On 11/07/2020 08:15, Chris Hogg wrote:
On Sat, 11 Jul 2020 08:02:32 +0100, Harry Bloomfield, Esq.
wrote:

BBC News article claiming a 13 year olds intricate calculations
persuaded them to fit 8 browning guns rather than the four planned.
Obviously chances are improved of bringing down an enemy aircraft, the
more guns are being fired, but what is the intricate calculation needed
to prove that?

It seems the guns were aimed to coincide at a range of 250 yards. 250
yards seems awfully distant to me, what they show of the films of the
time, it seems much less than 250 yards, maybe as little as 50 yards,
unless the used telephoto lens?

The convergence 'point' was actually a 12' x 8'rectangle, known as the
Dowding spread, as he believed it gave even a mediocre pilot a fair
chance of hitting the enemy. With Spitfire guns so widely spaced on the
wings, there was also quite a wide area of less concentrated fire both
before and after the convergence point. Aces would normally have the
guns set to their own personal preferences, while some Spitfire
squadrons set different convergence distances for each wing, to give a
greater depth of concentrated fire.


I rather wondered the same. And FMI what's the difference between a
Browning machine gun and a cannon, that some Spitfires were apparently
equipped with? Better for bringing down German bombers, the chap said,
which were more heavily armored than their fighters. Do cannons fire
heavier bullets, or have higher muzzle velocities, or what?


The Browning machine guns fitted to early Spitfires fired .303 rifle
rounds. The cannon fired 20mm (3/4") explosive rounds.

When the Germans started fitting armour to their bombers, the machine
guns proved to be inadequate and cannon were tried as an answer.
However, the early cannon were prone to jam, so, for a while the
standard fit became two cannon and four machine guns.

The later 'universal' or 'c' wing Spitfire could be fitted with either
eight machine guns, two cannon and four machine guns or four cannon, as
well as being able to carry 2 x 250lb bombs. The 'e' wing Spitfire had a
development of the 'c' wing and could carry either four cannon or two
0.5" machine guns and two cannon.

--
Colin Bignell