The following additional Information was
Mr. Bob Stoner, GMCM (SW), USNR
These were later versions of guns.
This Mark and Mod was not placed on the Battleship TEXAS.
20MM and 40MM Guns
20mm Oerlikon gun.
The first 20mm Oerlikons used pedestal that was adjustable (to
move the gun cradle up and down). The early marks through the Mk 4 gun
used this arrangement. However, the mount was very heavy, especially
when fitted with the gun shields. It wasn't long however before it was
supplemented and then replaced by the Mk 10. The Mk 10 was a simple
tripod design of a fixed height and was much lighter in weight. Aboard
ship it was accompanied by steel gun shields for the gunner and
loader. Aboard submarines and PT boats, the shields were removed to
reduce the size, drag, and lighten the gun even more. Attached is a
photo from the U.S. Navy Manual Gunner's Mate 3 and 2 from the 1950's.
The twin mount Mk 24 was an attempt to double the firepower of the 20mm
against the Japanese Kamikaze threat in the 1944 to 1945 period. The Mk
24 had two guns, with right and left magazines (the LH drum had a white
zig-zag mark painted on it for the loader's quick recognition). The twin
gun also used a lead computing sight to help the gunner get more hits on the
[NOTE: The post-Vietnam Mk 16 Mod 4 and Mod 5 guns were NOT
Oerlikon-type guns. These guns were AN/M3 or M24 aircraft guns modified
for shipboard use and were belt-fed from either a single 385-round box or two
200-round boxes on the mount. It was replaced by the Mk 38 25mm
Mk 10 20mm Oerlikon gun.
Mk 24 twin 20mm Oerlikon guns.
20mm Mk 10 and Mk 24 Oerlikon
The 20mm Oerlikon was an inter-World War Swiss development of
the World War 1 German Becker 20mm anti-tank gun.
The Mk 10 (single) and Mk 24 (twin) guns were very simple,
mechanical guns. They had a quick-change barrel that allowed an
overheated barrel to be easily replaced with a cool one. These guns were
blowback, advanced primer ignition in operation and required greased
ammunition. With blowback operation, the bolt of the gun is not
mechanically locked. Instead, the breech is kept closed by the force of
heavy springs. The Mk 10 and Mk 24 guns used concentric springs that
were fitted around the barrel. It required about 450 pounds of force to
cock a 20mm gun (the twin guns were cocked one gun at a time). To cock
the gun, the loader hooked the cocking bar onto the cocking stud that was
attached to the guide bar of the gun. Once hooked, the gunner hopped up
and put the shoulder rests under his armpits and used his weight to cock the
Ammunition was contained in a 60-round, snail shaped drum that
mounted on top of the gun's feed throat. There was a
triangular-shaped stand attached to the top of the 20mm magazine ready service
box that held 16 loaded drums. Empty drums were placed on the
loading stand, and individual rounds were inserted into the magazine
until it was full. There was a crank supplied for the drum to tension
the spring that fed the ammunition to the gun as it fired. Normal
spring tension was about 9 to 11 clicks. As the ammunition was loaded,
it was greased. Greased ammunition was required because of the gun's
method of operation.
As mentioned, the 20mm Oerlikon was blowback operated,
advanced primer ignition. Because there was no mechanical locking of the
breech and the breech had to remain closed until the pressures developed
during firing had dropped to a safe level, the gun used advanced primer
ignition. Advanced primer ignition means that the firing pin of bolt
fires the cartridge primer as the bolt is moving forward under the force of
the driving springs. The power of the springs combined with the inertia
developed by the moving bolt is sufficient to keep the bolt closed until
the pressures from firing drop to a safe level.
Unfortunately, this particular method of operation results in
very quick extraction of the fired case. It is so quick and violent that
some means has to be added to keep the recoiling bolt from ripping the back
end of the cartridge case off. [The cartridge case expands to seal
the breech on firing and requires a brief amount of time to contract so that
it can be extracted from the gun chamber without causing a jam.] The
20mm Oerlikon does not have any form of primary extraction that could loosen
the case before the bolt starts to move to the rear. The purpose of
the grease was to keep the cartridge case from sticking to the chamber
walls of the barrel and to assist extraction of the fired case. If the
gun was fired without greasing the rounds, a ruptured case was the inevitable
result with the fired case stuck in the barrel.
The only problem that I had with the Oerlikon was the magazine.
The 60-round drum was both heavy and awkward. It was relatively easy to
load if the gun was being used as an anti-aircraft weapon. However, when
used in the surface to surface role, it was difficult for the loader to hold
on while the boat pitched and rolled and raise to the magazine high
enough to reload the gun. The Oerlikon was a very reliable and robust
gun; it was a good compliment for the lighter .50 caliber machine gun.
20mm Mk 16 Mod 5
The Mk 16 Mod 4 and Mod 5 20mm guns were converted AN/M3 and
M24 aircraft guns. The original gun was developed from the Swiss
Hispano-Suiza HS404 cannon developed immediately before World War 2. The
HS404 was used to arm the cannon-equipped RAF Hurricane and Spitfire fighters
used during the Battle of Britain in 1940. Specimens of the HS404 were
exported to the United States for manufacture. These American-built guns
became the AN/M1 and AN/M2 20mm aircraft guns. Although overshadowed by
the venerable 50 Browning machine gun, the M1 and M2 guns had some teething
problems that were not worked out until the post-war AN/M3 gun. The
AN/M3 gun used lubricated, percussion-primed ammunition. AN/M3 guns
were the primary armament of many post-WW2 fighters and bombers. A
modification of the AN/M3, the M24, was designed by the USAF to fire
electric-primed ammunition because the service felt electric ignition was
faster and more reliable.
When the Vietnam War broke out, the old 20mm Oerlikons were
getting few and spares were becoming harder and harder to find. The
Navy decided to adapt the AN/M3 aircraft guns for use on small boats and other
craft. This gun was the Mk 16. The Mk 16 was a belt-fed 20mm gun
that used a round both larger and more powerful than that of the Oerlikon.
The gun itself was mounted in a cradle that it could recoil and counter-recoil
independent of its mounting. A mechanical feeder/link stripper was
attached to the top of the gun, and as the gun moved aft and forward in the
cradle, it would work the feeder to bring ammunition to the gun for firing.
During the Vietnam War, these Mk 16 guns were very common on the Riverine
Assault Craft that were converted from former landing craft.
After the Navy pulled out from Vietnam in 1972, it decided to
replace the remaining Oerlikon guns with the Mk 16 variants. The
difference between the Mk 16 Mod 4 and Mod 5 guns was the method of the sear's
release of the bolt. The Mod 4 used a solenoid and released the sear
electrically; the Mod 5 used a mechanical sear released by a cable. The
gun was secured in the Mk 67 Mod 0 or Mk 68 Mod 0 gun mounts. The
Mk 67 had a 385-round ready service box installed beneath the gun. The
Mk 68 had two 200-round ammunition boxes attached to the gun shields.
The Mk 68 gave the gunner the ability to switch to different types of
ammunition by simply swapping the ammo feed chutes to the feeder/link
However, the Mk 16 guns had several problems that were not
entirely solved. The cases had to be lubricated to function reliably
(the same as in the Oerlikon). Instead of grease, the Mk 16 used LSA
lubricating oil. This oil had a Teflon content and there was a neat
chamber lubricator added to the gun that operated during
recoil/counter-recoil. The problem was that it only held enough lube for
200-rounds. A Mk 67 mount had 385 rounds and the Mk 68 had 400 rounds.
This meant that the gunner had to replenish the oil supply in the middle of a
firefight. Instead gunners simply brushed LSA directly onto the linked
ammunition and that took care of the problem.
Another problem was not solved. The Mk 16 was an aircraft
gun adapted to surface use. The result was that the Mk 16 had lots of
lockwire to keep it from vibrating itself apart during firing. There
were also special lock tab washers that were used on fasteners where lockwire
could not be used. The result was it took only a few minutes to strip
the gun for cleaning, but it took about an hour to lockwire it together.
The other problem was that the special lock tab washers were always in short
supply. Standard procedure was to use them once and replace them; the
reality was that this was not always possible.
The last blow was that the Navy/Coast Guard eliminated the only
school for Mk 16 gun maintenance in the early 1980's. The Mk 16 needed
operators that understood its operation and maintenance requirements.
When the school closed, this caused lots of problems for ships and craft with
the Mk 16. The result was that the Mk 16 was withdrawn from service and
replaced by either the .50 Browning machine gun or the 25mm Mk 38 Mod 0 (aka
M242) "Bushmaster" cannon first fielded on the M2
"Bradley" armored fighting vehicle.
40mm Bofors gun.
The Mk 1 (twin) and Mk 2 (quad) guns were water-cooled guns
that were used on ships. They were heavy and so small craft such as LSTs,
submarines, and PT boats needed something lighter. The solution was to
make a Navy version of the Army's towed M1 40mm Bofors air-cooled single
The Navy version was called the Mk 3 Mod 0 or the Mk 3 Mod 4.
The former gun was all-manual operation; the later incorporated power drives
for elevation and traverse. The Mod 4 also had a more sophisticated lead
computing sight than the simple peep and ring type used on the Mod 0.
A 40mm Mk 3 Mod 0 Bofors gun aboard PTF-17 at Great Lakes, IL
A Mk 3 Mod 4 40mm Bofors gun from the Navy manual showing the
power drives and lead computing sight (with backup open sights).
40mm Mk 3 Bofors
The Mk 3 Bofors is a pre-WW2 Swedish design that is probably
the best light anti-aircraft gun ever designed. The gun is simple,
robust, and totally reliable. It was manufactured in twin and quad
(water-cooled) versions as well as single (air-cooled) versions. This
gun was unique that it was used by Allied and Axis powers during World War 2.
It has been modified many times. In U.S. Navy service, the Mk 3 gun is
to be found as either the Mk 3 Mod 0, Mod 4, or Mod 9. The Mod 0 gun's
operation requires a crew of at least 5 or 6: mount/gun captain [oversees gun
operation], 1st loader [loads ammunition into the loader], pointer [elevates
and fires the gun], trainer [traverses the gun], and one or two 2nd loaders
[to pass ammunition]. Mod 0 operation is strictly manual.
The Mod 4 gun adds power control to the elevation and traverse.
The pointer has a control that allows him to both elevate and traverse the gun
under power. The mount/gun captain is eliminated. If the power
fails, the gun can be operated manually by from the pointer and trainer seats.
The 1st and 2nd loaders are still required.
The Mod 9 gun retains the power control of the Mod 4 gun but
can be operated by one man. The loader is topped by an ammunition drum
that contains the ammunition to feed the loader. The pointer has total
control of the elevation and traverse. In case of power failure, the gun
can be operated in manual mode using a 1st and 2nd loader [ammunition drum
must be removed] and trainer along with the pointer. This gun was
used on Special Operations craft during the late 1970s through early 1990s.
The Navy has pretty well phased all models of 40mm Bofors out
of service at this time. Yet, as new systems have come about to replace
it, none have approached the simplicity and reliability of the Bofors design.
It is still used by many armed forces all around the world.