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

[jmcalli2]

If you were a fighter pilot in WWII, would you rather have the high rate of fire of the 50 cal, or the hitting power of the 20mm? I personally feel the 50 cal was plenty hard hitting enough to take out ANY aircraft, and its high rate of fire made it even more effective...the slow rate of fire for the 20mm meant you had to be a much better marksman...

The question has to go beyond the rate of fire and size of the shell. You have other important considerations, such as:
Reliability - Shoot, it won't shoot!
Production/parts availability
Ammunition capacity; how many seconds of shooting do you need?
What is you intended target, bomber, fighter, tank?

20mm ammo may be more effective than .50 cal, but how many rounds can you carry?
Take a P-51; ~300 rounds/gun for .50 cals, 60-120 rounds/gun for the 20mm. It's size and weight that limit the 20mm ammo.

Here's an interesting reference site:

The WWII Fighter Gun Debate: Gun Tables

 

[fastmongrel]

20mm ammo may be more effective than .50 cal, but how many rounds can you carry?
Take a P-51; ~300 rounds/gun for .50 cals, 60-120 rounds/gun for the 20mm. It's size and weight that limit the 20mm ammo.

Here's an interesting reference site:

The WWII Fighter Gun Debate: Gun Tables

P51 should have been able to carry more than 120 rounds per gun. The wing had similar volume to a Tempest which carried 200 rounds per gun.

 

[Vic Nighthorse]

I imagine that this has also been covered many times before but were more air to air fighter kills from attacks on unaware targets or against evasively maneuvering targets? If your target is unaware a lower velocity and lower cyclic rate wouldn't be as big of disadvantages but mineshell like damage would still be as much of an advantage as ever, no? These are sincere rather than rhetorical questions.

 

[jmcalli2]

P51 should have been able to carry more than 120 rounds per gun. The wing had similar volume to a Tempest which carried 200 rounds per gun.

The US 20mm Hispano round is ~1/2 inch longer than the .50cal Browning round and is ~ 3 x as heavy. The P-51D carried 1840 rounds of .50cal. That would limit the P-51 to ~600 total rounds for four 20mm by weight, about 150 rounds/gun.
The Tempest wing was thicker than the Mustang, so it's not surprising it would carry more ammo.

 

[jmcalli2]

P51 should have been able to carry more than 120 rounds per gun. The wing had similar volume to a Tempest which carried 200 rounds per gun.

The P-51 wing area was 235 sq ft, the Tempest was 302 sq ft. The Tempest wing was also thicker. The Tempest was a a much larger aircraft, weighing ~50% more empty than a P-51, ~11,400 lbs to ~7,600 lbs.

 

[Shortround6]

The P-51s with cannon could carry up to 125rpg. They came after the drums, no 60 round capacity problem. Weight shouldn't have been a big issue as the ones with cannon didn't carry bombs or drop tanks so there is nothing to trade off.

The A-36 and P-51A carried two .50s in each wing and carried 250rpg for the inner guns and 350rpg for the outer guns. As did P-51Bs & Cs.

What a hypothetical version of a P-51D might carry may be a different discussion.

 

[Glider]

The 20mm Hispano is ~1/2 inch longer than the .50cal Browning and is ~ 3 x as heavy. The P-51D carried 1840 rounds of .50cal. That would limit the P-51 to ~600 total rounds for four 20mm by weight, about 150 rounds/gun.
The Tempest wing was thicker than the Mustang, so it's not surprising it would carry more ammo.

I think it's worth remembering that the Tempest had the Hispano V not the Hispano II.

So 6 x 0.5 M2 weighs more than 4 x 20mm - 174kg vs 168kg
it has an almost identical M/V - 880 m/s vs 840 m/s
it also had a near identical ROF - 800 rpm vs 750 rpm
The 0.5 was shorter 1.65m vs 2.184 meters
plus the 20mm almost certainly had slightly better ballistics

Me, I would take the 4 x 20mm every time.

 

[jmcalli2]

The P-51s with cannon could carry up to 125rpg. They came after the drums, no 60 round capacity problem. Weight shouldn't have been a big issue as the ones with cannon didn't carry bombs or drop tanks so there is nothing to trade off.

The A-36 and P-51A carried two .50s in each wing and carried 250rpg for the inner guns and 350rpg for the outer guns. As did P-51Bs & Cs.

What a hypothetical version of a P-51D might carry may be a different discussion.

Thanks for the 20mm round capacity info; I couldn't find it and gave up after an hour of looking.

My P-51D was for comparison only, but is weight isn't an issue, why stop at 20mm? Just put 6 37mm into a P-51.

There was another armament outfit for the P-51, the original one of 2 x .50 cal in the lower nose, 2 x .50 cal in the wings, and 4 x .30 cal in the wings.

Here's another interesting tidbit: the UAAC combat evaluation of the P-51 made the suggestion the the four 20mm be replaced by four .50 cal. No reason was given, which is sad because I'd love to know what it was - vibration? - reliability? - aim/accuracy? - personal preference? He only references 'standardization' almost as an after thought.
Here's the link to the report:

P-51 Tactical Trials

The armament recommendation is in section 4b of the report.

My understanding is that the British did put four 20mm in a P-51D, but other than a photo (in which the cannon looked pointed about two degrees down. For staffing?) I haven't found much on that either. I blame Glenfiddich.

 

[jmcalli2]

I think it's worth remembering that the Tempest had the Hispano V not the Hispano II.

So 6 x 0.5 M2 weighs more than 4 x 20mm - 174kg vs 168kg
it has an almost identical M/V - 880 m/s vs 840 m/s
it also had a near identical ROF - 800 rpm vs 750 rpm
The 0.5 was shorter 1.65m vs 2.184 meters
plus the 20mm almost certainly had slightly better ballistics

Me, I would take the 4 x 20mm every time.

I think it depends on your mission. The British liked that set up, and the USN moved that way too. The USAAC even went with it in the P-61, and remember many of those did not have the 4 x .50cal turret mounted. In many mission though, more ammo may be what you need at the cost of reduced firepower; escort and interception of dive bombers come to mind.

It's like asking which is a better engine, one that gives you acceleration or one that gives you cruise; are you running the Indy 500 or a 1/4 mile NHRA race?

 

[PAT303]

So 6 x 0.5 M2 weighs more than 4 x 20mm - 174kg vs 168kg

The on target effect of twelve brownings in a lighter package than 6, whats not to like about four hispano's.

 

[PAT303]

It's like asking which is a better engine, one that gives you acceleration or one that gives you cruise; are you running the Indy 500 or a 1/4 mile NHRA race?

Four Hispano's are better than 4 or 6 Browning's every way you look at it, each 20mm hit is worth 3 .50 Cals so from single seat fighters to four engine bombers the Hispano's have them covered.

 

[BiffF15]

I think it's worth remembering that the Tempest had the Hispano V not the Hispano II.

So 6 x 0.5 M2 weighs more than 4 x 20mm - 174kg vs 168kg
it has an almost identical M/V - 880 m/s vs 840 m/s
it also had a near identical ROF - 800 rpm vs 750 rpm
The 0.5 was shorter 1.65m vs 2.184 meters
plus the 20mm almost certainly had slightly better ballistics

Me, I would take the 4 x 20mm every time.

I think there is room here for a few more variables. First, if instead of 6 x .50s one went with 4 but increased the ammo it would give a commensurate increase in trigger time. Second, I think in combat there is a "it depends". Reliability would be number one, or in other words knowing that when I squeezed the trigger that the guns would work. I would take reliability as my number one choice, then increased rounds count (longer trigger time / Mk14 type gunsight?) until I became a confident shooter, then would switch to heavier caliber.

Food for thought.

Cheers,
Biff

 

[jmcalli2]

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[gottschs]

I still stand by the key issue is what type of planes are you going up against and the experience of your pilot. If going up against a 4-engine bomber fleet, .303/.50 cal won't do the trick and you need 20 and 30mm. If you going up against a fighter force and twin engine light/medium bombers then I want more ammo and more guns firing (meaning .303 and .50 cal are perfect). Regarding pilot experience, the US had alot of average pilots that had many training hours so I would want them to have plenty of ammo and more guns firing. Stories from experienced Finnish, RAF, and German pilots showed they knew exactly where to hit and used a minimum amount of ammo.

 

[pbehn]

I think there is room here for a few more variables. First, if instead of 6 x .50s one went with 4 but increased the ammo it would give a commensurate increase in trigger time. Second, I think in combat there is a "it depends". Reliability would be number one, or in other words knowing that when I squeezed the trigger that the guns would work. I would take reliability as my number one choice, then increased rounds count (longer trigger time / Mk14 type gunsight?) until I became a confident shooter, then would switch to heavier caliber.

Food for thought.

Cheers,
Biff

That was the issue in 1940/41, no one had anything reliable with a high rate of fire. The UK and USA made different choices for different reasons. The discussion today is about the two types after they were sorted and while the cannon may be more effective in a fighter it didn't adapt well to a daylight bombers defence.

 

[GrumpyOldCrewChief]

I don't think I have seen a mention, except in passing, of one of the big reasons the US chose the M2. Production and logistics standardization. Yes, most upper echelon folks knew there were "better" solutions, but also had fears that production or logistics bottlenecks might be leaving fighting outfits with the wrong ammo. Or tools to fix, or whatever else. If everybody uses the same type of item, everybody always gets the right item. As our log tails got better organized, this was less of a real concern, but always remained as a BIG ghost in everybody's thinking. Standardization was viewed by many as a key to large production numbers. As an illustration - was the Sherman a good tank? Maybe, and certainly nowhere near as good, one for one, as many of the German tanks it met. But we could build thousands of them, and keep them in service, because of (relative) standardization. Assurance of meeting the level of performance required, at most times, trumped "best". The M2 performed well enough, often enough, to be the simplest solution. Again, it hadn't the highest RoF, or the highest projectile weight. But, it was the best compromise solution available.
And don't forget other production issues. If the US wants to use Hisso V cannons in everything, where is that huge number of weapons going to come from? License manufacture is an artform, from initial negotiations, to actual production line problem solving. A whole can of worms that many - right or wrong, better or worse - would not at that time willingly opened.

Most of what we are discussing here was also known to the production planners back then. If my goal is to present the "fightingest" weapons & force available, I would agree, mostly, with them. We get the Monday Morning Quarterback experience. They didn't have that option.

 

[A.G. Williams]

The USAAF - and even more the USN - were interested in cannon as fighter armament; they tested some 23 mm Madsens prewar and wanted to order them, but this was refused on the grounds that the US .9 inch (23 mm) cannon were in development and could do the job. These turned out to be dismal failures (the US did not appear to have any competent gun designers since the death of John Browning) so the US ended up following the UK lead and buying the Hispano. This was a problematic gun, as the UK had discovered, but the US managed to make an even bigger mess of it, and it was never regarded as reliable (see Modifications and Attempts at Standardization for the sorry details). So the US was fortunate that its opponents were usually fighter planes (and in the case of the Japanese, mostly lightly protected) so a battery of six or eight .50s turned out to be perfectly adequate for their needs.

 

[pbehn]

I don't think I have seen a mention, except in passing, of one of the big reasons the US chose the M2. Production and logistics standardization. Yes, most upper echelon folks knew there were "better" solutions, but also had fears that production or logistics bottlenecks might be leaving fighting outfits with the wrong ammo. Or tools to fix, or whatever else. If everybody uses the same type of item, everybody always gets the right item. As our log tails got better organized, this was less of a real concern, but always remained as a BIG ghost in everybody's thinking. Standardization was viewed by many as a key to large production numbers. As an illustration - was the Sherman a good tank? Maybe, and certainly nowhere near as good, one for one, as many of the German tanks it met. But we could build thousands of them, and keep them in service, because of (relative) standardization. Assurance of meeting the level of performance required, at most times, trumped "best". The M2 performed well enough, often enough, to be the simplest solution. Again, it hadn't the highest RoF, or the highest projectile weight. But, it was the best compromise solution available.
And don't forget other production issues. If the US wants to use Hisso V cannons in everything, where is that huge number of weapons going to come from? License manufacture is an artform, from initial negotiations, to actual production line problem solving. A whole can of worms that many - right or wrong, better or worse - would not at that time willingly opened.

Most of what we are discussing here was also known to the production planners back then. If my goal is to present the "fightingest" weapons & force available, I would agree, mostly, with them. We get the Monday Morning Quarterback experience. They didn't have that option.

I think that there was also another aspect to logistics, the amount of investment in production capacity and guns/ rounds of ammunition already made.

 

[Glider]

I think it depends on your mission. The British liked that set up, and the USN moved that way too. The USAAC even went with it in the P-61, and remember many of those did not have the 4 x .50cal turret mounted. In many mission though, more ammo may be what you need at the cost of reduced firepower; escort and interception of dive bombers come to mind.

It's like asking which is a better engine, one that gives you acceleration or one that gives you cruise; are you running the Indy 500 or a 1/4 mile NHRA race?

You certainly have a point in particular on long distance escort missions. That said, the Tempest gave you about 16 seconds of fire which isn't bad and not that many pilots used all their ammunition. You could also point out that a burst from the 4 x 20 would do far more damage than from the 6 x 0.50 so less ammunition would be needed

 

[Glider]

I think there is room here for a few more variables. First, if instead of 6 x .50s one went with 4 but increased the ammo it would give a commensurate increase in trigger time. Second, I think in combat there is a "it depends". Reliability would be number one, or in other words knowing that when I squeezed the trigger that the guns would work. I would take reliability as my number one choice, then increased rounds count (longer trigger time / Mk14 type gunsight?) until I became a confident shooter, then would switch to heavier caliber.

Food for thought.

Cheers,
Biff

As ever good points, but the P51 development went from 4 x 0.5 to 6 x 0.5 they would have done it for a reason, presumably because 4 x 0.5 was lacking. I am not aware of any significant complaints about a lack of 'trigger time' (I do like that phrase) of the P51