Notes on the BESA 15mm machine gun.

A set of tests were carried out firing 15mm BESA rounds at a Blenheim bomber on 8th January 1940 to assess the aircraft's vulnerability to HMG fire.  The BESA was used as captured/recovered stocks of German 15mm ammunition had been exhausted.
The gun was fired from directly astern at 200 yards range; 100 rounds were fired, with 90 striking the airframe, in bursts of 5 or 10.  A further 5 round burst was declared 'no test' as they missed the target due to a setting-up error.

Rear fuselage, protected by two V-shaped armour plates each 8mm thick with a gap of several inches between them.  24 strikes out of 25; 2 caused lethal damage to the elevators.
7 bullets penetrated the rear plate but none penetrated the forward plate.  Large holes were made in the fuselage sides by fragments.

Port inner wing, the petrol tanks protected by 21mm armour plate.  17 strikes out of 20.
8 bullets passed through the petrol tank, 4 causing 50% fuel loss and 4 causing 25% fuel loss.

Port engine, 37 strikes out of 40:
Engine out of action immediately 4 times.
Engine out of action in 5 minutes 5 times.
Engine out of action in 8 minutes 2 times.
Engine out of action in 10 minutes 1 time.
Engine out of action in 20 minutes 4 times.
5% drop in power 5 times.
20% drop in power 1 time.
Probable crash on landing 14 times.
Strength of wing reduced by about 20% once.

Starboard engine, 10 strikes out of 10.  5% drop in power once.
Probable crash on landing once.
Slow leakage of petrol once; this would be set on fire by at least one bullet of a later burst of 5 rounds.

Tailplane, 2 strikes out of 5.  Strength reduced by 20% once.

Penetration figures for AP bullet:
Range in yards  100  200  300  400 500  600
Striking velocity (in feet/sec)  2735 2634 2495 2343 2243 2089
Penetration at normal (in mm)   27 27 26.5 25.5 24 23
Penetration at 20° to normal  22  19 18 16.5 16 15.5
Penetration at 40° to normal  14.5 14 13 12.5 12 12

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Last updated: 14th September 2019.