'Other' countries adopt 8 LMG battery: pros cons

[tomo pauk]

The Hawker Hurricane introduced a, for the time, a powerful armament, that consisted of eight .303 machine guns. Fast firing and tightly packed (in the Hurry), they were able to harm even the heaviest bombers of the time, let alone a fighter-type A/C. The other benefit was a light weight of the guns their ammo. A better ammunition was introduced during the war.
The main shortcoming was that introduction of armor negated much of it's 'bite'.

How suitable would be the 8 LMG (light machine gun, or RCMG - rifle caliber MG) battery for other countries to adopt in the 1st half of the war, with their own guns of 7.5 - 8mm calibre?

 

[Shortround6]

Not sure about 'first half" of the war as this was pretty much a pre-war/1939-40 armament. By 1941 the British were moving to twelve .303 mg, four 20mm or mixed batteries.

While the Americans came to the party late (they kept one .50 and one .30 for far too long) , they leap frogged to fairly heavy batteries pretty quick.

The French were moving to one 20mm and four mgs so ditching the 20mm and adding more 7.5mm mg might be seen as going backwards. Took awhile for the French to adapt the Darne MG to belt feed instead of drums/magazines ( some of which were up to 300 rounds?)

Soviets likewise were moving to 20mm cannon through the prop and whatever MG they could fit in and still keep performance. Russian 7.62 mg was very high rate of fire but very expensive to produce and apparently prone to quite an array of jams.

Germans were looking for 20mm guns through the prop and 'settled' for a 20mm in each wing. Not sure what switching to three 7.92 mg instead of one 20mm MG/FF gets you except longer firing time.

Italians and Japanese both have production problems, they can't make enough guns as it is let alone supply 3-4 times as many guns per airplane. Japanese gun was also a copy of the Vickers MG and while very durable it too suffered from over two dozen different types of jams[in British service] (some of which could be remedied by a few good whacks to the gun). The British never tried to use the parent gun where the pilot or crewman could not get to the gun. At least one WW I British aircraft was fitted with 4 Lewis guns under wing ( and two Vickers) in the cowl rather than put Vickers in/under the wing. It was the lack of reliability of the Vickers that forced to the British to adopt the Browning for remote installations. Some early Gloster Gladiators got two Vickers guns and a Lewis gun under each wing when there was a shortage of Brownings.
Italians also had problems with having about the lowest powered fighter aircraft of any major power, any increases in weight/drag have to be considered very carefully.

 

[tomo pauk]

Going from a 'central battery' to the wing-mounted would involve the complications for the Soviet fighters, so no need to bother. For the Germans, at least in the B-109E, it would present more firing time than with MG FF, as you've noted - usable for a fighter that will try to snatch the air superiority from the enemy? Though, the lack of drop tank facility will reduce the benefits.
For the Japanese, at least for the Zero, it would also enable more of the effective firing time than with drum-fed cannons. Might come handy at Midway, even with an odd MG jammed?
The Americans - the heavy batteries were many times judged as a detriment for the ~1200 HP engines found at P-40s, P-39s and F4Fs. The 6 HMG batteries of the F4F-4 and most of the P-40s produced also featured a decreased number of rounds per gun, that severely reduced the firing time. The 8 LMGs battery would've weighted some 200 lbs, plus 260 lbs for 8 x 500 rds = 460 lbs. The 4 HMGs battery weighted 286 lbs (F4F-3) plus 360-516 lbs for the ammo = 646-802 lbs.
6 HMGs weighted 865 lbs, with 240 rpg; granted, a far more formidable firepower here.

 

[Garyt]

Just looking at the physics of the matter, from Navweaps -

20mm Hispano - 123gm projectile, 450 RPM, 835 meters/sec Muzzle velocity
.50 Caliber MG - 48.5gm projectile, 800 RPM, 800 m/sec
US .30 caliber MG - 9.8pm projectile, 550 RPM, 838 meters/sec

As it has been noted on another thread, the US war department felt a 20mm Hispano was worth about 3 .50 caliber MG's. There are similar numbers of disprepancy between the 20mm and .50 caliber as there are to the .50 caliber to the .30 MG.

I don't believe an exact measurement of mass truly shows how effective a round can be, as a well placed .30 can sever lines, kill pilots, etc. Though how to factor it is tough. The other factor is the explosive charge used in 20mm HE rounds.

a .30 round is about 1/5 the weight of a .50 cal, which is about 1/2.5 the weight of a 20mm. The shocking thing for me is the ROF of the M2 .50 Cal when used in aircraft.

I might be safe to say though that if it takes 3 .50 cals to equal a 20mm, it may take 3 .30 cals to egual a .50 cal.

That way an 8 LMG load is about like 2.67 .50 cals, seems to make sense.

I'd also point out that not all .30 or caliber machineguns are the ssame, nor all .50 cal/12.7mm, and of course 20mm can vary greatly.

The Japanese type 99-1 20mm cannon for instance had a decent rate of fire but a very low muzzle velocity, making both less accurate at range but also having very different ballistic qualities to the 7.7 LMG's carried by the Zero, whcih was not coorected until the A6M3 Model 22 was given the 99-2 20mm cannon which had a much better muzzle velocity.

 

[tomo pauk]

The AN/M2 version of the M1919 .30 cal was making 1200 rounds per minute, per this; that was the one used on the American ww2 aircraft.
Early .50 BMGs were good for 600 rpm (installed in early P-40s?), later (M2?) was at 800 rpm indeed; those RoF were severely cut down for syncronised installations (early P-40s, all P-39s).
The Hispano II was good for 600-700 rpm - maybe the Naval/shipborne version (provided there was one) was modified to fire at lower RoF?

 

[Garyt]

There seem to be many references as to varying ROF, some indicate whether it is cyclical or practical ROF, some do not. Makes it tough to really compare. However it seems as though there is a rough similarity between the ROF of the 3 weapons mentioned. I actually like hearing the aircraft .30 had a higher ROF than per Navweaps - it seems more in line, and it's a smaller round you would think a higher ROF.

In my opinion, sustained or practical rate of fire does not mean a ton with light AA or planes armaments - a few seconds is usually all you see for plane to plane fire, a long sustained burst will also rapidly deplete the ammo. And even for light AA, the target is usually only in effective range for a limited period of time, usually far less than a minute. I think Cyclical is the most important issue, that is unless for instance the size of the Magazine does not allow more than a 5-10 second burst (for ground to air light AA that is).

But is the ROF are similar, or at least similar enough, it boils down to the damage done and armor penetration. The 20mm is 2.5 times as heavy as the .50 - AND carries an explosive charge. The .50 is 5x heavier than the .30, but again it is not a straight weight or even kinetic energy or momentum to determine damage done.

I's still go with the 20mm equaling 3 .50's, and the .50 equaling 3 .30's. Maybe the .30 comes out even a bit worse, as it is the worst armor piercer of the bunch. And it's purely my guess here, but I'd think with the 20mm to .50 the ratio is closer to 2.5:1 than 3:1.

 

[tomo pauk]

Cyclical RoF is stated in most of the sources, when there is the practical RoF mentioned there is such a remark. For the belt-fed aircraft weapons, the cyclical RoF is the only one that matter.
Id agree with the 3:1 ratio.

 

[Shortround6]

The difference between AA work and aircraft work is that the air to air combat uses much less firing time. Keeping an enemy plane in the gun sight for more than 3 seconds was rather rare. AA gunners tended to hose the sky as the aircraft approached. Few guns exceeded their ammo supply in 3-4 seconds but many 20mm guns with 60 round drums could run dry in 6-8 seconds.

From Navweaps on the .50 cal, "Shipboard gunners of the 1930s-40s using water-cooled versions were trained to fire continuously in order to be able to "walk" the tracers onto the target. As the practical range against aircraft for this weapon was approximately 1,500 yards (1,400 m), an aircraft approaching at 200 knots would be under fire for about 14 seconds, or the rough equivalent of one belt of 100 rounds."

Some Axis guns (German, Japanese, Italian) used smaller magazines for their automatic AA guns, 12 to 30 rounds depending on type and caliber so the 'practical' rate of fire becomes much more important. But this is for AA work.

 

[Bad-Karma]

The difference between AA work and aircraft work is that the air to air combat uses much less firing time. Keeping an enemy plane in the gun sight for more than 3 seconds was rather rare. AA gunners tended to hose the sky as the aircraft approached. Few guns exceeded their ammo supply in 3-4 seconds but many 20mm guns with 60 round drums could run dry in 6-8 seconds.

From Navweaps on the .50 cal, "Shipboard gunners of the 1930s-40s using water-cooled versions were trained to fire continuously in order to be able to "walk" the tracers onto the target. As the practical range against aircraft for this weapon was approximately 1,500 yards (1,400 m), an aircraft approaching at 200 knots would be under fire for about 14 seconds, or the rough equivalent of one belt of 100 rounds."

Some Axis guns (German, Japanese, Italian) used smaller magazines for their automatic AA guns, 12 to 30 rounds depending on type and caliber so the 'practical' rate of fire becomes much more important. But this is for AA work.

Hey Shortround do you have any sources for the underlined? I actually asked a similar question in another thread and would like to read up on it. I'm very curious the difference between time on target and weapon system (if any).

 

[Garyt]

For the belt-fed aircraft weapons, the cyclical RoF is the only one that matter.

I'd agree with the short amount of time on target.

Few guns exceeded their ammo supply in 3-4 seconds but many 20mm guns with 60 round drums could run dry in 6-8 seconds.

One of the problems of the earlier Zekes for certain.

You know, factoring the 3:1 ratio, the relative difference in a 30mm and a 20mm is about the same as a 20mm to a .50 cal, so it would seem a 3:1 ratio might be accurate as well for a 30mm to 20mm given similar ROF.

Makes those ME262's with 4 x 30mm incredibly dangerous, using the 3:1 ratio about as dangerous as 36 .50 cals.

I think overkill would factor into the equation though. 1 30mm round could easily destroy a fighter by itself, so getting 2-5 rounds on target does not mean much more, you can only shoot down an aircraft one time.

And of course the low muzzle velocity is not a good thing, I've heard they were difficult score hits with at range.

 

[tomo pauk]

And of course the low muzzle velocity is not a good thing, I've heard they were difficult score hits with at range.

At 575 m/s of muzzle velocity, the trajectory was pretty curved for any distances bar the really short ones. Would take a real master (or a computing gun sight, like the K-14 used by the Allies at the last year of the ww2) to land hits on a small, fighter-sized A/C that is at some distance.

 

[Shortround6]

You have two major problems in air to air gunnery.

1. what is the actual range to the target.
2. what is the targets course and speed.

The 'computing' gun sights solved the second problem fairly well. It wasn't until they could put small radar units in fighters (like in the upper lip of the F-86) that the 1st problem was truly solved. Getting really close minimized the range problem but introduced a few of it's own.

Here is a page of instructions for the K-14.

K-14 Gyroscopic Gunsight « Lone Sentry Blog

 

[Garyt]

At 575 m/s of muzzle velocity, the trajectory was pretty curved for any distances bar the really short ones. Would take a real master (or a computing gun sight, like the K-14 used by the Allies at the last year of the ww2) to land hits on a small, fighter-sized A/C that is at some distance.

Another issue with the Japanese Type 99 Mark One 20mm used on the earlier Zeroes.

A muzzle velocity of 600 M/sec.

It seems as though while the Zero packed a pretty good punch in theory, several things hurt it. The ROF on the cannon was lower than the Hispano that the US based it's 3:1 ratio numbers on. Due to it's lower rate of fire I would not put the type 99 cannon any better than 2.5 to one in hitting power.

So you have perhaps the equivalent of 5-50 cals in the cannon, but with only 6-7 seconds of fire time, and a low muzzle velocity making it very difficult to aim at range.

The LMG's had about 33 seconds of fire time, but their ballistics were far different than the Type 99 20mm.

This compares to the F4F Wildcat that had about 22-23 seconds of firing time on target.

 

[Shortround6]

This compares to the F4F Wildcat that had about 22-23 seconds of firing time on target.

Depends on the F4F.

The -3s had around 30 seconds.
the -4s had around 18 seconds.
Assuming the guns fired at 800rpm. Adjust up or down a bit.

The difference in ballistics between the various guns ( of any nation) doesn't matter that much at close ranges. Usually 300yds or under and is a bigger problem with deflection shooting than firing from 6 O'clock or near to it.

The American .50 cal (and Russian 12.7 and German 15mm) with their high velocity and high retained velocity were probably the best for deflection shooting as they required the least amount of lead. The usually uniform batteries of the US planes also helped and at under 600yds the difference between the US .50 and 20mm Hispano is so small as to be disregarded.

Some aces were excellent shots who could almost instinctively figure out where they had to aim in order to get hits. Other Aces were mediocre shots but such excellent pilots that they could position their aircraft in such a good firing position that firing solution (aiming problem) was easy, "fill your windscreen with the enemy plane" being the advice of at least one ace.

Without some test figures trying to figure out some the 300-600yd differences gets a bit tricky, and only a hand full of pilots had any business firing at anything smaller than 4 engine bomber at over 600yds.

In general larger projectiles will retain velocity better than small ones, assuming somewhat equal density and shape. And this is were it goes to pot. Most 20mm shells had much poorer shapes than the 7.6-8mm mg bullets let alone the US .50 cal. And 20mm shell weights were all over the map. From a low of about 79 grams to a high (in the common guns) of 128-130 grams. A number of 20mm shells had enough mass (sectional density, weight per unit of frontal area) to overcome their poor shape and and so retained more of their initial velocity than the RCMG (Rifle caliber Machine Gun) ammo. This means the times of flight were not so far off as it might appear comparing MV only.

As an example using German guns (mostly because that is what I can find the figures for)

round.....................MV in M/s......V at 300m........Time to 300m in sec......V at 600m.......Time to 600m ""

7.92AP 10gr..............810...............538...................0.453........................348.....................1.159
13mm HEI 34gr.........750...............501...................0.49..........................337.....................1.22
15mm HEI 57.5 gr.....960...............743...................0.357........................583.....................0.816
20mm HEI 92 gr........695...............432...................0.551........................281.....................1.428 (1)
20mm HET 117gr.......720..............552....................0.447.......................422.....................1.101 (2)
30mm HEI 330gr........500..............370....................0.696.......................264.....................1.660 (3)
30mm HEI 330gr........500..............429....................0.649.......................370.....................1.403 (4)

Notes:
1, fired from MG-FFM
2, fired from MG 151/20 (change in nose shape and higher weight)
3, Ausf. A fired from MK 108
4, Ausf, C fired from MK 108 (notice change just due to nose shape)

From Tony Williams and Emmanual Gustin's book "Flying guns of World War II"

Please note that it is quite possible for the German 20mm projectiles even when fired from the same gun to arrive at rather different times at ranges around 600 meters.
These figures are for sea level and at higher altitudes the drag would be less and there would be less difference.
Shells fired by the Zero weighed about 128 grams and should hold velocity fairly well.
Shells fired by Japanese Army fighters weighed 79 grams and might retain velocity the worst of any 20mm shell.

Photo from Tony Williams website. What you can't see is how far down into the cartridge case the projectiles went. For instance the 13x64 German round and the British/Italian/Japanese 12.7X81 rounds all used bullets weighing within 3-4 grams of each (about 10%) other.

 

[Garyt]

The difference in ballistics between the various guns ( of any nation) doesn't matter that much at close ranges. Usually 300yds or under and is a bigger problem with deflection shooting than firing from 6 O'clock or near to it.

Yeah, understand about ballistics not being an issue at close ranges. Problem I see at any decent range - the 20mm and 7.7mm will not be hitting very close together, so at longer ranges it would seem to make sense to fire one or the other, not both. And the firepower of 2 7.7 MG's is almost negligible.

 

[tomo pauk]

...

As an example using German guns (mostly because that is what I can find the figures for)

...
20mm HEI 92 gr........695...............432...................0.551........................281.....................1.428 (1)
20mm HET 117gr.......720..............552....................0.447.......................422.....................1.101 (2)
...

Notes:
1, fired from MG-FFM
2, fired from MG 151/20 (change in nose shape and higher weight)
3, Ausf. A fired from MK 108
4, Ausf, C fired from MK 108 (notice change just due to nose shape)

From Tony Williams and Emmanual Gustin's book "Flying guns of World War II"

Please note that it is quite possible for the German 20mm projectiles even when fired from the same gun to arrive at rather different times at ranges around 600 meters.
...

The 92 gram 20mm shell would be the Mine shell, the 115 gram is the 'usual' one?

 

[Shortround6]

Yeah, understand about ballistics not being an issue at close ranges. Problem I see at any decent range -..........

Define "decent range" please? Germans figured the "effective" range against 4 engine bombers for the majority of their aircraft guns as 400 meters (leaving out the 15mm MG 151, 30mm MK 103 and the 5cm BK 5).

Max range differed with altitude (to to air density) but generally did not exceed 1100 meters at 6000 meters altitude.

And the firepower of 2 7.7 MG's is almost negligible.

Well, that "negligible firepower" accounted for a fair number of allied aircraft from Dec 1941 to the summer of 1942. A large number of the KI 43s having only two 7.7s. Problems with the 12.7mm Ho-103 (or ammo supply ?) means a lot fewer Ki 43s had one gun of each caliber than was commonly thought.
Granted you needed very good pilots to get the pair of 7.7 guns into a good firing position. And you had to be close. No figures for the British or Japanese .303/7.7mm but the German 7.9mm lost 33% of it's velocity in the first 300 meters. Since striking energy (armor or airframe structural penetration) is dependent on the square of the striking velocity long range shots don't do a lot of damage even if they do hit.

 

[Shortround6]

The 92 gram 20mm shell would be the Mine shell, the 115 gram is the 'usual' one?

That's right.

Please note that if firing shells that have even a 0.2 second difference in time of flight against a 200mph bomber (300fps) from the 3 or 9 o'clock position the impact points will be 60 ft apart. And since the heavier (115-117g) shells are the ones with tracer elements that means the mine shells mixed in the belt land where if the pilot is using tracers to correct fire?

And against a fighter

 

[Garyt]

Well, that "negligible firepower" accounted for a fair number of allied aircraft from Dec 1941 to the summer of 1942. A large number of the KI 43s having only two 7.7s. Problems with the 12.7mm Ho-103 (or ammo supply ?) means a lot fewer Ki 43s had one gun of each caliber than was commonly thought.

Negligible may not have been the best choice of words. But do you agree with the findings of Commander J.P. Monroe, head of the armament branch of the Bureau of Aeronautics, who stated a 20mm was worth about 3 .50's? And it looks like from a comparison standpoint you could make a similar comparison to a .50 cal vs. a .30 or 7.7mm or other comparable LMG's.

If that is the case, the 2 7.7 Zero MG's have about 1/9 the firepower of a standard US 6x.50 caliber rig. Negligible? Perhaps not. Severely outclassed from a firepower standpoint? Yes.

Define "decent range" please? Germans figured the "effective" range against 4 engine bombers for the majority of their aircraft guns as 400 meters (leaving out the 15mm MG 151, 30mm MK 103 and the 5cm BK 5).

I'd say maybe over 300m or so, as fighter to fighter combat is different than trying to knockdown a four engine bomber.

And here you actually help illustrate my point:

Please note that if firing shells that have even a 0.2 second difference in time of flight against a 200mph bomber (300fps) from the 3 or 9 o'clock position the impact points will be 60 ft apart. And since the heavier (115-117g) shells are the ones with tracer elements that means the mine shells mixed in the belt land where if the pilot is using tracers to correct fire?

Look a trying to fire both the 20mm and the 7.7mm in a zero. Bad idea as because if at anything other than vary close range, the 2 types of armament won't be converging on the target at all well.

Now if you combine the maneuvering abilities of a zero with a skillful pilot, they could probably get into a close advantageous firing position fairly well.

Of course, bearing in mind the 6-7 seconds of firing time they had with their cannons, long range spraying fire is probably not the best idea.

 

[Shortround6]

The Japanese 7.7mg had a MV of about 750m/s and the 20mm gun used in the early Zero fired a heavier projectile than the German MG-FF and a slightly higher velocity. They probably matched up better than the guns in the Bf 109E did. Only advantage the 109E had over the Zero was twice as much ammo for the RCMGs. Of course once you are much over 30 seconds of firing time the extra ammo starts heading for just being extra weight.

Germans "boosted" the MV of the MG-FFM by using the lighter Mine shell but since it looses velocity quicker than the standard heavy shell it doesn't do anything for long range shooting.

The big advantage the 20mm had over the US .50 cal was that the 20mm used exploding ammunition or in the case of incendiary ammunition carried a much greater payload. Sometimes as much as 10 times the incendiary material.

The US .50 was certainly much more effective than the RCMGs. It did hit a lot harder but depended (like the smaller rounds) pretty much on kinetic energy. One fast firing .50 was probably worth 3-4 RCMGs on average. And that is partly because the US .50 was easier to hit with (shorter flight times to target). It is also because few of the RCMGs used exploding ammunition (and in those that did, it was more of a target marker (shows bullets were hitting) than actual destructive effect). Please note however that while a wing mounted .50 was quite capable of firing at 800rpm or a bit above the synchronized guns (early P-40s, P-39s, Buffalos, early P-51s) often failed to get passed 500rpm.

Effective range against fighters is shorter than bombers for several reasons. Smaller target is one. Target is flying a less predictable flight path and is usually moving much faster. From the above chart and picking a worst case round, it took over 1/2 second for the 20mm MG-FFM mine shell to cover 300 meters. A fighter doing 300mph would move about (rounding off) 225 ft in that amount of time or about (36ft fuselage ?) 6 1/2 fuselage lengths. Faster planes and longer distances make things get really difficult really quick. A slower bomber with a 72ft long fuselage makes things a lot easier to get hits. Of course you need more hits to bring the bomber down.