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"How was the MG131 any better than the MG15?" Topic

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4th Cuirassier11 Dec 2023 5:49 a.m. PST

The MG131 was a 13mm belt-fed aircraft machine-gun that fired 900rpm of a 34g round at a muzzle velocity of 750 m/s.

It largely supplanted the MG15, which was a 7.92mm drum-fed aircraft machine-gun that fired 1,000rpm of a 35g round at a muzzle velocity of 755 m/s.

There appears to be no material difference in the ballistic performance of the two weapons. The 131 was 4kg heavier but its larger calibre appears not to have increased the round weight, which I thought was the point of going to heavier calibres. Does anyone know why Germany bothered with it? Was it cheaper to make, or was the belt feed hyper important or something? I don't follow why you would design a whole new machine gun simply to have a belt feed instead of designing one you can retrofit to an existing gun.

Personal logo ColCampbell Supporting Member of TMP11 Dec 2023 7:59 a.m. PST

I would surmise, from reading the Wiki articles on both guns, that the 13mm rounds of the MG131 were preferred because they inclusded high-explosive variants while the MG15 rounds were all solid shot.

MG131 -- link See "Ammunition Specifications" about 1/2 down the page.

MG15 --


Griefbringer11 Dec 2023 9:05 a.m. PST

It largely supplanted the MG15, which was a 7.92mm drum-fed aircraft machine-gun that fired 1,000rpm of a 35g round at a muzzle velocity of 755 m/s.

What is your source for the weight of that 7.92 mm cartridge? This Wikipedia page lists 14.6 g as the bullet weight for the original cartidge, and less for the later variants:


4th Cuirassier11 Dec 2023 10:09 a.m. PST

A different Wikipedia page:

Calibre: 7.9 +/- .04 mm
Cartridge: 7.9257mm Mauser
Round weight: 35.5 grams (cartridge 24 grams, bullet 11.5 grams)
Muzzle velocity: 755 metres per second (2,480 ft/s)
Rate of fire: 1,000 (possibly up to 1,050) rpm
Length : 1,078 millimetres (42.4 in) (without attachments)
Barrel length: 600 millimetres (24 in)
Weight unloaded with gunsight and cartridge bag: 8.1 kg (18 lb)
Weight loaded with gunsight and cartridge bag: 12.4 kg (27 lb)
75-round magazine unloaded: 2.27 kg (5.0 lb)
75-round magazine loaded: 4.24 kg (9.3 lb)
Weight of the 2-part loader: 0.72 kg (1.6 lb)

-- but it appears that for the MG131, 35g was the weight of the bullet, whereas for the M15, it was the weight of the entire round including the cartridge. So that makes more sense.

On a simple proportionate basis, a round that was 13mm rather than 7.92mm but otherwise the same shape would weigh about 4.4x more. This one appears to weigh 3x more but nonetheless that makes a lot more sense than a 13mm round that somehow did not weigh any more.

Griefbringer11 Dec 2023 10:24 a.m. PST

It is easy to mix up the weights of bullet and cartridge if not being careful.

I would presume that the added round weight of MG15 was the main advantage for it in air-to-air combat.

The prospect of high explosive rounds may also sound interesting, though I do have a feeling that the amount of explosive that you could actually fit inside a 13 mm bullet would be rather limited.

4th Cuirassier11 Dec 2023 10:57 a.m. PST

The belt feed would also help, in that if you're using drums the cyclic rate of 1,000rpm or whatever is completely unattainable whereas with 250-round belts you get bursts long enough that you more or less do attain it. I was struggling to see why it made more sense to design a heavier new gun than to design a belt feed or a different type of ammo.

Personal logo Mserafin Supporting Member of TMP11 Dec 2023 3:05 p.m. PST

I suspect part of the reason for the switch was to gain additional range. Attacking U.S. bombers armed with .50 caliber MGs would've daunting if you're using .30 caliber ammo.

Andrew Walters12 Dec 2023 10:29 a.m. PST

The MG34 was used in tanks even after the MG42 was widely available because you could do a barrel swap on a MG34 inside a tank but not on a MG42. Many times these decisions are based on what it's feasible to manufacture or maintain, or what it's easier to keep supplied. It may simply have been easier to handle the belts than work with the drums.

We're naturally more aware of the combat characteristics of these things, but the logistical side is often the decision making factor.

Joe Legan13 Dec 2023 3:29 p.m. PST

While Andrew is correct about logistics being important I suspect in this case it was weight of fire as griefbringer alluded too. As you have stated the 131 will throw more weight at the target in a given burst. Pre war this was debated, more smaller caliber guns verses fewer larger caliber. By mid war it was apparent that weight of fire was the answer.
Of course reliability and ammo (logistics again) played a part in deciding what guns to use internally as well.


Andy ONeill15 Dec 2023 12:44 p.m. PST

I think the mg131 was designed pre war.
There's a British report on effectiveness written 1940.
My understanding is the idea was bigger bullets, bigger holes, better armour penetration and a variety of ammo type. Some he being better than none.
And all that in a weapon could fit in space intended for smaller calibre machine guns.
It's that backward compatibility limited size and weight.
I read somewhere the electronic ignition was supposed to be particularly good at enabling high rof for a weapon firing through the prop.
The fuzes on the early he shells were too sensitive so they went off on contact.

DBS30316 Dec 2023 10:14 a.m. PST

Whilst the MG131's 13x64mm round was the smallest of WW2 aviation HMG rounds, Williams & Gustin still calculate the MG131's effectiveness at around two and a half to three times that of the MG15. As mentioned above, HE and Incendiary natures play a part, especially as the Luftwaffe realised that AP rounds were less effective even in the early war years. (Their fighters were less well protected than the RAF fighters in the Battle of Britain.)

Also worth remembering that the 131 was introduced on fighters that were otherwise quite lightly armed the Bf109Fs had only one 15 or 20mm cannon compared to the pair on the later E models, so having more powerful cowling guns helped restore the balance. Equally, the first 190s only had MGs, so, combined with the rising challenge of bombers, any extra gunpower that could be squeezed in was welcome. Remember that the very reason the MG131 was relatively small and weak was that it had been designed from the start as an easy compact weapon to fit, a lot less bother than an MG151.

Plus the first to use electric priming for improved synchronisation.

4th Cuirassier18 Dec 2023 6:43 a.m. PST


Interesting. By my reckoning, if an E had two MG15s and two 20mm cannon with 60 rounds each, then a G6 on a comparable basis had six machine guns and three cannon (because the engine mounted 20mm had 200 rounds). Add in two more cannon in gun pods and the G was something three times as heavily armed as its BoB predecessor.

DBS30318 Dec 2023 2:54 p.m. PST

I would disregard gun pods as being a burden which helped against USAAF heavy bombers but made the already overweight Bf109G more vulnerable to escorts. The key difference is that the single engine cannon on the Fs onward was not so much ammunition capacity but that the MG151/20 was a lot more powerful than the MG-FFs used by the Es, even once the Es received the minengeschoss shells. There was a penalty in that the MG151 was also a lot bigger and heavier.

Galland did not like the move to one cannon and two MGs, even if more powerful weapons, because he reckoned that whilst good pilots like himself could get the best out of the armament, more moderate pilots would be better served with more weapons to improve chance of hitting with a short burst – the same reason the RAF had gone with eight RCMGs in its Battle of Britain vintage fighters. That said, the MG-FF and the MG17 were not a great match ballistically.

4th Cuirassier19 Dec 2023 8:57 a.m. PST

Yes, that's often overlooked in the claims you hear sometimes to the effect that it was a huge error not to give Spitfires and Hurricanes four 0.50" guns instead of eight 0.303". Most RAF pilots could not hit the floor if they fell out of bed. Many opened fire at 600 to 700 yards because they thought they were at 200. It is far from obvious that missing the target almost all the time with four guns would have produced better results than missing the target almost all the time with eight. The actual losses appear to have been inflicted mostly by those nutters who opened fire at 150 to 150 yards' range, i.e. they did not start to fire until after they were supposed to have broken away.

Personal logo Mserafin Supporting Member of TMP19 Dec 2023 12:37 p.m. PST

those nutters who opened fire at 150 to 150 yards' range

You mean the Poles of 303 squadron. They had gotten used to getting that close in 1939, when their planes only had 2 .30 cal mgs. They got close to ensure hits. They did the same in BoB, but were delighted by the effect that 8 guns had at that range.

4th Cuirassier19 Dec 2023 6:07 p.m. PST

Those and others. It must have taken nerves of titanium to close to within 50 yards of an enemy aircraft at 300mph.

Joe Legan19 Dec 2023 6:47 p.m. PST

4th, agreed. To be an ace required special skills. I looked up the weight of fire for a ME 109f and it varies between 1.06 and 1.6 kg/sec of fire. The Me 109g-5 with 131 guns has as weight of fire of 2.46 kg/sec; big difference in a 2 to 4 second burst. For comparison an 8 gun spitfire had 1.72 kg/sec.


4th Cuirassier20 Dec 2023 3:04 a.m. PST

@ Joe

The other interesting thing about the Es for me is that although they had 20mm cannon those cannon only had 60 rounds each. ISTR that's about 6 seconds of fire, which is two to three bursts. After that they were down to the two machine guns. As DBS303 points out above, the MGs weren't a lot of use in helping you to hit with the cannon because they weren't much of a match ballistically.

The weapon combination that always amazes me is the 37mm / 0.30 cal on the Airacobra of which the Russians were so fond. It must have been impossible – except at suicidally short range – to hit any target with both the 37mm and the MGs. It would surely have been one or the other given the ballistic mismatch.

DBS30320 Dec 2023 7:27 a.m. PST

William & Gustin observed that arguably the RAF would have been better suited in the Battle with the 109E-3's armament, as their most important target was bombers, and the Luftwaffe would have done better with the Spitfire/Hurricane armament of eight RCMGs as they were only bothered with fighters…

If the later model Spitfires had been able to cope better with the weight of the later marks of Merlins and Griffons, the RAF would have gone for four Hispanos for all its fighters from 1942. The Spitfire Vc did fly with four Hispanos in some units, especially the SAAF in Italy.

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