Large gun Me 109

[tomo pauk]

The issue with 30mm guns in wing roots is the combination of the variability of the action and the variability of the power burn.
With higher rate of fire guns, both wing root guns are guaranteed ready when the 'signal' is given to fire at reasonable of fire for synchronized guns. With both guns firing, the aircraft remains a steady gun platform..But when you get into larger individual weapons, you have to slow the rate of fire way down to ensure both guns are charged - which slows firing rate way down.The other challenge is not shooting your own propeller off. Unfortunately, with the amount of power being burned in 30mm casing, and the difference in burn times between cold and hot temperatures, it becomes more than the time difference between propeller blades coming into line of fire. More of issue with MK 103 as it has more powder, but definitely concern. For some reason, pilots have real concerns about their propellers being shot off....

Note that my suggested 'baby MK 103' is firing a much weaker load than the historical MK 103, thus the variation between the propellant weights in different ammo will be smaller. Germans moved to the electricly-primed MK 103 by 1943, that insures as uniform ignition and lower loss of RoF on the synchronised guns.

FWIW, Germans (FW actually) were willing to give a go to the idea of two historical MK 103s to be installed within the Ta 152 wing roots; not that I'm subscribing to the idea 100%, but it probably warranted a look back then:

 

[z42]

The issue with 30mm guns in wing roots is the combination of the variability of the action and the variability of the power burn.

Note that my suggested 'baby MK 103' is firing a much weaker load than the historical MK 103, thus the variation between the propellant weights in different ammo will be smaller. Germans moved to the electricly-primed MK 103 by 1943, that insures as uniform ignition and lower loss of RoF on the synchronised guns.

Depending on how chonky the 'baby Mk 103' is, could fit in the outer wing positions on the FW 190 wings? Seems from the A-6 onwards the wing was able to mount Mg 151/20'ies in the outer positions, so maybe 'baby Mk 103's could also fit? No worries about synchronization and shooting away your propeller (also, with a 30mm shell I suppose there's a fair chance the blast or shrapnel would set the engine on fire as well, in addition to destroying the prop).

 

[Shortround6]

For the Germans and the Japanese (another major proponent of 30mm guns in WW II) there were a number of conflicting requirements like cost of building the actual guns vs performance. Cost also included materials like high grade steel. The MK 108 was rather cheap to build which allowed to be deployed in large numbers.

Photo from Anthony William's web site.
MK 103...................................MK 108..............IJN type 2..............JIA Ho-155...................JIN type 5
The two German shells and the three Japanese shells. Japanese traded shell weight for velocity in the two smaller rounds.
Gun......................weight........rate of fire......shell weight...........Velocity......................ME
MK 103................141kg.......360-420............440g.....................800m/s..................141,000J.
MK 108................60kg.........600-650............330g.....................505m/s....................42,100J
Type 2..................51kg.................400..............273g......................710m/s....................68,800J
Ho-155................50kg.................400..............235g.......................700m/s...................57,600J
Type 5..................66kg.................500..............350g.......................750m/s...................98,400J
Post war
Aden
MK 3M LV..........87kg.............1200................273g....................604m/s................49,800J
revolver

Please note that the Aden gun had a barrel about 50% longer than the MK 108 barrel and the Japanese type barrel was almost twice as long.
If you lengthen the barrel on the MK 108 you may have to reduce the rate of fire as the shell has to clear the muzzle to drop the pressure before the breech block moves very far.
Blow-back guns cannot be synchronized or rather API guns cannot be synchronized (or any gun that is not firing from a closed bolt).

Post war guns may have had better propellent (depending on year) and had better steel/alloys and better machining.

 

[z42]

For the Germans and the Japanese (another major proponent of 30mm guns in WW II) there were a number of conflicting requirements like cost of building the actual guns vs performance. Cost also included materials like high grade steel. The MK 108 was rather cheap to build which allowed to be deployed in large numbers.

Wiki mentions the Mk 108 was made mostly from stamped parts which indeed made it pretty cheap. And while I'm sure the 30 mm shell was more expensive than a corresponding 20mm one, presumably the difference wasn't huge. Taking into account the much lower numbers of 30mm shells to shoot down a plane, on average, than 20mm ones (say, a factor of four or so), and it seems the Mk 108 gun along with its ammunition was pretty cheap on a "cost per plane shot down" metric.

MK 103...................................MK 108..............IJN type 2..............JIA Ho-155...................JIN type 5
The two German shells and the three Japanese shells. Japanese traded shell weight for velocity in the two smaller rounds.
Gun......................weight........rate of fire......shell weight...........Velocity......................ME
MK 103................141kg.......360-420............440g.....................800m/s..................141,000J.
MK 108................60kg.........600-650............330g.....................505m/s....................42,100J
Type 2..................51kg.................400..............273g......................710m/s....................68,800J
Ho-155................50kg.................400..............235g.......................700m/s...................57,600J
Type 5..................66kg.................500..............350g.......................750m/s...................98,400J
Post war
Aden
MK 3M LV..........87kg.............1200................273g....................604m/s................49,800J
revolver

Please note that the Aden gun had a barrel about 50% longer than the MK 108 barrel and the Japanese type barrel was almost twice as long.
If you lengthen the barrel on the MK 108 you may have to reduce the rate of fire as the shell has to clear the muzzle to drop the pressure before the breech block moves very far.
Blow-back guns cannot be synchronized or rather API guns cannot be synchronized (or any gun that is not firing from a closed bolt).

Post war guns may have had better propellent (depending on year) and had better steel/alloys and better machining.

Though one should note the ADEN LV was quite a short-lived affair, evidently not being seen as successful. The later higher velocity version (using a common 30x113 shell with the French DEFA gun, and subsequently used for various Helicopter chainguns and whatnot) became the highly successful ADEN cannon. The manufacturer's website as well as wikipedia mentions a muzzle velocity of 790 m/s. Wiki also mentions a shell weight of 220g, vs. around 240g for a couple of random manufacturer websites I found. So presumably wiki is in the ballpark. Giving a muzzle energy of 68700J.

I wonder how the post-war HE shell is so light at 220g vs. 330g for the German WWII mine shell, which already should be pretty light? The ADEN shell doesn't seem to be excessively short either? Better manufacturing tech allowing making even thinner shell casings than the WWII mine shells, to pack more HE? EDIT: Actually, looking at the picture above, the German shells do seem to be quite long?

 

[Shortround6]

I wonder how the post-war HE shell is so light at 220g vs. 330g for the German WWII mine shell, which already should be pretty light? The ADEN shell doesn't seem to be excessively short either? Better manufacturing tech allowing making even thinner shell casings than the WWII mine shells, to pack more HE? EDIT: Actually, looking at the picture above, the German shells do seem to be quite long?

What is sometimes hard to see is how far into the case the Projectile goes.

Not a whole lot of room for the propellent.
An extra 10-20mm of case length can make a lot of difference.
Also make sure you are comparing like to like.

Also note that sometimes post war fuses are made of Aluminum and not brass like many war time fuses. In 20mm ammo this was worth 10-15gram per shell.

 

[z42]

I wonder how the post-war HE shell is so light at 220g vs. 330g for the German WWII mine shell, which already should be pretty light? The ADEN shell doesn't seem to be excessively short either? Better manufacturing tech allowing making even thinner shell casings than the WWII mine shells, to pack more HE? EDIT: Actually, looking at the picture above, the German shells do seem to be quite long?

I went spelunking on the interwebs for some comparison of projectile length/diameter ratios. Some findings, part from various data I found, part measured from images:

Gun / Shell	Length/diameter ratio
30x113 (ADEN, chainguns, etc.) M792 projectile	3.47
30x90RB Mk 108 M-shell	4.89
20x82 Mg 151/20 M-shell	4.33
20x102 post-war US e.g. Vulcan cannon, M56	3.85
20x110 Hispano	4.10
25x137 M792 (M242 chaingun, GAU-12)	4.68
7.62 NATO M80 / M80A1	3.75 / 3.96
155mm M107 / M795	3.92 / 5.42

One can see that the 30mm shell used by the ADEN gun is indeed unusually short. For shooting a ~220g shell, they could have used something around a 25mm size and gotten a shell with better ballistic coefficient. But maybe it doesn't matter that much at the relatively short ranges expected for air-to-air combat.

Another explanation might be that it seems the total length of the shell+cartridge is the same as for the ADEN LV, so it might be that they just didn't want to redesign the action of the gun and the barrel when they decided they wanted more muzzle velocity? Picture below from Anthony Williams ( A A.G. Williams ) site; the ADEN LV is slightly to the right of middle, with silver tip and black bottom half of the projectile, the ADEN 30x113 is two steps to the right with a yellow shell, same total length (inbetween them is the shell for the French DEFA 540, the later DEFA 550 switched to the same 30x113 as ADEN):

In any case, a ~220g shell with a muzzle velocity of 800 m/s would have been pretty sweet in WWII, whether the diameter is 30mm, 25mm, or something in between those.

Conversely, the German WWII mine shells are unusually long, perhaps an attempt to achieve a decent ballistic coefficient despite the lightweight filling of those shells?

As an aside, as for why not make the length/diameter even longer to get a better ballistic coefficient, turns out that there are limits how high that ratio can go for a spin stabilized projectile. The higher the length/diameter ratio, the faster the projectile must spin lest it becomes unstable in flight. Also one reason why tank guns went to smoothbore guns in the post-WWII era, as that allowed usage of fin-stabilized darts with very high sectional density for maximum kinetic energy penetration.

 

[peceed]

I wonder how the post-war HE shell is so light at 220g vs. 330g for the German WWII mine shell, which already should be pretty light? The ADEN shell doesn't seem to be excessively short either?

The reference shell is Ausf.C
General recipe:
2/3 of "payload volume" using the same type of steel case linearly shrunk, and a fuse made partially from light alloys that lowered its weight to 2/3.
3/2 (50% more) of propellant using a 50% longer barrel.
Exactly the same momentum of recoil and length of cartridges.

P.S. Linear shrinking of the case can work only on the section of the case above the ring. Fortunately, there can be additional savings due to the lighter fuse section.

I went spelunking on the interwebs for some comparison of projectile length/diameter ratios.

2*1.41+0.65 =3.47
3*1.41+0.65 = 4.88~=4.89

P.S. Clearly simple fit doesn't work. Pixel counting on Ausf. C sectional view is required.

 

[Engineman]

I had a stoppage on a 30mm ADEN on Live fire Air to Air . I was presented with the guilty case and the other one (last to fire) still in the revolving breech. The deformed case had jammed during the second stage of extraction. These cases are exactly 110mm from flat base to top rim.

Eng