50 cal (high rate of fire) vs 20mm cannon (hitting power)

[Glider]

No they wouldn't have, the shot gun effect of the eight .303's was an attempt to make up for a lack of shooting ability, that's why bombers flew back to France peppered with holes, the average pilot just aimed at big black blob rapidly filling up their windscreen and let rip, the use of cannons in that situation would just have pilots running out of ammo quicker and hitting nothing.

The use of 8 x 303 was because the RAF knew that with the much faster aircraft a more powerful 'punch' was needed and they were aware that four x 303 on its own wasn't sufficient. The RAF had decided pre war that the 20mm was the way to go but didn't have a suitable weapon, hence the multiple 303 solution.
I notice that you didn't comment on the example I gave which would have allowed for the same level of marksmanship. Clearly far fewer 20mm hits would have been needed and this would have used less ammunition and been a quicker solution. You also didn't comment on my observation that most pilots of all nations tended to fire at too long a range. It is often said that a pilot holding fire until they were close was the sign of an experienced pilot.

No you are wrong, 0.5 cals were only fitted after gyro gunsights became standard and reliable ammunition was developed, until that happened two
Hispano's with HEI and SAPI and four .303's with AP and Incendiary was the standard fitment.

On this we will have to agree to disagree.
A couple of points.
a) The GGS and the Spit XIV were introduced at roughly the same time but one was definitely not dependent on the other. Many Spitfires were built with the E wing yet didn't have the GGS. Plus of course many other aircraft were fitted with the GGS as well
b) To believe that the 0.5 didn't have reliable ammunition until so late in the war flies in the face of all the evidence. The USAAF were of course using the 0.5 for a long time before as were the RAF in the American aircraft used by them.
c) I notice that do agree that the later Spit XIV carried two x 0.5. If the 4 x 303 had been so effective there would be nothing to stop the RAF fitting the GGS and keeping the 4 x 303

 

[PAT303]

I notice that you didn't comment on the example I gave which would have allowed for the same level of marksmanship

Dowding himself commented on the lack of marksmanship in the RAF, that's why the official Spitfire and Hurricane gun divergence of 12 feet by 8 feet pattern was called the ''Dowding Spread'', because it gave average pilots a chance to hit something. Fitting cannons to aircraft, and remember cannons at that time such as the Mauser FF/M only had 7 seconds of firing time, had ammunition that didn't work properly and you had to get within 150-200m to have a chance of hitting anything, if you look at it they offered no advantage over MG's at that stage of the war.

 

[PAT303]

To believe that the 0.5 didn't have reliable ammunition until so late in the war flies in the face of all the evidence.

The .50 BMG only became reliable as a weapon, both gun and ammunition after 1943, you could almost say 1944, the Americans even reversed engineered a scaled up version of the De Wilde .303 round to get an incendiary in production they were so desperate, the AP ammunition likewise tumbled after striking the target limiting it's penetration, there was lots of issue's with aerial weapons, guns cannons ammunition sighting arrangements and training throughout WW2.

 

[PAT303]

c) I notice that do agree that the later Spit XIV carried two x 0.5. If the 4 x 303 had been so effective there would be nothing to stop the RAF fitting the GGS and keeping the 4 x 303

They did, huge numbers of MkXIV's had the 2 Hispano 4 .303 armament, it's a whole other argument as to whether 80 AP/incendiary .303's hitting a FW190 or Me109 are going to have the same effect as 25 .50BMG's.

 

[A.G. Williams]

A few comments:

Cannon shells stopped being purely HE quite early in WW2, when it was realised that fire was the main plane-killer. After that, incendiary material was added to the HE, either mixed in a chemical compound or in a separate capsule. After 1942, the standard RAF load-out for the Hispanos was 50/50 HEI and SAPI. The SAPI consisted of the standard HEI shell stuffed with incendiary material and with a hard steel nose cap instead of a fuze - they ignited on impact. So they held around 11g of incendiary, compared with 0.9g for the .50 API (the standard type of late-war .50 ammo). Incidentally, the actual armour penetration of the .50 API was about the same as the 20mm SAPI (both very good), but the behind-armour effectiveness of the 20mm was an order of magnitude greater.

All this talk about an RAF fighter pouring hundreds of .303 rounds into the bombers needs qualifying, since only a very small percentage of the shots fired actually hit. The Luftwaffe increased the calibre of their late-war cannon from 20mm to 30mm as a result of a simple calculation: it took about 20 hits from 20mm cannon to down a heavy bomber, only about three hits from a 30mm. About 5% of shots fired scored hits, which meant that on average the fighter needed to fire 400 20mm rounds at the target to bring it down - rather more than most German fighters could carry. With 30mm, some 60 rounds needed to be fired, which is much more feasible.

A similar calculation could be done for 20mm cannon vs .50 or .303 MGs, only I don't think that the RAF collected and analysed such data quite as thoroughly as the Luftwaffe. It is worth pointing out, however, that they had .303 and 20mm guns in service together for several years, which gave them a good basis for comparison. Their conclusion was that 4 x 20mm was the optimum fighter armament. Other air forces (e.g. the USSR) also preferred 20mm cannon even though they had a very good 12.7mm HMG.

 

[A.G. Williams]

Common myth say's the .303's weren't effective against German aircraft in the BoB because they were fitted with armor, what people fail to realise is the armor was fitted because of the number of pilot injuries and deaths suffered in the Battle of France.

Yes of course, but what mattered was that armour was added before and during the BoB, resulting in the .303" and 7.9mm MGs becoming less effective.

 

[fastmongrel]

In 1939/40 the Browning .303 had a big advantage over the .50 and 20mm.

IT WORKED

The Hispano 20mm and the.50 BMG didn't work when fitted in a flexible wing. No good if you can pump out 600rpm of shells if the damn thing jams as soon as it gets some G forces. Fill the bomber with holes or not fire more than a couple of rounds I know which the pilots would have chosen.

 

[A.G. Williams]

In 1939/40 the Browning .303 had a big advantage over the .50 and 20mm.

IT WORKED

The Hispano 20mm and the.50 BMG didn't work when fitted in a flexible wing. No good if you can pump out 600rpm of shells if the damn thing jams as soon as it gets some G forces. Fill the bomber with holes or not fire more than a couple of rounds I know which the pilots would have chosen.

Well, if we're just focusing on 1939/40 then I would agree that, with the benefit of hindsight, the Hispano was the wrong choice - simply because it wasn't ready in time to be useful in the BoB. My choice would have been the 20mm Oerlikon FFL - basically like the MG-FF but with a longer barrel and firing long-cased, higher-velocity ammunition. It still only weighed 33 kg so you could have six for the same weight as four Hispanos. And the MV was the same as the .303, so the trajectories would match. The Japanese Navy adopted it as the 20mm Type 99-2 and showed how it could be kept competitive by developing a belt feed, plus increasing the rate of fire from c.500 rpm to 620 and then 720 rpm right at the end of the war.

The Oerlikons were thoroughly developed and renowned for their ruggedness and reliability. They suffered from the same too-fast fuzing problem as other cannon ammo at the time, but that was rapidly solved by the RAF and the Luftwaffe by modifying the fuzes.

 

[PAT303]

The Hispano 20mm and the.50 BMG didn't work when fitted in a flexible wing.

When the British tested the .50BMG in the Martlet they jammed immediately, they later found out the Americans test fired them with the planes flying straight and level, in 1940 the .50BMG was worthless.

In 1939/40 the Browning .303 had a big advantage over the .50 and 20mm.

IT WORKED

Yep, the squadron that had cannon armed Spitfires in the BoB, I think it was 606 Squadron demanded to be re-equipped with .303 armed fighters because the Hispano's didn't work.

 

[PAT303]

My choice would have been the 20mm Oerlikon FFL - basically like the MG-FF but with a longer barrel and firing long-cased, higher-velocity ammunition. It still only weighed 33 kg

You could only have 2 Oerlikons with 60 rounds and two .303's with 300 rounds in a 1940's Spit, your going to have the same problem as the early 109's and A6M, not enough ammo for your primary weapons.

 

[PAT303]

Well, if we're just focusing on 1939/40

We aren't, it just drifted that way, it took time for the .50BMG and various 20mm's to mature into reliable weapons, until they did the .303 8mm 30/06 guns were the best choice.

 

[Greyman]

... I don't think that the RAF collected and analysed such data quite as thoroughly as the Luftwaffe.

ORS studied this in terms of length/number of bursts required to destroy an enemy aircraft (counting strikes on combat film not being practical). It was based on Spitfire, Typhoon, Tempest and Mustang combats vs. 109s and 190s (sample size vs. other LW aircraft was too small to draw conclusions).

Now, it sounds like a flippant conclusion by a disinterested party -- but it really did come down remarkably close to:

1 x 20-mm Hispano was as lethal as 2 x .5-inch Brownings​

1 x .5-inch Browning was as lethal as 2 x .303-inch Brownings​

 

[tomo pauk]

FWIW, German data and calculations about the effectiveness of guns and guns' set-ups. The number of rounds required is, to the best of my knowledge, for a 4-engined bomber. Translation by your truly.

 

[A.G. Williams]

FWIW, German data and calculations about the effectiveness of guns and guns' set-ups. The number of rounds required is, to the best of my knowledge, for a 4-engined bomber. Translation by your truly.

Very interesting, thanks for posting this. I am slightly confused about the note in brackets underneath the title: I assume that all the ammo consisted of mine shells except for the MG 151/15? It would otherwise be very odd that the MG 151/15 required almost four times as many hits as the MG 151/20 to down the target. It also seems odd that the number of guns assumed for the MG 151/15 is six, compared with only four for the MG 151/20. I don't know of any Luftwaffe planes which carried six MG 151/15!

 

[A.G. Williams]

You could only have 2 Oerlikons with 60 rounds and two .303's with 300 rounds in a 1940's Spit, your going to have the same problem as the early 109's and A6M, not enough ammo for your primary weapons.

The advantage of the trajectory matching is that the MGs could be used to correct the aim with the cannon only joining in when hits were being scored. With two .303s, one would be loaded with the B. Mk IV incendiary tracers (which left a smoke trail) while the other could have B. Mk VI (Dixon "De Wilde") incendiaries which flashed on impact.

Besides, in an alternative universe, if the Oerlikon had been chosen instead of the Hispano, there would have been plenty of time to develop a belt-feed before the war, instead of all of the time wasted negotiating, testing and debugging the Hispano.

 

[tomo pauk]

Very interesting, thanks for posting this. I am slightly confused about the note in brackets underneath the title: I assume that all the ammo consisted of mine shells except for the MG 151/15? It would otherwise be very odd that the MG 151/15 required almost four times as many hits as the MG 151/20 to down the target.

Yes, you're right wrt. the ammo type used. A bit clumsy translation on my part.

It also seems odd that the number of guns assumed for the MG 151/15 is six, compared with only four for the MG 151/20. I don't know of any Luftwaffe planes which carried six MG 151/15!

Table is probably both of theoretic and practical value? In theory, a six 15mm battery could be carried by Fw 190, Ta 152 or many of 2-engined fighters, but for practical & obvious reasons it was never attempted.
Probably the main lesson for the readers of the table was, back in winter of 1944/45, that 15mm is beyond obsolete for the target type Germans were most interested in killing?

 

[tomo pauk]

...
Besides, in an alternative universe, if the Oerlikon had been chosen instead of the Hispano, there would have been plenty of time to develop a belt-feed before the war, instead of all of the time wasted negotiating, testing and debugging the Hispano.

Bingo.

 

[Greyman]

... all of the time wasted negotiating, testing and debugging the Hispano.

To be fair to the British, I wonder how much development was lost with the fall of France. In 1939 it ruled by the Director of Armament Development that British and French guns were to be completely interchangeable for installation -- so my assumption is that development would be in tandem, and that the French firm would have the stronger grasp on the situation.

Maybe another one of those 'gee 1938 Britain, didn't you know there were going to be Messerschmitt bases in Calais?' situations.

 

[tomo pauk]

Far more time was wasted before the fall of France. Oerlikon was selling both actual cannons and licences to countries years before ww2 - both Germany and France were buying, Poland was offering PZL P.24 fighters with Oerlikon cannon.

Germans were already in 1914 at the shores of the Channel, crushing Belgium in process. Anyone want a bet that next time they will not do it, and then some?

 

[PAT303]

The advantage of the trajectory matching is that the MGs could be used to correct the aim with the cannon only joining in when hits were being scored. With two .303s, one would be loaded with the B. Mk IV incendiary tracers (which left a smoke trail) while the other could have B. Mk VI (Dixon "De Wilde") incendiaries which flashed on impact.

Besides, in an alternative universe, if the Oerlikon had been chosen instead of the Hispano, there would have been plenty of time to develop a belt-feed before the war, instead of all of the time wasted negotiating, testing and debugging the Hispano.

You have made some very good points, not picking the Oerlikon can go up there with not fitting aux fuel tanks to Spitfires as two ''shakes my head'' moments regarding the RAF.