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

[A.G. Williams]

Barrel life is how many rounds you can fire through a barrel before enough rifling gets worn/burned away to affect accuracy and velocity to a certain limit. Perhaps also a variable between nations?

There's another aspect to this affecting the barrel life of aircraft guns: the length of the burst fired. A barrel will last far longer if the gun is fired in short bursts, preferably with a cooling-off period between bursts.

I can't vouch for the accuracy of this, but I was once told that if a plane armed with a 27mm Mauser BK 27 revolver cannon fired off all 150 rounds (normal ammo load) in one burst at 1,700 rpm, the barrel would need to be replaced.

 

[Shortround6]

You are quite right.
US manuals for the .50 go into some detail about burst length and cooling times. Like initial burst can be 75 rounds and following bursts are more like 25 rounds (?) with a certain number of seconds between bursts, obliviously impractical if not impossible in air to air combat.

For ground guns Melvin Johnson described destroying a BAR in under 800 rounds. Gun was fastened down and trigger fasten back with magazines changed as fast as possible. Wooden fore end started smoking in under 400 rounds, then bust into flames and gun stopped firing when mainspring lost it's temper and failed to return the bolt forward. Of course he was trying to sell his LMG at the time

Chrome plating barrels could give them much longer life.

But I am just pointing out that these are all different measures and often one measure has nothing to do with another. The Vickers ground gun being a great example, the gun itself lasting for hundreds of thousands of rounds, the barrels lasting for about 10,000rounds (assuming water jacket is kept filled) but the gunners manual detailing 27 different ways it could jam.

 

[Clayton Magnet]

And when the RAF realized they would have no choice but to use Spitfires as fighter bombers after the invasion of Normandy they pushed to remove the four .303 guns and replace them with twin .50 cal as soon as possible.

My understanding was that the "E" wing Spitfires with 2x 20mm and 2x 50 Cal were fielded because the similar trajectories between the projectiles were a better match for the new gyro gunsights, not for any ground attack purpose.

 

[MIflyer]

My understanding was that the "E" wing Spitfires with 2x 20mm and 2x 50 Cal were fielded because the similar trajectories between the projectiles were a better match for the new gyro gunsights, not for any ground attack purpose.

According to an article in Flight Journal on the Spitfire fighter bomber, the push for the .50 cal was based on the ground attack mission. See attached with mention of 2 TAF..

 

[pbehn]

According to an article in Flight Journal on the Spitfire fighter bomber, the push for the .50 cal was based on the ground attack mission. See attached with mention of 2 TAF..

View attachment 616258View attachment 616259

That is basically what this page says too. spitfiresite.com/2010/04/sorting-out-the-e-american-armament-for-the-spitfire-mk-ixxvi.html There were other issues too, By 1944 there were US 0.5" Mgs all over UK and it was more effective in ground attack. I also seem to remember that putting the 0.5 on the inside allowed a longer firing time than the .303s on the outside, the page also mentions the need on some spitfires for more space for oxygen. The comment by Edgar Brooks is also informative.

 

[Glider]

One additional factor for the replacement of the 303 with the 0.5 was that the space saved. It allowed certain items of equipment to be placed where the 303 had been installed. These items allowed the rear internal fuel tank to be fitted increasing the range of the Spitfire.
If the 0.303 had been retained they would have had to redesign the rear tank. The additional oxygen tank was needed because of the extra range of the Spit with the rear tank.

 

[Greyman]

According to an article in Flight Journal on the Spitfire fighter bomber, the push for the .50 cal was based on the ground attack mission.

This goes against what was being said in the discussion between the big-wigs. What I've seen of it, anyway.

One of the arguments in keeping the .303 was the fact that it was better for ground attack -- throwing about three times as many bullets in the air (or at the ground, in this case).

 

[Shortround6]

This goes against what was being said in the discussion between the big-wigs. What I've seen of it, anyway.

One of the arguments in keeping the .303 was the fact that it was better for ground attack -- throwing about three times as many bullets in the air (or at the ground, in this case).

This rather depends on the intended target/s. Shooting up railroad steam engines and railroad cars might require the .50 cal bullets. Shooting up marching/running soldiers, normal trucks, wagons, etc can be done by either one pretty well, more bullets being an asset.
The .50 will shoot through more timber, light masonry, dirt/sand and such.

 

[Greyman]

From their writing the secondary armament (MGs) were more for things like radiators, men and horses.

The Hispanos handled the heavier stuff.

 

[Darthtabby]

This may help. From USAAF microfilm report.

View attachment 600945
View attachment 600946

Hope you don't mind me reaching this far back, but I'm kind of curious what the scale at the bottom of the second image refers to. Weight of metal? The results of some sort of special formula cooking up for measuring destructive power?

 

[Deleted member 68059]

Hope you don't mind me reaching this far back, but I'm kind of curious what the scale at the bottom of the second image refers to. Weight of metal? The results of some sort of special formula cooking up for measuring destructive power?

 

[Darthtabby]

View attachment 641414

Belated thanks, D Deleted member 68059 .

Has anyone here actually read the oft mentioned USN evaluation that rated 20mm three time as effective as .50? I'm kind of curious about it since it appears to evaluate the 20mm's advantage as much more significant then the RAF Operational Research Greyman mentioned, and the second of Snowygrouch's graphs also seems to suggest a less significant difference (though its comparing to planes armed with German cannons rather then the 20mm Hispano).

 

[fastmongrel]

Could be the USN report was talking about the greater HEI content which iirc was roughly three times greater than a .50 BMG round.

 

[A.G. Williams]

Could be the USN report was talking about the greater HEI content which iirc was roughly three times greater than a .50 BMG round.

The US never fielded any explosive .50 bullets as far as I'm aware (although they did a lot of experimental work on them). By 1944, the standard .50 aircraft loading was the M8 API, which had only about one-tenth of the incendiary content of the 20mm Hispano SAPI (which matched its penetration).

 

[pbehn]

This rather depends on the intended target/s. Shooting up railroad steam engines and railroad cars might require the .50 cal bullets. Shooting up marching/running soldiers, normal trucks, wagons, etc can be done by either one pretty well, more bullets being an asset.
The .50 will shoot through more timber, light masonry, dirt/sand and such.

The V1 was very difficult to take down with MGs, just because of its shape and construction.

 

[Greyman]

I'm kind of curious about it since it appears to evaluate the 20mm's advantage as much more significant then the RAF Operational Research Greyman mentioned

The ORS observation was based on German single-seat fighters only. My guess is that if the scope is expanded to twin and four-engine aircraft -- the disparities would grow.

 

[MIflyer]

Anybody look at this video that addresses that question:

The V1 was very difficult to take down with MGs, just because of its shape and construction.

And because it tended to BLOW UP with such force it could shoot down the interceptor. One problem with intercepting the V-1 at night was that its blowtorch was easy to see but very hard to estimate rnage on. The RAF experimented with using tail warning radars - such as "Monica" or APS-13 - to tell the range to the V-1

 

[pbehn]

Anybody look at this video that addresses that question:


And because it tended to BLOW UP with such force it could shoot down the interceptor. One problem with intercepting the V-1 at night was that its blowtorch was easy to see but very hard to estimate rnage on. The RAF experimented with using tail warning radars - such as "Monica" or APS-13 - to tell the range to the V-1

If you hit the warhead it would explode for MGs and cannon. The V1 was faster in level flight than almost all interceptors so there wasnt much time to hit coming out of a dive. From behind the kill areas were very small and MG bullets tended to bounce away from the V1 fuselage surface.

 

[MIflyer]

The V-1 airframe was steel and was pretty tough.