.303 vs 20mm

[Airspeed]

Hi All,
Collected a batch of 20 reproduction 20mm aircraft cannon ammo. yesterday.
Had these made to display in AFV.
Top is a genuine .303 cartridge case, minus projectile.
Lower is the 20mm beast.
I will soon start experimenting with electroplating to make the case appear more realistic. Materials on order.
There's quite some difference, as I'm sure you'll agree.
Now that I've established contact with a willing engineering works, I'll look into other calibres.
I've wanted for a long time to show the physical comparison, and it's finally coming close.
Hope you don't mind me sharing....I'm pretty happy!

[DaveB]

Imagine firing a rifle which took 20mm shells! I wouldn't fancy having to carry the ammo either

ATB
DaveB

[SkippyBing]

Plus the 20mm is filled with explosive so it makes a nice big hole...

[Nigel H-J]

Our poor Spitties and Hurries only had the .303 calibre guns, could never work out why they never designed the Spitefire to also carry the cannons as the ME109's did.

Nigel.

[petermcleland]

Here are a couple of 20mm that I have here:-



The projectiles are unfired as can be seen by the condition of the copper driving bands on them...If they had been fired then the copper would be deeply scored by the bite of the barrel rifling. They are practice "Ball" rounds and are shaped to represent high explosive incendiary and semi armour piercing incendiary rounds. The cases are old fired ones that I picked up on a firing range.

The unfired projectiles have sad memories...A friend on my squadron (266 in 2nd TAF) was returning to Wunstorf from the range, in his Venom FB1. The aircraft caught fire and Greg tried to put it down on the crash strip alongside the runway...All was well until the last moments, as he was rounding out for the belly landing, his control cables burnt through and the aircraft nosed in and broke up. Pilot Officer Gregory was killed and among all the fragments of his Venom were some 20mm rounds which had been wrenched out of their cases and lay there in the debris. I picked up these two practice rounds...I left the cases as they still had live primers and charges in them. Later on, I collected a couple of used cases from a range to put them in. They stand on top of my bookcase in the "Flightdeck" and I think of Greg every time I see them.

[Airspeed]

Thanks for posting that pic, Peter.
I'll do some comparisons with mine.
Sorry to hear about your sad memories though.

[Dev One]

Our poor Spitties and Hurries only had the .303 calibre guns, could never work out why they never designed the Spitefire to also carry the cannons as the ME109's did.

Nigel.

IIRC there was a study made before WWII that compared the mass of 'lead' that was needed to down an aircraft, & it appears that because of the rate of fire of machine guns vs cannon that it was better to fit machine guns. Obviously battle experience proved otherwise, because of explosive power I wonder?
Keith

[Chris Trott]

The biggest reason for lack of Cannon early in the war for the Allies was simple - our Cannon sucked. They all had serious problems with freezing at altitude (even with heaters) and they weren't very accurate. It was only after significant development of the Hispano design in the US by several armament companies and the US Army in conjunction with experience and feedback by US & UK armorers and pilots that got them reliable by the end of '41 when they started being seen in the newer Spitfire designs and the US Navy began fielding them in their Night Fighter variants of the F6F and F4U.

The other issue was that for the same weight, a sixtet of .50BMG or octet of .303 MG resulted more firing time (despite the higher firing rate) and a higher total energy delivered to the target than a quartet of 20mm Cannon. In post-war evaluations, it was found that the Bf-109's 2 cannon and 2 MG were vastly outperformed by the all-MG armament of the mid-war Allied fighters both in accuracy and damage. It wasn't until high velocity, high rate of fire revolver and "Vulcan"-type cannon were developed in the 1950's and the massive jump in payload of jet-powered fighters that allowed the cannon to become the primary armament of aircraft.

Tony Williams has a great site that includes multiple articles on Cannon vs Machine Guns and the effectiveness of the various weapon loadouts of WWII (and other era) aircraft for comparison.

http://www.quarryhs.co.uk/

An article by him that speaks directly on the Cannon v MG issue can be found here - http://www.quarryhs.co.uk/CannonMGs.htm

[Airspeed]

I'm absorbing all of this, gents, so any information is very welcome and appreciated! Thank you.

Peter's photo has me somewhat deflated at present, as it suggests that the example which I had obtained may be inaccurate; too short and wide
If I've scaled up your photo properly, Peter, the case is only 25mm diameter, and the overall length would be 200mm. Trim off a couple for the flat - nosed bullet.
Would you kindly confirm or correct, please?
I have a couple of print outs on the wall, showing a variety of lengths, but I have to check the diameters, as nothing was specified in the pictures which I found.
Now have to read those links from Chris to see if there's more detail available.

EDIT: Yes, the Anthony Williams article is where I extracted the comparative images for .303" & .50" Brownings and the Mk108 30mm, which are full-size on the wall of AFV.
From this re-reading and further looking at the available pictures, it does seem that I let myself be too easily led by the example which I bought.

[petermcleland]

Mike the overall length of the round with the pointed head is 183mm and the diameter measured at its base is 24mm. The one with the flat on its tip is a slightly longer round by about 2mm. You would think that the flat tipped round would be shorter than the pointed head but it is in fact longer