WWII aircraft armament machine guns and cannon

[44 AMP]

One of the many things to remember (and prioritize) when discussing WWII aircraft, performance, armament, tactical doctrine, and use, effectiveness, design philosophies, or anything else about them is the specific time period involved.

Every nation entered the war armed with pre war designs, and design philosophies. The mid 1930s saw the introduction of a new generation of aircraft, with the main types being first fielded in the later 30s, though it was not a smooth even process until the urgency of war made that a priority.

Every pre war design got improved, upgraded and modified as the war went on, until the design reached its limits, and even then, some of those designs remained in service until the end of the war, sometimes beyond.

So, looking at what fought in 1940 and what was fighting 4 years later, even if the basic airframes are still the same, many aircraft had changed so much that its becomes and apples vs. oranges comparison.

Those first P-51’s must have been a nasty surprise when they showed up so deep in German skies.

The first p-51s were Allison engine powered, and had 4 .50 cal wing guns, with a noted tendency to jam during high G maneuvers, due to design flaws in the ammo feed chute system. This was fixed, of course, but the P-51 A, B and C models were not in the same performance class as the more famous and more effective D model.

Why did the Brits choose to not have ventral gun positions on their bombers?

Quite possibly the answer is simple inertia. And, by that I mean design and bureaucratic inertia. Look at the pre war designs, partiularly medium bombers (twin engined). Almost nobody had a ventral gun position on their twin engined bombers, other than the Germans who had only one single rifle caliber machine gun there, which, by pre war thought, was considered adequate. US heavy bombers (4 engine) were designed from the start with a belly gun position of some kind.

According to WIKI, there was a ventral gun turret on early Lancasters, but it was removed, because it was "dead weight", being operated by a periscope with a very limited field of view (20 degrees) and was "too slow to keep a target in its sights". After the prototype B.II version, it was almost never used.

[105kw]

Another reason for no belly gun in British 4 engine bombers, they had their bombing radar mounted there.
On later (1942-43) Lancaster, look for a bulge on the ventral fuselage.
The USAAF system replaced the ball turret, and was called BTO, BOMBING THROUGH OVERCAST.

[105kw]

My question was why didn't replace their 4 .303 gun turrets with .50s?
Right after the war ended, the next heavy bombers they built did.

[Pumpkin]

But were the first P-51's used as long range escorts?

I should have been more clear, I was referring to the Merlin/Packard versions, starting with the B.

And I agree, the D was the best one.

[davidsog]

How would the same Spit have faired with a short blast from 6/8 50’s?

A lot of .50 cal sized holes punched in the skin. As the RAF vulnerability reports states....about a zero percent chance to be effective in bringing down the aircraft.



We put holes in structural bearing aluminum aircraft parts all the time to lighten the structure without weakening it.

The max ord on the .50cals mounted on the P47 was 116cm compared to the Mk108's 65 cm for those interested.



Understand, its not the movies...
You cannot just hold the button down and spray tons of ammo. There were burst limits that where surprisingly small and the dispersion on the .50 wasn't anything outstanding. It's not a sniper rifle at all and certainly not a laser.

Out of the P-51D Operators Manual:



That is one reason why the Germans chose the path of going with a cannon. It was so hard to land just one round in an aerial gunnery solution it was better to ensure that one round would do maximum damage as opposed to hitting an aircraft multiple times while doing very little damage per hit with a rifle caliber machinegun. That is also why we went with cannons too.

Good illustration of aircraft gunnery disperson:



I actually have the dispersion charts for the P51D and F6F if anyone is interested.

[davidsog]

And I agree, the D was the best one.

In terms of aircraft performance, the B series was much better. It was lighter and less drag for the same amount of power. The D series added a lot of extra weight and the bubble canopy has much more drag than a fastback.

[Pumpkin]

So we’re assuming none of those 50’s would have penetrated:
Pilots seat armor
Engine blocks, cylinder barrels, radiators, fuel systems, hydraulic systems, etc.
Why would we think frame work and aluminum skin would be the only parts damaged?

[davidsog]

So we’re assuming none of those 50’s would have penetrated:
Pilots seat armor
Engine blocks, cylinder barrels, radiators, fuel systems, hydraulic systems, etc.
Why would we think frame work and aluminum skin would be the only parts damaged?

Your question on the armor is answered in the first paragraph of the RAF testing. From directly behind it a .50 caliber AP round in not likely to effectively damage the aircraft or injure the pilot.

The discussion isn't "could the .50 caliber" equipped aircraft shoot down other planes. Yes, we defeated the Luftwaffe with rifle caliber equipped fighters. That is a fact. The fact we could launch missions with more airplanes in one mission than the entire Luftwaffe had aircraft had much to do with that too. It was a lot more to winning air superiority than a rifle caliber gun.



If you want, I can provide you with The 8th USAAF After Action Review so you can read it from the horses' mouth.

Once more our pilots were significantly better trained with more experience on average than the German pilots. We rotated and rested our pilots whereas the Germans flew until they died without respite.



It is a fact that the Mk108 was a significantly better weapon for aerial combat delivering 10 times the destructive power of a single .50 caliber M2 Aerial Weapon.

[bamaranger]

Might just be wordplay or a typo, but am I the only one that does not consider the .50 BMG "rifle caliber"?

The US cal .30, the Brit .303 , the Russian 7.62x54r and for the Axis the Japanese 7.7 and 6.5, the German 7.92 and the Italian 6.5 are rifle caliber, but the .50 BMG is in another class don't ya think. Yes there are .50 BMG rifles, these days but the .50 BMG, especially then, was another animal, and side by side, still is.

There is far too much gun camera footage, available for all to see, that demonstrates the destructive power of concentrated air to air .50 BMG fire on WWII Axis aircraft , Yes, there are holes placed in aircraft structure to lighten them. When you put more holes in that structure, violently, and its going 200-300 mph mph under stress of maneuver its apparent really bad things happen.

I'm no pilot, no engineer, and only half well read, but I can see the destructiveness of the .50 bmg for myself.

[Pumpkin]

Agreed, or a hunting caliber, for big game.

Some rifle ranges will turn 50BMG’s away.

[44 AMP]

Traditionally, (and during WWII) "rifle caliber" refers to the cartridges used in each nation's standard infantry service rifles. And pistol calibers were those in each nation's service pistols.

It was during WWII that the intermediate class of cartridges were developed, and used in carbines and assault rifles. More powerful than the standard pistol rounds but not as powerful as the standard infantry rifle rounds.

The .50 BMG was not a rifle round in WWII, it was a heavy machine gun round, and still is today. The fact that, today, there are rifles which fire it does not change that.

I have a question about the "continuous firing limits" chart, or rather about the manual it came from. Specifically I'm interested in if the manual states the M2s tested are firing from the open, or the closed bolt.

Since the chart mentions the risk of cook off, that indicates firing from the closed bolt, since cook off is nearly impossible in an open bolt gun. The entire point of an open bolt firing system is to prevent a cook off.

Unlike many other machine guns, the M2 can be set to fire either way, open or closed bolt operation. The trigger and the bolt release are separate mechanical controls on the backplate. The aircraft version has these replaced by an electrical solenoid, which performs the dual function of operating the internal parts for the trigger and the bolt release. When electrical power comes off the solenoid (when the cockpit trigger is released), the bolt comes back from firing and is held back until/unless the bolt release is tripped.

On the infantry gun (M2HB) there is a hook on a sleeve over the buffer tube that can be used to lock the bolt release down, allowing firing from the closed bolt, but the solenoid activated aircraft gun does not have that part. Firing from the closed bolt has infantry uses, as it provides the ability to fire well aimed single shots or bursts without disturbing the flex mounted gun's aim due to the weight of the internal parts moving. This is of no concern to a fixed gun mount, such as in the wing or nose of a fighter plane.

So, I wonder just how is there a risk of cook off??

[davidsog]

So, I wonder just how is there a risk of cook off??

There is good engineering reason why the Operating Instructions are as published. What Browning and North American's specific reason is I can't say.

What I can tell you from having restored some of these beautiful warbirds is I would bet it has to do with the thermal stress of the environment the guns are required to operate in.

Guns stuffed into wings have some general characteristics that set them apart from just shooting a weapon on a range.

They are all stuffed into tight fitting bays that traps heat in a very cold environment at altitude. The biggest issue in getting them to work properly is the fact those bays can freeze up. That means almost all of them have a weapon heating system to ensure the gun is warm and able to operate. Firing the gun in a warm environment means rapid heat build up inside an enclosed bay.



It's not so much the guns are over cooled, it is that they are heated before they even fire. Once more, even in rare operating environments that a heater is not required...

They are all stuffed into tight fitting bays that traps heat so your problem remains getting rid of the heat. It's like Bleed Air used to heat the wing leading edges in a modern Transport Category Aircraft Anti-Ice has to be used cautiously because you can overwarm the wings and cause a fire.

[davidsog]

From the 1942 TM on the M2 Aerial Weapon:



These are the generic weapon instructions. Each installation and POH will have any changes to this noted. The table posted earlier comes from the P51D series.

[Pumpkin]

How much if any cooling would come from surrounding ambient air flow?
Maybe this was already taken into account when the instructions were written?

[44 AMP]

There is good engineering reason why the Operating Instructions are as published. What Browning and North American's specific reason is I can't say.

Ok, so you don't know, which is a valid answer, but does not resolve the question. I am well aware of the heat range and thermal stresses the guns are required to operate in, from 100F+ ground temps to -40F a few miles up, and back again in a single mission.

I also understand the difference in heat stress, firing and cooling times between the 10lb aircraft gun barrel and the 28lb infantry gun barrel. Neither of which answers my question about "cook off".

The Army Air Corps (and later the USAF) use the same terms for the same things. Naturally, the Air Corps uses a lot of terms unique to aircraft, but cook off is not one of them.

Cook off refers to the heat from firing being enough to ignite the powder of an unfired chambered round. This is not possible with a gun firing from the open bolt, as there is no round in the chamber to be cooked off. The only way a cook off can happen in an open bolt gun is for the A) the gun to be hot enough, AND B) at the same time having the firing pin fail to function, due to an inert primer, or physical damage preventing the firing pin strike. Then, and only then will there be a round in the chamber, unfired, and able to be cooked off.

I was rather forcibly instructed about this by a senior gunner Sgt at the US Army Ordnance Center & School in 1975. Since the M2 can be set up to run either way, but usually isn't swapped back and forth, and since a cook off is nearly impossible from an open bolt but is possible firing from a closed bolt, and you have posted pages from the instruction manual, I was asking if that manual stated which way the tested guns were set up.

Personally, I would expect the gun in the wings to be set for open bolt firing, just common sense, it seems, but I do know the Army, and common sense isn't always the way they do things.

[davidsog]

The entire point of an open bolt firing system is to prevent a cook off.

Open bolt machinegun's cook off all the time if overheated if the gun experiences a stoppage. Cook offs often result in gun damage in an open bolt due to the fact the bolt is open.

https://www.youtube.com/watch?v=seZZoddtlH8


https://www.youtube.com/watch?v=2IY9tR7B8dE

Some aircraft in WWII had the ability to reset the guns in flight. I believe the P51 series could do that and I know the BF-109/FW-190 could do so.

If you have a cook off, not only will that stop your one gun but you run the risk of losing all of them if the damage impedes the cocking system.

[davidsog]

Ok, so you don't know, which is a valid answer,

That's why I posted the manual which says exactly what I said....

Are you reading my posts' or is something else going on?


If you look at the chart I posted from the P-51 POH:



It follows exactly what is said here:



The guns cooked off because the guns get hot. They are stuck in little cramped aluminum bays in a very tough thermal environment.

[davidsog]

The Army Air Corps (and later the USAF) use the same terms for the same things. Naturally, the Air Corps uses a lot of terms unique to aircraft, but cook off is not one of them.

What? They seem to use "cook off" quite a bit in the USAAF, USA, USMC, USN, and USAF manuals when it come to Open Bolt Machineguns.

That second video is an MG-3 in the IDF experiencing a cook off right before your very eyes.

I was rather forcibly instructed about this by a senior gunner Sgt at the US Army Ordnance Center & School in 1975.

I was very forcibly instructed in exactly the opposite throughout my career including my duties as an M60 Gunner in B co. 1/75th.

If you heat the gun up hot enough, it will set the powder in the cartridge off without the primer being struck.

[44 AMP]

Open bolt machinegun's cook off all the time if overheated if the gun experiences a stoppage. Cook offs often result in gun damage in an open bolt due to the fact the both is open.

A stoppage is a malfunction, and damage to the gun results when the stoppage results in the bolt not being fully closed and locked, and a cook off detonates the cartridge. The system isn't perfect.

[davidsog]

The M2 Aerial Weapon in a fixed aircraft mount fires from a closed bolt making cook offs even more likely.

[davidsog]

A stoppage is a malfunction

A cook off is a type of malfunction.....

[davidsog]

Personally, I would expect the gun in the wings to be set for open bolt firing

Open Bolt guns are much more prone to misfires due to the fact they are open bolt. Drop safe is not a requirement. I wouldn't want to fly an airplane that could misfire its weapons with every hard landing or turbulence.

[bamaranger]

The gun bay picture is slick......thanks.

[44 AMP]

The M2 Aerial Weapon in a fixed aircraft mount fires from a closed bolt making cook offs even more likely.

Why didn't you say that when I asked??

[davidsog]

The gun bay picture is slick......thanks.

You are welcome. I got a huge library of original documents on WWII aviation. I got lucky enough to work on a number of WWII aircraft restorations when I was getting my A&P. I got to rebuild the heads on a Merlin V-1650-7 from a P-51D, the fuel pump on a BMW801D2 engine from a Focke Wulf FW-190, Disassemble a DB605, and a lay out a crated FW-190A5 crash dug out of Russia for future restoration.

Since I was in the Army at the time, I worked on my pilot qualifications at the same time so that by the time I could retire, I could change careers to being a pilot. I built a lot of my time flying to and from restoration works and archives.

If I have it, you are welcome to it.

Why didn't you say that when I asked??

You did not ask me about open or closed bolts....you made a statement.

Your question was never asked but you seemed to be musing on the validity and source of the chart on continuous firing limits.

Open bolt guns do cook off and it does not matter one bit how the gun worked. The valid limitations are on it regardless and are legitimate. It does not need me or you, lol.

44 AMP says:

I have a question about the "continuous firing limits" chart, or rather about the manual it came from. Specifically I'm interested in if the manual states the M2s tested are firing from the open, or the closed bolt.

Since the chart mentions the risk of cook off, that indicates firing from the closed bolt, since cook off is nearly impossible in an open bolt gun. The entire point of an open bolt firing system is to prevent a cook off.

Unlike many other machine guns, the M2 can be set to fire either way, open or closed bolt operation. The trigger and the bolt release are separate mechanical controls on the backplate. The aircraft version has these replaced by an electrical solenoid, which performs the dual function of operating the internal parts for the trigger and the bolt release. When electrical power comes off the solenoid (when the cockpit trigger is released), the bolt comes back from firing and is held back until/unless the bolt release is tripped.

On the infantry gun (M2HB) there is a hook on a sleeve over the buffer tube that can be used to lock the bolt release down, allowing firing from the closed bolt, but the solenoid activated aircraft gun does not have that part. Firing from the closed bolt has infantry uses, as it provides the ability to fire well aimed single shots or bursts without disturbing the flex mounted gun's aim due to the weight of the internal parts moving. This is of no concern to a fixed gun mount, such as in the wing or nose of a fighter plane.

So, I wonder just how is there a risk of cook off??

The fact you went on to explain the operation of an M2 served to confirm my conclusion your question wasn't about the M2 itself.

So, it was a misunderstanding. All good?

So back to the original topic...

Yes, a .50 caliber M2 is a rifle caliber weapon. It's a big rifle caliber no doubt. There are their hunting rounds that are larger than a .50 caliber BMG but by definition, a rifle fires from a rifled barrel and relies upon kinetic energy of the projectile's velocity.

A cannon on the other hand, relies on the potential energy of the rounds chemical explosive to create damage.

Monocoque construction is how fighter aircraft were made at the time the Mk108 was developed. That means the skin of the aircraft was designed to be a structural bearing component of the airframe.

The .50 cal was designed to pierce the structural bearing skin of a monocoque construction aircraft and attack the components within.

The Mk108 was designed to attack the structural bearing skin and removed it in large sections so that the structure failed in flight. The fuze allowed the round to penetrate 1/3 of its length thru the aluminum and then explode with the force of a full pound of TNT.

Most aircraft even today can't take a pound of TNT going off inside their structure and still fly.

The .50cal M2 wasn't ideal and it certainly wasn't better than a Mk108. That does not mean it did not get the job done.
To me, it's just one more thing our Grandfathers got done despite the odds. Egypt did not have cranes or bulldozers yet they built the pyramids. Our Grandfathers did not have the best fighter aircraft armament but they won the war and cleared the skies over Germany with what they had.