Rheinmetall-Borsig MK 108 30mm cannon
In many ways, the Rheinmetall-Borsig MK 108 30mm cannon was considered
to be a masterpiece of weapons engineering, due to it's compact size, ease
of manufacture and hitting power. Although it was first designed by Rheinmetall-Borsig
in 1940 as a private venture, the design was finalized in 1942. It met
a later RLM requirement for a new aircraft cannon that could knock down
enemy bombers with the lowest expenditure of ammunition and stay beyond
the range of enemy defensive fire.
In short, the
MK 108 was a blow-back operated, rear-seared, belt fed 30 mm cannon using
electric ignition and was charged and triggered by simple compressed air.
One drawback was that once installed, there was no method to adjust the
gun's harmonization. One distinctive physical feature was the very short
gun barrel, which gave the MK 108 a low muzzle velocity of 500-540 meters
(1640-1770 feet) per second. The maximum rate of fire was 650 rounds per
minute. The operating sequence for the MK 108 went like this:
-
An ammunition can fed the rounds to the gun by means of a disintegrating
belt
-
Once the sear was released, the bolt went forward under the action of the
two driving springs
-
A projection on the top of the bolt passed through the ring, thus extracting
a round
-
This then forced the round into the chamber and fired the round while the
heavy bolt was still moving forwards
-
The empty cartridge case reinserted itself in its link after firing
-
Ejection was achieved by means of pawls activated by camming grooves that
were cut into the top of the bolt
-
Finally, the new round slipped into position and the sequence repeated
An interesting feature was that neither the barrel or receiver moved in
recoil, the entire force of firing was absorbed by the rearward movement
of the bolt against the driving springs, which buffered against the recoil.
No locking mechanism was needed, because by the time the fired round had
overcome the inertia of the firing bolt, the round had left the barrel
and the pressure had dropped.
There were two
main types of ammunition for the MK 108 to use, a 30 mm high-explosive
self-destroying tracer ("M-Shell" or "Mine-Shell") and a 30 mm incendiary
shell. The first type was designed to cause a maximum blast effect by 
combining
a very thin shell casing with the maximum load of explosive. Tests carried
out at Rechlin (where most of the Luftwaffe aircraft and weapons tests
were done) showed that with a "M-Shell" with 85 grains of explosive, five
hits could destroy a B-17 or B-24 bomber. The second type of shell, the
incendiary, was meant to be targeted at the fuel tanks of the enemy plane.
Since some penetrating force was still needed to overcome the armor or
airframe of the target, and not have the shell break up or explode upon
contact, a hydrodynamic fuse was fitted so that the shell only exploded
once it came into contact with liquid.
Although there
were drawbacks with the MK 108, such as an insufficient flat trajectory
(the shell "arced" too much due 
to
the low muzzle velocity) and that the ammunition belts sometimes ripped
when the guns were fired while the aircraft was banking sharply, the MK
108 was chosen to be fitted in a wide variety of Luftwaffe aircraft (please
see table below) before the war's end. It was also planned to be used
in many of the project aircraft also (please see table
below), for the reasons given above: ease of manufacture, compact size
and weight and destructive power. The MK 108 30mm cannon earned a fearsome
reputation among Allied bomber crews, who named it the "pneumatic hammer"
due to its distinctive firing sound.
Rheinmetall-Borsig MK 108 30mm cannon data
| Designation |
Maunfacturer |
Caliber |
Length |
Weight |
Rounds/
Minute |
Muzzle
Velocity |
Projectile
Weight |
Cartridge
Weight |
| MK 108 |
Rheinmetall-Borsig |
30 mm
1.18" |
1057 mm
3' 5.6" |
58 kg
127.9 lbs |
660* |
500-540 m
1640-1770' |
330 g
0.73 lbs |
480 g
1.05 lbs |
* The rate of fire was to be improved to 850 rounds/minute later
Cutaway Schematic of the MK 108 Firing Operation
 |
 |
Please
click here for a large format drawing of the Rheinmetall-Borsig MK 108
30mm cannon
(opens in a new browser window)
The Rheinmetall-Borsig
MK 108 30mm cannon
Side and Top views
|————————— 1057 mm (3' 5.6") ———————–|
|
Although not a high quality photo, the small size of the MK 108 is
apparent (note hands on left)
 |
Luftwaffe Aircraft that used the MK 108
| Aircraft |
Number |
Location |
Notes |
| Bv 155 |
1 |
engine |
prototype flown |
| Bv 40 |
2 |
wing roots |
glider - 6 examples built |
| Fw 190 |
2 |
wings |
various 190 variants |
| Ta 152 |
1 |
engine |
C and H models |
| Ta 154 |
2 |
fuselage |
|
| He 219 |
2 |
wing roots |
2 additional firing upwards |
| Ho 229 |
4 |
wing |
prototype flown |
| He 162 |
2 |
fuselage |
later replaced with Mg 151 |
| Ju 388 |
2 |
ventral bulge |
J-1 night fighter version |
| Bf 109 |
1 or 2 |
engine or
under wing |
various models from
G onwards |
| Bf 110 |
2 |
cockpit |
firing upwards |
| Me 163 |
2 |
wing roots |
|
| Me 209 |
1 |
engine |
never entered service |
| Me 262 |
4 |
nose |
standard armament |
| Me 410 |
2 |
nose |
B-2/U2/R2 version |
|
|
Note: Neither list above encompasses all the actual aircraft
or aircraft projects that used the Mk 108 30mm cannon