20mm cannon, best, worst, specs, comparison to LMG, HMG etc.

[davparlr]

I can think of a lot of good reasons for the Navy "keeping" the .50s.

1. We have a lot of left over .50 cal ammo.
2. We have a lot of left over parts for .50 cal guns.
3. We would have to retrain our armorers.
4. We would have to print new (more) training manuals.

And probably more along those those lines.
The truth is the Navy "kept" the .50 cal for a very short space of time and only for planes whose design was well advanced.

There were 297 F4U-4Bs with 20mm guns and about 550 F4U-5s with 20mm guns completed post war along with about 200 close support models.

The 652 Bearcats with .50 guns were followed by another 600 with 20mm cannon.

While some F7Fs carried both the majority used four 20mm cannon.

The 60 FH-1 Phantoms (1600lb thrust engines) with .50 cal guns were followed by 762 FH-2 Banshee's (3000lb thrust engines to start) with 20mm cannon. Prototype contract was placed March of 1945, 5 months after the conference. the FH-1 can be traced back to Jan 1943, with final configuration decided on in early 1944.

The 30 NA FJ-1s were followed by the FJ-2s with 20mm guns but these were actually modified F-86Es so don't count.

Vought, however, did come up with the unlamented F6U-1 during this time frame (Navy request for proposals issued in Sept 1944) Plane had four 20mm cannon but might have been better with the .50 cal guns because of serious under power issues.

No Navy fighter after the ones mentions used .50 cal guns.

All correct and all miss the point. There was no apparent ergency at the fleet to replace the .50s for the reasons you mentioned and mainly,....the .50s worked fine for the job they had to do, at least to the men whose life depended on them.

 

[Shortround6]

All correct and all miss the point. There was no apparent ergency at the fleet to replace the .50s for the reasons you mentioned and mainly,....the .50s worked fine for the job they had to do, at least to the men whose life depended on them.

No, it does not miss the point. unless you are sending out refit kits to existing aircraft the replacement of .50 cal armed planes by 20mm armed planes didn't take that long, all things considered. It could take several months to get a plane from a factory door to an operational unit in the far pacific. Even refitting existing aircraft could have taken months from when the decision was made.
The US Navy bought 90 jets armed with .50 cal guns, the FH-1 design was started in 1943 and it was pretty much finalized months before the conference. The FH-2 Banshee with the 20mm cannon was being worked on in the Spring of 1945 with a full size mockup being completed in April 1945. It doesn't sound like the Navy was wasting much time in going for the 20mm cannon.

The work on the NA Fury was also well under way at the time of the conference. With prototypes ordered within weeks of the conference, preliminary design work would have going on for a number of weeks or months before hand, there is also the question of how much NA borrowed from the P-51 for this design for example it might have speeded things up to to copy as much of armament installation as possible (Fury 1 guns were in the wings).

As an example of how long it some times took things to get into service, the first F4U-4s came off the production line in Dec of 1944, but the first ones didn't reach the Philippines until April. at this point 500 had been produced. First operations of the F4U-4 aren't until May 1945. The Navy had ordered 300 F4U-4s with 20mm cannon back in Jan. of 1945. Now consider that the navy had issued a letter of intent to Vought to develop the F4U-4 model in Jan of 1944. 16-17 months from letter of intent to first combat use. While changing the guns isn't any were as big a project it wasn't going to be a matter of weeks to rearm the fleet.

 

[davparlr]

No, it does not miss the point. unless you are sending out refit kits to existing aircraft the replacement of .50 cal armed planes by 20mm armed planes didn't take that long, all things considered. It could take several months to get a plane from a factory door to an operational unit in the far pacific. Even refitting existing aircraft could have taken months from when the decision was made.
The US Navy bought 90 jets armed with .50 cal guns, the FH-1 design was started in 1943 and it was pretty much finalized months before the conference. The FH-2 Banshee with the 20mm cannon was being worked on in the Spring of 1945 with a full size mockup being completed in April 1945. It doesn't sound like the Navy was wasting much time in going for the 20mm cannon.

The work on the NA Fury was also well under way at the time of the conference. With prototypes ordered within weeks of the conference, preliminary design work would have going on for a number of weeks or months before hand, there is also the question of how much NA borrowed from the P-51 for this design for example it might have speeded things up to to copy as much of armament installation as possible (Fury 1 guns were in the wings).

As an example of how long it some times took things to get into service, the first F4U-4s came off the production line in Dec of 1944, but the first ones didn't reach the Philippines until April. at this point 500 had been produced. First operations of the F4U-4 aren't until May 1945. The Navy had ordered 300 F4U-4s with 20mm cannon back in Jan. of 1945. Now consider that the navy had issued a letter of intent to Vought to develop the F4U-4 model in Jan of 1944. 16-17 months from letter of intent to first combat use. While changing the guns isn't any were as big a project it wasn't going to be a matter of weeks to rearm the fleet.

Let me quote from the report. This quote is from Commander J.P. Monroe, head of the Armament Branch of the Bureau of Aeronautics, U.S. Navy.

I think that was a very good presentation of the arguments against the 20 mm., and it shows the Army's point of view, which I have been wondering about for a long time. We are going to let our case for the 20 rest entirely on the way the Fleet wants it. If the 20's are wanted, they will get them; if not, we will stick with the 50's. I think the 20's are coming in here shortly.

My opinion is that, since they were not there shortly, the fleet must have felt that the 50's were adequate for job. If they had determined that they were inadequate or ineffective, I think the 20's would have already been in the pipeline. Apparently you disagree.

 

[claidemore]

When you say, "shot down 5 in one day", you mean they shot down 5 in one flight correct?

When Emil Lang shot down 18 planes in one day, how many missions did it take for him to do that? My point on 50 caliber armamament for use against fighters only, is that 400 rounds per gun give enough ammo to shoot down 5 or more on a single flight without landing to reload.

Some of those listed got 5 in one mission,(Bert Houle for instance) others flew more than one mission.
If Wally McLeod could shoot down a 109 with 3 cannon rounds, and the Mk IX carried 240 20mm rounds, he could theoretically shoot down 80 planes without landing to reload. Without using his machineguns.
There were 19 Mustang pilots who got 5 kills/claims in one day (afaik), and given the mission most of them were flying, they probably did it in one sortie. So we know the Mustang could do it, but that doesn't mean another plane couldn't. We're comparing a long range escort fighter and mission that spends hours over enemy territory (target rich environment), vs (if we compare to a Spitfire) a short range fighter with an entirely different mission. Swap weapons in the two planes....do you now get 19 Spitfire aces in a day and only a handfull of Mustang aces in a day? Not likely, cause the weapons are not the limiting factor, the plane, the mission, and the opportunity at multiple targets is.
If the six gun .50 armed plane was the magic forumla for ace in a day status, there would be a whole bunch of Kittyhawk and Warhawk ace in a day guys from North Africa, but there isn't. The only RAF one was Caldwell and his P40 had two .50s and six .303s.
'Ace in a day', as a measurement of gun effectiveness, just doesn't work unless you remove all the variables. You need to put the same planes in the same missions with different armament and you might be able to draw some conclusions.

 

[claidemore]

Question: how many fighter sorties were flown by American pilots (airforce, navy and marines) during WWII?

 

[Milosh]

Question: how many fighter sorties were flown by American pilots (airforce, navy and marines) during WWII?

United States Army Air Forces in World War II

 

[Ratsel]

so what I'm getting from this thread is that:

2x MG131 + 1x MG151/20 is roughly = to 6x .50 or 8x .303.

so in comparison:

1x Mk108 would be roughly = to 2x MG131 + 1x MG151/20 or 6X .50 or 8x .303
1x Mk108 + 2x MG131 would be roughly =/> 8x .50

or am I off the mark here?

 

[Shortround6]

My opinion is that, since they were not there shortly, the fleet must have felt that the 50's were adequate for job. If they had determined that they were inadequate or ineffective, I think the 20's would have already been in the pipeline. Apparently you disagree.

I guess it depends on how you define "shortly". It certainly looks like within a few months of that report the Navy was no longer ordering fighters with .50 cal MGs. Accepting fighters already on order with them yes. They did not refit any fighters with 20mm cannon.
While it took until late 1948 to get those 90 Early Jets delivered, the follow up designs, already being worked on in the spring of 1945 were equipped with 20mm guns. Some of this overlap was due to the end of the war and mass cancellations of contracts. We have already gone over the over 1600 Piston engined fighters armed with 20mm cannon delivered from about the end of the war on, and ordered earlier, in spite of the mass cancellations.

Now compare "shortly" to the Air Force. While they did use 20mm cannon on the F-89, continuing their tradition of using 20mm guns on night fighters, and did fit a prototype F-88 with 20mm cannon they didn't finally abandon the .50 cal MG for day fighters until the F-100. About 5 -6 years after the navy, based on new designs and not using old planes as reserves.

My opinion is that the 20mm guns were in the pipeline for the Navy. They just didn't rewrite existing contracts and change existing delivery schedules to get them.

 

[Shortround6]

so what I'm getting from this thread is that:

2x MG131 + 1x MG151/20 is roughly = to 6x .50 or 8x .303.

so in comparison:

1x Mk108 would be roughly = to 2x MG131 + 1x MG151/20 or 6X .50 or 8x .303
1x Mk108 + 2x MG131 would be roughly =/> 8x .50

or am I off the mark here?

This is one of those great "it depends" questions

From a Chart in Tony Williams ans E. Gustins book.
Ratings for fire power ONLY, for one gun.

.303 MG=21
MG131=45
.50cal=58
151/20=204
MK108=580

These ratings depend on how much you value explosives vs kinetic energy.
They do not take into account efficiency (weight of installation vs firepower).

They most especially DO NOT take in probability of getting hits, which is going to change drastically with range, size of target and speed of target due to velocity of projectiles and shape of projectiles (flight time to a given distance).

They also do not take in combat duration (firing time) as that is also a function of the weight of the installation.

Edit> if you go back to the second post in the thread, Jabberwocky posted some very good links, the second one should answer your question very well.

 

[claidemore]

Thanks Milosh, very informative site.
Looking at Jan 43 to May 45,for USAAF it looks like 1,346,113 fighter sorties, from which there resulted 43 USAAF ace in a day (AIAD) pilots. If we assume that all those 43 AIADs were earned in a single sortie, that would be 3 onethousands of a percent AIADs per sortie, or one AIAD per 31,304 sorties. And that's without 1942 sorties. Unless my math is wrong, math was never my strong suit.

 

[Glider]

My opinion is that, since they were not there shortly, the fleet must have felt that the 50's were adequate for job. If they had determined that they were inadequate or ineffective, I think the 20's would have already been in the pipeline. Apparently you disagree.

Its worth remembring that the F6-F5 Hellcat used in WW2 was designed to be armed with 2 x 20mm and 4 x HMG. The reason why the aircraft were armed with 6 x HMG was that the US built 20mm was very unreliable. I tend to look at it with a view that the 20mm may well have been better, but the 0.5 wasn't ineffective or inadequate.

 

[Ratsel]

This is one of those great "it depends" questions

yes, I was looking at those charts. and everyone that mentions the MG131 says it has a rate of 900rpm, but I beleive with the electronic syncronization (becouse the guns fired through the prop arc) it was reduced to around 810rpm. another thing none list a 13mm projectile which the MG131 used.

one more odd thing, they used the example of 'gun power' in a P-40E WarHawk as 6800, then the Mustang as 8800, funny becouse they use the exact same 6x M2 50s. different rounds perhaps? I hope not becouse those charts would not be a good comparison. its should utilize the std. round if thats the case.

 

[Shortround6]

In the third chart the rate of fire is taken into account, the > (s) < indicating that the gun is synchronized to account for the difference in the "gun power" and "time to fire 2320" columns.

The first chart lists two different projectiles for the MG131 but in the last column they are averaged together. Lots of guns had a variety of ammunition they 'could' fire but some of it was not produced or used in large quantities.

As to the > 'gun power' in a P-40E WarHawk as 6800, then the Mustang as 8800< I believe you misread the column title. Those numbers appear in the "AmmoPower" column which is a total of the power of the rounds the aircraft carried. The P-51s carried more rounds per gun and so would have more total power (firing time ) available to them. This also indicated buy the "weight " column which is gun weight+ammo weight. P-40E weight is 332KG and P-51 weight is 385KG, since the guns didn't change (much ) the difference is in the amount of ammo carried.

 

[Ratsel]

Thanks for the explanation. Personally, I`d very much doubt that in terms of damage done, there would be much difference between any of the 12.7mm 13mm calibers, regardless of kinetic energy. The more powerful rounds simply made a neater hole in the thin alumiunium skin.. of course ballistics were better with the bigger munition. The MG 131 had OTOH much higher Rate of Fire than the M2 Brownings, and it was much lighter as well. The Soviet UBS was probably the best of the lot, good ballistics, light weight, high RoF.

 

[Shortround6]

People do tend to make a lot out of minor differences but there was quite a difference between the big 12.7-13mm rounds and small ones.
The small ones (German, Italian, Japanese Army) were around 9600-10600 joules of muzzle energy. The big ones (US,Soviet, French, Japanese Navy, Belgian) were around 15,000 to 19.200 joules. Their heavier, pointier bullets held velocity better too. The German 13mm HE bullet lost 33% of it's velocity by the time it reached 300 meters and 55% of it's velocity by the time it reached 600 meters. I believe this is at sea level and would be much less of a reduction at high altitudes.

While the big ones do punch neater holes in sheet aluminium they can break things like spars, ribs, landing gear struts, engine mounts., etc better than the smaller rounds. Certainly not always and with the lower rates of fire (except for the Soviet gun) there is a bit less chance of getting the hit any way. They are better at penetrating armor but which round actually works best after FIRST penetrating aircraft skin or other obstacles before hitting the armor takes a lot of experimental trials to figure out.

And As I have been pointing out ONE big .50 may not equal two smaller guns.

The Big 12.7-13mm guns did have advantages is aiming and hitting too. Not really in the curve of the trajectory (if you are worried about that you are firing from too far away) but in the time of flight to a given distance. For example (again at sea level?) the German 15mm MG 151 (MV 960m/s) had a time of flight to 300 meters of 0.357 seconds, the MK 108 using the type A mine round (mv 500m/s) had a time of flight of 0.696 sec. just under twice as long. Target aircraft could move twice as far between round leaving the muzzle and getting to the target. Without getting into a lot of calculations and what ifs, the MK 108 needed to lead the target aircraft by about twice the distance as the MG151/15. A plane ding 300mph was moving about 300ft in 0.7 seconds.

The difference between the big and little 12.7-13mm guns may not have been that bad but the difference was there.
I am not sure arming a plane with 6-8 MG 131s would have been a good idea if other guns were available.

 

[renrich]

I believe that one reason that some in the USN liked the 20 MM over the 50 cal was that their fighters were more and more being used for ground attack. In an air to ground role, shorter firing times were not as critical as they could be in the interceptor role. There was discussion during the Conference about, in the strafing role, the tendency of the pilots to hold down the trigger and burn out the barrels in one run, inadvertently of course. The slower rate of fire and shorter firing time of the 20 mm made burning out the barrels a little less likely, not to mention its greater destructive power.

One important factor to keep in mind was that as an interceptor or escort fighter, the airplane has only one reason to exist and that is to carry a weapon or weapons into the sky and shoot down E/A. A fighter in those roles is useless without ammo so long firing times were a big advantage. Interestingly though the Navy planes could carry a lot of ammo and their firing times were pretty high. Typical firing times of the premier US fighters were:

P47B-N-17.8 seconds ( that was carrying 267 rounds per gun. It could carry more but may not have done so always)
P51B-C-21 seconds
P51D,K-20.9 seconds
F4F-3-28.7 seconds
F6F3,5-26.7 seconds
F4U1,4-26.7 seconds
The firing time of the two cannon on the F6F5 was 22.5 seconds( not a whole lot less than the 50 cals) It carried 225 rounds per gun of cannon ammo.
The lowly F4F3 had the longest firing time which must have helped Butch O Hare when he was credited with five Betties (now reduced to four but he damaged and stopped the bomb runs of at least two more)
Another interesting point is that the throw weight per second of the F4U1C with four 20mms was 11.6 whereas the F4U1 with six 50s had a throw weight per second of 9.54. Not a huge difference.

To me, a useful analogy in this debate is a dove hunt. One dove hunter is equipped with a 12 gauge, improved cylinder and fifteen number six shells. The other is carrying a twenty gauge improved cylinder and has twenty five number 7.5 shells. Both guns are hump backed Brownings with magazines choked to two capacity. Both are equally good shots and it is opening day and they both are staked out on the same good tanque and there are going to be a lot of birds. Which one is going to be more likely to limit out with ten birds? We all know number six shot is lethal on doves, but 7.5s will work also. My bet is that the hunter with the 20 gauge is more likely to limit out. He can miss fifteen times whereas the twelve gauge man can have only five misses. To me, that is a smilar situation to the escort fighter or interceptor in fighter versus fighter ACM comparing the 20 mm to the 50 cal.

 

[Ratsel]

I am not sure arming a plane with 6-8 MG 131s would have been a good idea if other guns were available.

yep. not a good idea at all. The MG 131 had a reliable explosive round which made it formidable and easily the 'match' of the M2 w/API. Two of those firing from the central axis of the 109 should be more effective than two M2 50's (of four or of six) on a Mustang. OTOH the two MG 131's together make up less than 30% of the firepower of a 109 (with Mg 151/20 motorcannon). As long as you have 20mm ammo the 131's provide some extra power. When this is depleted you better get your butt out of dodge. But if somehow you are forced to engage, two 131's w/ explosive rounds is still a lot better than two M2 w/API.

 

[claidemore]

renrich: I get a throw weight from 6 x .50 of 8.1lbs/sec @850 rpm and 7.1 lbs/sec @750rpm. 4 x 20mm Hispano Mk II I get 11.4 lbs per second. Thats 28% more for the 20mm (vs the higher 8.1 figure), which is pretty significant.

The big difference for me (without even considering the exploding shell vs api) is that the 20mm is throwing a 130 gram projectile @880 m/sec, while the .50 is only throwing a 44 gram projectile @ 880 m/sec.

For me a better hunting analogy would be comparing a .25-35 Winchester to a .375 H&H.
Both have similar velocities (approx 900 m/sec), and .375 bullets are about 3 times as heavy as .25-35s... just as 20mm are about 3 times as heavy as .50s.
You can take coyotes or antelope with a .25-35 reliably, but anything bigger than that and you are undergunned. The .375 will also take coyotes and antelope (bit messy), but it will also do the job on any other animal you might choose to shoot, up to and including large pachyderms.

It's simple ballistics, given a similar velocity, the heavier bullet hits harder, penetrates better, retains it's energy better, and retains it's structural integrity better.

 

[Shortround6]

I believe that one reason that some in the USN liked the 20 MM over the 50 cal was that their fighters were more and more being used for ground attack. In an air to ground role, shorter firing times were not as critical as they could be in the interceptor role. There was discussion during the Conference about, in the strafing role, the tendency of the pilots to hold down the trigger and burn out the barrels in one run, inadvertently of course. The slower rate of fire and shorter firing time of the 20 mm made burning out the barrels a little less likely, not to mention its greater destructive power.

The lower rate of fire helped but the shorter firing firing time did nothing. 75 rounds, the recommended 1st burst, takes about 5.3 seconds to fire. Even a slow flying strafing plane doing 250mph will cover just under 700yds in that time. giving the pilot a even 100yds to pull up means he opened fire at just under 800yds. Even if our tyro pilot opened fire 1150 yds he would only be firing for 7.9-8 seconds. Even a Spitfire with 120rpg would have to open fire at 1600yds to run it's guns dry (assuming a 100yd pull up and about 250mph) for firing time to have any meaning on a first pass (one run). Second or third runs would have an effect. The bigger bore of the 20mm gun allows the propellant gases to get out of the barrel while heating the inner barrel surface less.

One important factor to keep in mind was that as an interceptor or escort fighter, the airplane has only one reason to exist and that is to carry a weapon or weapons into the sky and shoot down E/A. A fighter in those roles is useless without ammo so long firing times were a big advantage. Interestingly though the Navy planes could carry a lot of ammo and their firing times were pretty high.

This quite true, but both the US Navy and the US navy favored 20mm guns for night fighters so as to inflict the most damage in the shorter firing times expected during night engagements. In other words they expected a plane with four 20mm cannon to be able to inflict fatal damage to an enemy plane with less firing time during the engagement than a plane with six .50s. Now perhaps they did not envision a night fighter engaging 3-6 targets in a single flight but it means that a 20mm armed fighter does not need the same firing time as a .50 cal armed fighter, on average, to bring the fight to a satisfactory conclusion.

Another interesting point is that the throw weight per second of the F4U1C with four 20mms was 11.6 whereas the F4U1 with six 50s had a throw weight per second of 9.54. Not a huge difference.

As good as "America's Hundred Thousand" is the gun tables are just a little off in some places. This is one of them. Bear with me, 9.54lbs is 66780 grains total, divide by 6 to get one gun. 11130 grains divided by 743 (heaviest bullet list by AHT on Page 133)gives 14.98 rounds per second or 898.8 rpm. Now this book does claim that the Browning .50 fired at 800-900rpm. I am not sure if any other source does, most claim 750-850rpm. Now for the 9.54lb per second to be true you need all six guns firing at the peak speed list by AHT and you need them loaded exclusively with the heaviest bullet used by the Americans. Neither condition is realistic. Dropping to even 850rpm for all six guns drops the throw weight to 8.97 lbs and dropping the average bullet weight to 700 grains drops the throw weight to 8.43lbs. US belts were usually a mixture of AP. Incendiary and tracer, proportions changed as the war went on but the Incendiary and tracer bullets were well under 700 grains. Even the AP bullets were only just over 700 grains.

To me, a useful analogy in this debate is a dove hunt. One dove hunter is equipped with a 12 gauge, improved cylinder and fifteen number six shells. The other is carrying a twenty gauge improved cylinder and has twenty five number 7.5 shells....... My bet is that the hunter with the 20 gauge is more likely to limit out. He can miss fifteen times whereas the twelve gauge man can have only five misses. To me, that is a smilar situation to the escort fighter or interceptor in fighter versus fighter ACM comparing the 20 mm to the 50 cal.

A flawed analogy. The "hunter" is allowed more "bursts" of the same duration using the smaller gauge gun. Against the aircraft the 20mm guns may require a shorter burst at times. The target plane is is seen to be smoking, on fire, or large pieces falling off in 2 seconds vs the (perhaps) 3 seconds needed by a .50 cal armed plane. The hunter with his shotgun cannot adjust his "bursts", he cannot fire only 2/3s of a shell.

 

[Shortround6]

yep. not a good idea at all. The MG 131 had a reliable explosive round which made it formidable and easily the 'match' of the M2 w/API. Two of those firing from the central axis of the 109 should be more effective than two M2 50's (of four or of six) on a Mustang. OTOH the two MG 131's together make up less than 30% of the firepower of a 109 (with Mg 151/20 motorcannon). As long as you have 20mm ammo the 131's provide some extra power. When this is depleted you better get your butt out of dodge. But if somehow you are forced to engage, two 131's w/ explosive rounds is still a lot better than two M2 w/API.

I think you are expecting an awful lot from those explosive bullets. They carried about 1 gram of explosive. about 1/10th of what a 20mm Hispano round carried or 1/18th of of what the 20mm MG/151 mine shell carried. Maybe they carried 1.2 grams, 1/8th and 1/15th ? The .50 cal used either M2 AP and M1 Incendiary mixed about 50/50 in the belts (with some tracer thrown in, up to 20% depending on year) or it used M8 API which had about 1 gram of incendiary material in each bullet. Either way the German 13mm guns don't really have a big advantage in the amount of chemicals being thrown.
The Germans also mixed their belts, the HE rounds were mixed with with AP/T either 1:1 or 2:1 which also cuts into the amount of HE fired per second.
Even using the 2:1 belt mix and 1.2 grams per bullet you would need to hit with 50% of the rounds fired in 1 full second to deliver 10.8 grams of explosive.

The .50cal Browning is usually rated at about 25-33% better than the 13mm mg131 in hitting power, and this is taking into account chemical fillers.

The problem with the Browning wasn't it's actual firepower but it's firepower to weight ratio. not including ammo you can get 6.8 MG131s for the weight of four .50cal Brownings. and for every 100 rounds of .50 cal ammo you can get 150 rounds of MG131 ammo.

The American fighter planes in general (from early 1943 on) were big enough and powerful enough that they could carry a large weight of armament even if it wasn't the most efficient.