16.

A motor mac that served with BSU-1 in the early days (actually years) told me about the 50s being mounted at the fantail.  He said they used (and probably reinforced) the corner stanchions for the lifeline/side rail as mounts.

I am interested in getting information on the "tins" the 40mm ammo was stored
and shipped in.  You say these tins were opened and set into the ready racks
on either side of the 40mm mount?  Just trying to get all the pieces of the
puzzle together.

One thing I noticed on your fax, the ready service box for the 81mm/50cal is
one big box.  How was the box compartmented to separate the 50cal ammo from
the mortar rounds?  The official drawings (unconfirmed by photos) show two
separate smaller boxes which I faithfully drew as accurately as I could.

I am trying to be as accurate as I can with the drawings.  Considering the
many modifications made to the boats over their operational life, this may
turn out to be quite an adventure.  One thing I have learned from
construction and architecture is the details will nail you in the ass if ya
ain't lookin' fer it.  Any drawings or pictorial information on the boats is
always helpful and welcome.

Thanks again for the information,

Chip Marshall


I have great news!  I found my copies of the OPs on the Mk 2 Mod 1 81mm/.50 BMG [OP 1743 of 15 July 1972], Mk 67 Mod 1 mount [OP 3990 of 15 July 1972], Mk68 Mod 0 mount [OP 4011 of 15 March 1973], and Mk16 Mod 4 and Mod 5  [OP 4014 of 15 July 1971].  I am culling out the better illustrations to send to you on Monday morning.

  1. Any .50 caliber mounts that were mounted on stanchions back aft would have to be pretty beefed-up to take the recoil of the gun.  An easy way to do this would have been to install the M4 pedestal mount from an M151 jeep as it is designed to accommodate the M60, M2HB, and Mk19 machine guns.  The M4 even has outrigger legs for additional support.  However, the MRT (mobile repair team) could have taken the cheap and dirty route of using heavy pipe.
  2. The 40mm ammo "tins" were the standard 40mm shipping containers.  They were about 18 inches on a side and about 36 inches high, made of steel, with two carrying handles on the sides, and a top that had a dogging latch mechanism that was locked by four thumbscrews.  The loaded box weighted 114 pounds and carried 16 rounds of 40mm in four clips.  I remember them well because my original ship, USS NUECES (APB-40) had two quad 40mm mounts fore and aft.  We were usually re-supplied by an LST on Sundays.  This meant that we would off load the empty 40mm brass and clips stowed in the shipping containers to the LST and then move the loaded containers to our ship's ready service magazines.  It was a whole 3rd division evolution.  We would form a daisy-chain and lug the cans to the ready service magazines.  The carrying handles were really rough on your hands and we didn't have any work gloves.  We never really knew how much 40mm we had in the ship's magazines until we did a complete off load at West Loch NAD, Pearl Harbor.  It took all hands eight hours to manhandle all those cans out of the magazines and band them to pallets so they could be put on the pier by a crane.  Ugh!

    When we would go on a gun shoot with the 40mm, we would have the ready racks loaded with the 40mm cans.  On the way to the exercise area, we would remove the safety wire on the thumb screws and take the tops of the cans off.  We would load the ready racks on the back of the M3 mount with loaded clips.  The rest of the ammo stayed in the cans until we were ready to use it.  Because the safety spring was difficult for the second loader to work when he was pulling clips from the can, I would pull out two clips about half way and position them so the loader could easily pull them the rest of the way.  After two clips had been removed from the can, the other safety springs were easy to move away to pull the loaded clips out.

  3. The ready service box for the Mk 2 Mod 1 was located in front of the superstructure on the port side behind the mortar.  This box was the same size as used for the 20mm ready service magazines.  There was a fuse setter attached to one side so the gunner could set the time fuse on illumination mortar rounds.  The fuse setter looks like a triangular box with a plate attached to it.  The plate has a hole with notches cut in it.  The notches grab corresponding lugs on the fuse when it is being set.  If you go to the PTF-17 photos at ptfnasty, you will clearly see the ready service box and may be able to pick out the fuse setter box on the side.  [As an aside, the 20mm ready service boxes are the same construction as the 81mm/.50 ready service box; only the internal arrangement is different and the 20mm boxes do not have the fuse setter box.]

    If memory serves correctly, the box was divided into two ready service areas.  Facing the box with cover open, on the left were stacked 100-round ammo boxes for the .50 BMG.  To the right were the 81mm rounds.  They were held vertically by a removable bracket that resembled a grid pattern.  The rounds were loaded nose first in their cardboard ammunition containers.  For peacetime purposes, we left the cardboard shipping caps in place until we were ready to fire.  In wartime, the caps were discarded to make it easy to pull the round out by its fins.  I seem to remember that the grid could also be removed.  If that were done, then the 81mm rounds were stacked in their containers on their sides.  My memory fails me as to what the total ready service load for the box was.

    My operational experience was on PTF-13 and PTF-17 through PTF-19.  Some of the early Nastys may have used the segregated arrangement for .50 and 81mm as shown on your drawing.  However, all my experience was with the single multi-use ready service box.  [By the way, this is the same kind of box that is used on PCF (Swift) boats for the Mk 2 Mod 1.]

    One other thought.  The box itself was actually a box with an a box.  That is, the inner box carried the ammo.  Attached to the outside skin of the inner box was inch spacers and attached to the spacers was the outer skin of the box.  The idea was to keep the inner box a more or less constant internal temperature by allowing air to circulate and to protect the inner skin from the direct rays of the sun.  We took temperatures of the ammo boxes every day to see what the minimum and maximum temperatures inside them were as temperature directly affects the life and performance of the ammunition.

Weapon Systems - Page 3