20mm MG FF. Was it so bad?

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I think there has always been a tendency for different organisation to have a preference for an emphasis on Kinetic or chemical energy.

In the 50's to the 70's Europe tended to go for the Chemical preference (think DEFTA and ADEN 30mm) whereas the USAF tended to go for the Kinetic approach (20mm of various types).

Today the preference seems to be in the 25-27mm sized cannon

You pay your money and take your choice.
 
The Americans tended to favor short time of flight, which almost automatically means high kinetic energy.

I am not sure which was the primary driver/desire.

You don't get short time of flight without high velocity and high velocity means high kinetic energy.

High velocity also means less chemical energy as a high velocity projectile needs thicker walls to stand up to the stress of firing, and/or the higher rotational speed although that might be able to adjusted a bit with the rate of twist in the rifling.

You kind of have to start with one objective in mind and the rest kind of follows, trying to mix the two might not end well, neither one or the other.

The Germans only got the mine shell to work because of Rheinmetall's expertise in deep drawing steel that allowed for a thin but strong shell body but even they didn't try to use it in high velocity guns like the 20mm flak guns.
 

If momentum is the important factor, the MG 151/20 has 14% more momentum than the MG FF.

And 30% more kinetic energy.
 
Again, explosive content gets a sizeable bonus. I'm sure the much higher RoF and ammo capacity were big attractions, aside from eliminating harmonization effects.
 
Higher energy means higher velocity which means stronger recoil which means higher stress on the airframe. On the other hand heavier gun means better absorption of the recoil...strength of the recoil might influence accuracy...
 
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Some more details on ammo itself
 

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Well those pieces are rarely being found around here so I am not much into it as I am in aircraft bombs. But in this publication you have data on rounds being used in both guns especially distance of self-destruct which is different. For example 2cm HEI-SD self destruct time is 3 sec. On FF it will go off after 1100m and on 151 on 1300m. And you have to be careful when watching the images as FF cartridge case is 80.6mm long and 151 is 81,8mm long. Also FF cartridge case is narower. Compare on pages 121-132.
 
Here are both cartridges with Mine-Geschoss or mine round, left for MG FF and right for MG 151. Notice the difference on cartridge sizes. Hispano and Oerlikon rounds had even longer cases both being 110mm long.

 
alsaad said:
Higher energy means higher velocity which means stronger recoil which means higher stress on the airframe. On the other hand heavier gun means better absorption of the recoil...strength of the recoil might influence accuracy...
Click to expand...
Engine mountings inherently absorb recoil stress better. Wing guns are "pressing" against the spars, which are perpendicular to the recoil. This inherently allows greater vibration than engine mountings. The M.S.406 was handicapped by this--they had no experience with wing-mounted cannon and the Hispanos had tremendous recoil compared to the MG-FF.
 
MG-FF (and the Japanese Type 99) were light enough that they could also be used as defense guns in bombers, at a time when this role was usually covered by weapons firing rifle caliber cartridges. I was looking at the ballistic table in that 'German Airborne Ordnance' doc linked by Alsaad and I was a bit surprised to see that the lighter Minengeschloss employed in the FFM had worse ballistic characteristics that the slower 115g round: despite starting out at 695m/s versus 585m/s, it bled speed much faster and, after only 500m, it was already slower than the heavier round; it must have had a worse BC, since momentum should be more or less the same taking into account speed and weight differences. I'm curious how the heavier Japanese bullets (128g) fared.

On a side note, I found this video of a guy firing a Oerlikon FFL light AA gun, the 'bigger brother' of the MG FF and Type 99-I.

The cyclic rate is truly low: the motion of the obturator return spring is fully resolved in the video (while for many MGs it's so fast it appears blurred). Also changing the drum magazine requires a bit of effort (imagine being the tail/waist gunner on a Betty bomber having to change the drum while under attack by fighters!)
 

The standard 115g HE shell had a similar aerodynamic (or rather, non-aerodynamic!) shape to the 92g M-Geschoss. For projectiles of the same calibre and shape like these, the rate at which they lose velocity is directly linked to the difference in weight.


The RoF was officially 470 rpm. This compares with 450-600 rpm for the .50 M2HB.

Yes, the full magazines were not easy to manoeuvre, especially on board a small ship in stormy weather...
 

MS.406 have had engine-mounted cannon, not wing-mounted.
The 1st aircraft with wing-mounted Hispanos were the MB.150 series fighters.

msxyz said:
MG-FF (and the Japanese Type 99) were light enough that they could also be used as defense guns in bombers, at a time when this role was usually covered by weapons firing rifle caliber cartridges.
Click to expand...

French were installing the big Hispano as a defensive weapon on their new bombers. Granted, the FF series will be far handier, being 1/2 of the weight, and with lighter ammo drum; indeed one will probably want a belt-fed weapon instead of a drum- or magazine-fed one.
 
The Japanese Navy, when using the MG FF as a defensive gun, used 15 round magazines. A split between rounds fired before reloading and being able to reload faster and have an easier to aim gun.
Germans used 30 round magazine/drum in the forward firing positions on the HE 111, FW 200 and other bombers tasked with strike missions. Their actual utility as defensive weapons is questionable.

French dorsal gun mount for the big Hispano was hydraulic powered.

Belt feed was always desired, achieving it was the problem. For a flexible gun you needed a flexible feed chute and auxiliary feed motors in the chute/system. Fixed guns (prop hub, wing, Schräge Musik ) was a much easier system to figure out for belt feed.

BC, (Ballistic Coefficient ) takes into account both the shape/form of the projectile and the mass, mass is figured as sectional density. Sectional Density is the weight per unit of frontal area. A heavier projectile of a given caliber/diameter will always have a higher SD, whether the lighter projectile can make up for it with better shape is a question.
 
tomo pauk said:
MS.406 have had engine-mounted cannon, not wing-mounted.
The 1st aircraft with wing-mounted Hispanos were the MB.150 series fighters.
Click to expand...
Ha! I know that so well and still managed to just type the wrong thing for no reason. Even had an image of the M.B.152 in my mind as I did.

The French installed the HS-404s in non-enclosed turrets--hydraulic powered mounts. Apparently not even close to practical otherwise.

The installations were failures because it was essentially impossible to reload the HS-404s. The length of the weapon made it very had to reach the drum to remove one and install another. The weight of the drums was far greater than the MG-FF, so they were very hard to lift and maneuver, and trying to get one into place with your arms at full extension ... well, imagine trying to hold 20+kg with your arms fully extended in front of you.

Edit: Ninja'd by Shortround!
 
Some data from my forthcoming book on auto cannon and their ammo:

Examples of the weight of empty drum magazines from WW2 are as follows:

2 cm Oerlikon L (20 x 99RB): 15 rounds, 2.0 kg

2 cm MG-FF (20 x 80RB): 90 rounds, 11.5 kg

20 mm Hispano (20 x 110): 60 rounds, 10.8 kg

20 mm Oerlikon S (20 x 110RB): 60 rounds, 14.5 kg (29 kg filled)

The Hispano drum was significantly lighter than the Oerlikon S's (although less satisfactory for various reasons). The ammo weighed about the same, so a full Hispano 60-round drum would weigh around 25 kg.
 
I should perhaps add that Hispano knew the HS 404 was too big for a flexible mounting. They designed a much smaller gun around its own ammunition which had a much shorter case and a lot less power - it was designated the HS 405. I know some of the ammo was made, but I don't know if they got as far as making a gun.
 
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