UK goes all-in on a HMG class gun in the mid-30'ies

[A.G. Williams]

The British banked on the Hispano and that was very nearly ready for the BOB, being common fairly soon after it. The FFL was an inferior weapon to the Hispano so in my mind the decision which had to be taken before 1938 to go for an interim heavy battery of 8 x LMG was the right one.

True of course. But only if your 20mm are available and on this note its worth remembering that quite a surprising number of the Bf 109's used at the start of the BOB only had 4 x LMG. Now if the Germans who had bet the farm on the 20mm FF couldn't get all their fighters equipped with the 20mm. It would be wrong to assume that the UK could do any better with the FFL which was a later design. There is a good chance that the RAF introduction dates for the FFL and Hispano, wouldn't have been much different.

Good points. The crucial question is at what times would the armament choice be made?

The Luftwaffe did not start with the MG-FF as their first cannon, they went in a very different direction at first with fitting the very powerful but heavy and slow-firing MG C/30L in 20 x 138B calibre, just one gun fitted in an engine mounting of a prototype He 112 V5 which was evaluated in combat conditions in Spain. This concept proved to be a failure which left the Luftwaffe without an effective cannon - they planned to adopt the MG 151 but it wasn't ready for the start of the war. So they chose the wing-mounted MG-FF as an interim solution (that wasn't ready for the Battle of France, but made it just in time for the BoB), and modified the MG 151 to use 20mm ammunition in due course.

As far as the RAF is concerned they were clear that they wanted to replace the .303 with 20mm cannon, but we all know what happened there. They did evaluate earlier versions of various Oerlikons (they were out in the late 1920s) but it seems did not pay much attention to the much-improved FF, FFL and FFS which emerged in c.1935. Certainly the battery of .303 Brownings was the right decision to go for in the early/mid-1930s as they were very reliable, but if at the same time they had chosen an Oerlikon and put the development effort into that, it is reasonable to suggest that it would have emerged earlier than the Hispano. Ironically, as well as being too late for the BoB, the performance of the Hisso was reduced in the interest of reliability, while the Oerlikons were improved.

 

[tomo pauk]

This concept proved to be a failure which left the Luftwaffe without an effective cannon - they planned to adopt the MG 151 but it wasn't ready for the start of the war. So they chose the wing-mounted MG-FF as an interim solution (that wasn't ready for the Battle of France, but made it just in time for the BoB), and modified the MG 151 to use 20mm ammunition in due course.

The 1st cannon-carrying Emils (Bf 109E-3, armed with two MG FF + two LMGs) were delivered in late 1938. Total of 153 of the E-3s were delivered before 1939, or 325 before April of 1939, or 603 before October of 1939.
They were the majority of 109s manufactured between late 1938 and mid-1940, by what time the E-4 was introduced (main change that has bearing on this thread being the introduction of MG FFM instead of the MG FF.

Source for the production numbers: "German Aircraft Industry and Production 1933-1945" by Vajda and Dancey.

All-LMG armed Bf 109E-1s were produced in the same time frame as the E-3, but in lower numbers, and a lot of survivors 'lost' the wing-mounted LMGs and were retrofitted with MG FFM cannons by mid-1940, and with drop tanks by late 1940, making them equivalents of the E-4 and E-7, respectively. Source: Messrschmitt Me 109 by Radinger and Shick.

MG FF was standard outfit for the Bf 110 before mid-ww2, starting with 109B from 1938. Also the He 112B was with MG FFs.

 

[A.G. Williams]

The 1st cannon-carrying Emils (Bf 109E-3, armed with two MG FF + two LMGs) were delivered in late 1938. Total of 153 of the E-3s were delivered before 1939, or 325 before April of 1939, or 603 before October of 1939.
They were the majority of 109s manufactured between late 1938 and mid-1940, by what time the E-4 was introduced (main change that has bearing on this thread being the introduction of MG FFM instead of the MG FF.

Source for the production numbers: "German Aircraft Industry and Production 1933-1945" by Vajda and Dancey.

All-LMG armed Bf 109E-1s were produced in the same time frame as the E-3, but in lower numbers, and a lot of survivors 'lost' the wing-mounted LMGs and were retrofitted with MG FFM cannons by mid-1940, and with drop tanks by late 1940, making them equivalents of the E-4 and E-7, respectively. Source: Messrschmitt Me 109 by Radinger and Shick.

MG FF was standard outfit for the Bf 110 before mid-ww2, starting with 109B from 1938. Also the He 112B was with MG FFs.

Thanks for the clarification.

 

[Greyman]

... old wives tale ...

It's tough to take more than a step or two into official material on WWII aircraft weapons without running into concerns about deflected bullets. I won't be able to comb through for the best, most relevant excerpts but I did a quick skim through a few documents and scooped up what I came across.


Comparison of British .303" A.P. and German 7.92 m.m. A.P. ammunition
(Orfordness, October 1940)
...
(3) Attack of armour through duralumin structure.
The bullet on meeting aircraft duralumin skin and structure is deflected and retarded. This has a pronounced effect on the subsequent damaging capacity of the bullet. The magnitude of the deflection depends on the design of the bullet and on its degree of stability. The evidence tabulated below indicates that the German bullet is more adversely affected by impact on the aircraft structure than the British bullet.

In the testing referenced in this report, a Blenheim was attacked at 200 yards from dead astern using each type of ammunition. The rounds striking the rear fuselage within the projected area of the gunner's armour were examined.

Percentage deflected away from and out of the fuselage: British: 38%, German: 29%
Percentage reaching armour through the fuselage: British: 33%, German: 23%

For comparison's sake, here are the deflection percentages of British ammunition from different angles of attack:

10° off: 8%
10° above: 24%
10° below: 2%​

*****​

Notes on Types of .303 Ammunition for use in Aircraft
(Gunnery Sub Committee, Jan 1939)

(i) Mark VII Ball has reasonable penetrative capacity against light metal structures, but it is readily stopped by armour plate or heavy fittings. At flat angles of impact, e.g. astern attack, about 30% of the bullets are deflected off the skin of modern aircraft.

(ii) Mark I A.P. is slightly more susceptible to deflection at flat angles than Ball, but has superior piercing qualities when confronted by armour plate or heavy fittings at steep angles of impact. This ammunition causes rather heavier barrel wear than Ball ammunition.
...
Recent trials have proved that only a small percentage of Ball or A.P. .303" ammunition fired at a modern bomber from astern penetrates sufficiently to do vital damage, and that A.P. is slightly less effective than Ball owing to its greater susceptibility to deflection.

*****​

The estimated vulnerability of a F.W. 190 A. 3 from direct astern
(Orfordness, Dec 1942)
...
The angle of the fuselage with the line of attack is 6° and this gives the 0.303" B. Mk VII and 0.5" B. Mk.II no chance of causing a fire. These ammunitions will be for the most part deflected or broken up on the 19 s.w.g. fuselage skin; angles off astern would make these ammunitions more effective.

*****​

The vulnerability of the German Flying Bomb to fighter attack
(Farnborough, Jun 1945)
...
As a result of ground firing trials a variety of 20 m.m ammunitions, of both Service and experimental types, against flying bomb components the following conclusions were reached by C.E.A.D.
...

(ii) Plain A.P. 20 m.m. ammunitions may be rather less effective than the less rigid 20 m.m. S.A.P./I. owing to the greater ease with which the former type appears to be deflected.​

*****​

Firing Trials of Manganese steel deflector plates.
(Orfordness, January 1942)
...
The deflecting properties of 18 and 20 s.w.g. manganese steel, when attacked with 0.303" W. Mk.I A.P., are comparable with 14 and 18 s.w.g. respectively of dural. (For what it's worth, as far as I can tell with limited research, 18-22 s.w.g dural is a reasonable average for most WWII aircraft skins)

Critical angle of deflection

18 s.w.g.: 8½° – 9°​

20 s.w.g.: 5°​

 

[PAT303]

it seems the Oerlikon 20mm family was available and fairly mature by then,

The FF used in the 109 wasn't reliable, the drums were loaded with only 55 rounds instead of 60 to limit jams likewise the ammunition was faulty, the cast iron bodies of the shells either exploded to dust or into a few large chunks or not at all, furthermore all the shells into the FF/M's mine shells needed a graze fuse to work on shallow strike angles resulting in direct hits either exploded prematurely on alloy skins or again not exploding at all when striking fabric coverings. Like posted earlier the Germans replaced the FF FF/M pretty quickly after the BoB.

 

[Shortround6]

the cast iron bodies of the shells either exploded to dust or into a few large chunks

Any proof that the shells were 'cast iron'?
Cast iron has been used for mortar bombs and hand grenades. It doesn't work very well on actual cannon shells as it tends to sometimes break up in the barrel or disintegrate upom leaving the barrel due to centrifugal force. Low grade steel is not cast iron. Cast steel is not cast iron.

furthermore all the shells into the FF/M's mine shells needed a graze fuse to work on shallow strike angles

just about anybody's explosive shells had problems with grazing/sensitivity. British even used inert shells for a while (in a mix) because the HE shells were too sensitive and not penetrating.

Like posted earlier the Germans replaced the FF FF/M pretty quickly after the BoB.

They did but the MG 151/20 used the same projectiles/fuses fired out of different cartridge case/propelling charge.
The MG 151/20 fired faster and the shells left the muzzle at higher velocity. But the projectiles were the same.

 

[A.G. Williams]

Any proof that the shells were 'cast iron'?

Most cannon shells in smaller calibres like this were made by machining from solid steel bars. The M-Geschoss were different, in that they were "drawn" in the same way as the brass cartridge cases were. This enabled the shell walls to be very thin, reducing the shell weight while increasing the HE capacity.

See the photo below showing sectioned 20 x 80 RB rounds for the MG-FF(M). A standard HE-T is on the left, an M-Geschoss in the middle and an API on the right.

 

[tomo pauk]

The FF used in the 109 wasn't reliable, the drums were loaded with only 55 rounds instead of 60 to limit jams

Some good source to back up the unreliability claim?

furthermore all the shells into the FF/M's mine shells needed a graze fuse to work on shallow strike angles resulting in direct hits either exploded prematurely on alloy skins or again not exploding at all when striking fabric coverings.

What fuzes from other countries worked on fabric coverings (with source, preferably)?
Who was downing all these Hurricanes? Swordfishes during the Channel dash?

Like posted earlier the Germans replaced the FF FF/M pretty quickly after the BoB.

Not true.
FFM was in use on Fw 190s from 1941 until well into 1943 - it even gotten the 90 rd drum in the meantime -. Ditto for the the nightfighters, like eg. on Do 217s where it received the belt feed motor (200 rds per gun there) and as Shraege Musik on Bf 110s, since it could fit and was light.

 

[Shortround6]

Most cannon shells in smaller calibres like this were made by machining from solid steel bars. The M-Geschoss were different, in that they were "drawn" in the same way as the brass cartridge cases were. This enabled the shell walls to be very thin, reducing the shell weight while increasing the HE capacity.

See the photo below showing sectioned 20 x 80 RB rounds for the MG-FF(M). A standard HE-T is on the left, an M-Geschoss in the middle and an API on the right.

Thank you.
I believe you already know the following.
I believe only the Germans succeeded in making shells using the "draw" process.
The British tried for a number of years to make cast iron shells for the 25pdr and finally gave up, none were released for service. And the 25pdr was on lower end internal pressures and rate of spin.
Not all steels have the same strength which is part alloy and part heat treatment and part "work hardening". Not a lot of the last on a machined steel shell unless the speed and feed are pretty high. The Drawn steel case can have a lot of work hardening. Rifle brass in often annealed twice in manufacture as it gets very hard and brittle in the drawing process (many steps) and many bottle neck cases show a considerable variation in hardness over the length of the case. They want a tough rim/head of case material for strength but they want a somewhat softer, more flexible material for the upper body and neck for easy extraction and to conform to chamber tolerances.
Again using the 25pdr as an example there was an experimental HE shell made of higher grade steel and the weight dropped to 21lbs and amount of explosive more than doubled.
Somebody may have tried to use cast iron in WW I during the shell shortage of 1914/15 but they blew up enough guns and killed enough gunners to put a stop to the less that standard production pretty quickly, not just cast iron but really low grade steel or attempts at fabricating bodies from pieces.

It also seems that the Germans either never made "mine" or only introduced it very late (perhaps on the 30mm guns?) as there is a bit of problem with getting the whole projectile to move as one. A heavy nose section and a heavy rear section joined by a thin tube does not behave quite the same as heavy nose and light everything else. The rounded rear end of the mine shell is also less likely to concentrate stress than sharp angles or sudden changes in shape.

 

[PAT303]

Any proof that the shells were 'cast iron'?

That's my mistake, the bodies were cast steel and machined or machined from steel bar, either way the bodies weren't consistent in the way they performed when they exploded, the same was true of the Hispano shells.

 

[PAT303]

What fuzes from other countries worked on fabric coverings (with source, preferably)?
Who was downing all these Hurricanes? Swordfishes during the Channel dash?

None did, like everything it took time for both the guns and ammunition to mature, cannons of all types were underdeveloped in 1940.

 

[PAT303]

Not true.
FFM was in use on Fw 190s from 1941 until well into 1943 - it even gotten the 90 rd drum in the meantime -. Ditto for the the nightfighters, like eg. on Do 217s where it received the belt feed motor (200 rds per gun there) and as Shraege Musik on Bf 110s, since it could fit and was light.

It's more of a case that they were available and especially as shraege music guns were used at very short ranged that kept them in service, according to wiki they didn't get much love in the FW190 MG FF cannon - Wikipedia

 

[PAT303]

It's tough to take more than a step or two into official material on WWII aircraft weapons without running into concerns about deflected bullets. I won't be able to comb through for the best, most relevant excerpts but I did a quick skim through a few documents and scooped up what I came across.


Comparison of British .303" A.P. and German 7.92 m.m. A.P. ammunition
(Orfordness, October 1940)
...
(3) Attack of armour through duralumin structure.
The bullet on meeting aircraft duralumin skin and structure is deflected and retarded. This has a pronounced effect on the subsequent damaging capacity of the bullet. The magnitude of the deflection depends on the design of the bullet and on its degree of stability. The evidence tabulated below indicates that the German bullet is more adversely affected by impact on the aircraft structure than the British bullet.

In the testing referenced in this report, a Blenheim was attacked at 200 yards from dead astern using each type of ammunition. The rounds striking the rear fuselage within the projected area of the gunner's armour were examined.

Percentage deflected away from and out of the fuselage: British: 38%, German: 29%
Percentage reaching armour through the fuselage: British: 33%, German: 23%

For comparison's sake, here are the deflection percentages of British ammunition from different angles of attack:

10° off: 8%
10° above: 24%
10° below: 2%​

*****​

Notes on Types of .303 Ammunition for use in Aircraft
(Gunnery Sub Committee, Jan 1939)

(i) Mark VII Ball has reasonable penetrative capacity against light metal structures, but it is readily stopped by armour plate or heavy fittings. At flat angles of impact, e.g. astern attack, about 30% of the bullets are deflected off the skin of modern aircraft.

(ii) Mark I A.P. is slightly more susceptible to deflection at flat angles than Ball, but has superior piercing qualities when confronted by armour plate or heavy fittings at steep angles of impact. This ammunition causes rather heavier barrel wear than Ball ammunition.
...
Recent trials have proved that only a small percentage of Ball or A.P. .303" ammunition fired at a modern bomber from astern penetrates sufficiently to do vital damage, and that A.P. is slightly less effective than Ball owing to its greater susceptibility to deflection.

*****​

The estimated vulnerability of a F.W. 190 A. 3 from direct astern
(Orfordness, Dec 1942)
...
The angle of the fuselage with the line of attack is 6° and this gives the 0.303" B. Mk VII and 0.5" B. Mk.II no chance of causing a fire. These ammunitions will be for the most part deflected or broken up on the 19 s.w.g. fuselage skin; angles off astern would make these ammunitions more effective.

*****​

The vulnerability of the German Flying Bomb to fighter attack
(Farnborough, Jun 1945)
...
As a result of ground firing trials a variety of 20 m.m ammunitions, of both Service and experimental types, against flying bomb components the following conclusions were reached by C.E.A.D.
...

(ii) Plain A.P. 20 m.m. ammunitions may be rather less effective than the less rigid 20 m.m. S.A.P./I. owing to the greater ease with which the former type appears to be deflected.​

*****​

Firing Trials of Manganese steel deflector plates.
(Orfordness, January 1942)
...
The deflecting properties of 18 and 20 s.w.g. manganese steel, when attacked with 0.303" W. Mk.I A.P., are comparable with 14 and 18 s.w.g. respectively of dural. (For what it's worth, as far as I can tell with limited research, 18-22 s.w.g dural is a reasonable average for most WWII aircraft skins)

Critical angle of deflection

18 s.w.g.: 8½° – 9°
20 s.w.g.: 5°​

Looking at this photograph you can see both holes and tears from the different impact angles.

 

[SaparotRob]

Thank you.
I believe you already know the following.
I believe only the Germans succeeded in making shells using the "draw" process.
The British tried for a number of years to make cast iron shells for the 25pdr and finally gave up, none were released for service. And the 25pdr was on lower end internal pressures and rate of spin.
Not all steels have the same strength which is part alloy and part heat treatment and part "work hardening". Not a lot of the last on a machined steel shell unless the speed and feed are pretty high. The Drawn steel case can have a lot of work hardening. Rifle brass in often annealed twice in manufacture as it gets very hard and brittle in the drawing process (many steps) and many bottle neck cases show a considerable variation in hardness over the length of the case. They want a tough rim/head of case material for strength but they want a somewhat softer, more flexible material for the upper body and neck for easy extraction and to conform to chamber tolerances.
Again using the 25pdr as an example there was an experimental HE shell made of higher grade steel and the weight dropped to 21lbs and amount of explosive more than doubled.
Somebody may have tried to use cast iron in WW I during the shell shortage of 1914/15 but they blew up enough guns and killed enough gunners to put a stop to the less that standard production pretty quickly, not just cast iron but really low grade steel or attempts at fabricating bodies from pieces.

It also seems that the Germans either never made "mine" or only introduced it very late (perhaps on the 30mm guns?) as there is a bit of problem with getting the whole projectile to move as one. A heavy nose section and a heavy rear section joined by a thin tube does not behave quite the same as heavy nose and light everything else. The rounded rear end of the mine shell is also less likely to concentrate stress than sharp angles or sudden changes in shape.

I was in the wire business for a while so I enjoyed this post.

 

[Shortround6]

the bodies were cast steel and machined or machined from steel bar

trying to cast them would be a lot of work and will give you different results.
There is a difference between hot rolled sheet/bar steel and cold rolled even of the same alloy.
The grain structure is different and the process of rolling the steel work hardens it a a bit.
If you machine enough off the outside you get rid of the work hardened part but the different grain structure goes all the way through.

Go back the pictures of the shells. The left hand one will break up into one or more big pieces and a number of little ones. At times the entire rear section came off in one piece. Not sure what happened to the fuse. British fuses were brass, The German ones may have been.
The Mine shell is never going to give any big pieces (or very, very rarely).
A British HE shell (it it was without tracer) might give you a lot medium fragments and a few fragments from the rear. With a tracer compartment there is a lot more steel in the back.

Shells changed over time. Both sides got better at matching the types of explosive to the type of steel to help optimize the fragment size. Assuming that the best preforming explosive was safe (not too sensitive) and and not too hard/expensive to make. In 1939-40-41 it may have been enough just to have shells that would go bang. Things got even better during the 1950s and 60s. They had the time to short out the grain structure/hardness of the shell bodies and pair it with an explosive mixture to give better optimized fragmentation.

Fuses had to go bang in somewhere well over 1/100 of a second and closer to 1/1000 of second. Too quick and they went off on the skin and too slow ( at 2000fps) and they were going out the other side. Took a bit of juggling between spring tension/restraining bands and part weights and distances. Obviously flight time could make a big deference in fuse function.

 

[Shortround6]

I was in the wire business for a while so I enjoyed this post.

You can correct me when I mess up
We got a lot of plant tours at the fire department.
"Yes the tiny flames coming out of that 20ft long box the rolled steel is going though are supposed to be there".
Heat treating/annealing in controlled atmospheres.
Or other things to stay away from.

 

[PAT303]

Fuses had to go bang in somewhere well over 1/100 of a second and closer to 1/1000 of second. Too quick and they went off on the skin and too slow ( at 2000fps) and they were going out the other side. Took a bit of juggling between spring tension/restraining bands and part weights and distances. Obviously flight time could make a big deference in fuse function.

I don't has access to a lot of the info on ammunition because I either can't find it on my computer or it's at home, I work in remote outback OZ, I have results from RAF testing that said the best ammunition was the inert practice ammunition because it just blasted it's way through wrecking everything in its path because there was no fuze to go wrong, I really need to organise my files.

 

[PAT303]

Some good source to back up the unreliability claim?

Somewhere yes but can't find it, one thing that also had me thinking was why the cannon shells were blunt, I found info on the Hispano that showed it didn't work with pointed shells in the drum loaded models, the tips would dig into the side of the drum and jam, I can image the same issue could also be attributed to the Mauser, Madsen made pointed spitzer style shells but in testing the number of jams increased with them.

 

[A.G. Williams]

It also seems that the Germans either never made "mine" or only introduced it very late (perhaps on the 30mm guns?) as there is a bit of problem with getting the whole projectile to move as one. A heavy nose section and a heavy rear section joined by a thin tube does not behave quite the same as heavy nose and light everything else. The rounded rear end of the mine shell is also less likely to concentrate stress than sharp angles or sudden changes in shape.

All I can say as a collector is that 20mm M-Geschoss are very common, in both the 20 x 80RB (MG-FF/M) and 20 x 82 (MG 151/20 cases). Light-alloy fuzes were developed (although speaking from memory I think that they were mainly used with the conventional HE-T shells because M-Geschoss shells were not well suited for tracers). So there was an "intermediate" weight loading, in between the "full weight" 134 g HE-T of the MG-FF and the M-Geschoss 92 g of the MG-FFM; the 115 g HE-T. Propellant loads were adjusted to meet the recoil requirements when using the 115 g shells in the MG-FFM.

In 30 mm for aerial combat (i.e. in the 30 x 90RB MK 108) I believe that M-Geschoss were the only shells used. Earlier generations of 30 mm HE shells were used in the big, mainly ground-attack, MK 101 and MK 103 cannon (30 x 184B ammo).

 

[tomo pauk]

one thing that also had me thinking was why the cannon shells were blunt, I found info on the Hispano that showed it didn't work with pointed shells in the drum loaded models, the tips would dig into the side of the drum and jam, I can image the same issue could also be attributed to the Mauser, Madsen made pointed spitzer style shells but in testing the number of jams increased with them.

I'm not sure what Mauser has to do with MG FF.
The 20mm HE shell is bound to look blunt since the design and working principle of the fuse dictates it. the bigger the caliber, the HE shell looks less blunt.

It's more of a case that they were available and especially as shraege music guns were used at very short ranged that kept them in service, according to wiki they didn't get much love in the FW190 MG FF cannon - Wikipedia

The main quality of a weapon of war is 'it is available'.
Where is specifically said that 'they didn't get much love in the Fw 190'?