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There was a requirement in 1939 for both 7.92mm incendiary and observation bullets and trials took place with an observation bullet to design DD/L/ 9971 based on the .303 inch Incendiary B Mark VI but filled with phosphorus. This was abandoned in July 1940 but work continued on an incendiary to design DD/L/11392, still based on the .303 inch B Mark VI. This did not receive final approval and work continued on a new design.
Problems arose though with propellant as it only proved possible to achieve 2,480 fps at 90 feet compared to the desired velocity of 2,530 fps if the bullet was to match the ball round at 600 yards. This was accepted as a temporary measure whilst a new propellant was developed.
Incendiary B Mark Iz
"Cartridge S.A. Incendiary 7.92mm B Mark Iz" was approved to design DD/L/11849A in 1942 and was not shown in Lists of Changes.
The case was as for the Ball Mark Iz but with a blue primer annulus and the headstamp included the code "BIZ".
The bullet was based on the .303 inch B Mark VII with a slight boat tail and had a GMCS envelope with a single cannelure. The bullet had a lead antimony sheath inside the envelope with a steel sleeve containing the incendiary mix of 6 grains of SR.365. The base of the bullet was closed with a steel disc and a lead plug and the nose was specially thinned to contain 1 grain of QF composition. Weight was specified to be 180 grains but actual examples weigh 172-175 grains.
Propellant was specified as 42 grains of nitrocellulose but records show that the load was normally 38 grains.
Observed velocity at 90 feet was 2,480 fps due to propellant problems rather than the specified 2,530 fps.
Almost as soon as the B Mark Iz entered service in February 1942 problems were encountered with premature breech and barrel explosions found to be caused by the presence of the QF composition in the nose of the bullet. It was decided to omit this in future production and to introduce the new round as the B Mark IIz.
Incendiary B Mark IIz
"Cartridge S.A. Incendiary 7.92mm B Mark IIz" was approved to design DD/L/12698B2 in 1943 and was not shown in Lists of
Changes.
The case was as for the Ball Mark Iz but with a blue primer annulus and the headstamp included the code "BIIZ" or after 1944 "B2Z". A small number were also produced with blue bullet tips.
The bullet was based on the B Mark I withot the QF composition in the nose. The incendiary filling was increased to 7 grains of SR.365. The base of the bullet was closed with a steel disc and a lead plug..
Propellant was 45 grains of nitrocellulose.
Observed velocity at 90 feet was 2,480 fps. This was ordered to ICI Kynoch depite a previous desire to achieve 2.530 fps.
Trade Pattern Incendiary
The Incendiary B Mark Iz did not enter service until some three years after the BESA was introduced. Although not mentioned in the records, there is evidence that trade pattern incendiaries were supplied by ICI Kynoch to cover the deficiency.
The label illustrated below shows 7.92mm "Smoke Tracer" as included in the belt mix. "Smoke Tracer" was the ICI Kynoch description for their Buckingham style incendiaries filled with phosphorus. That this was non standard ammunition is shown by the fact that it is printed in black on white paper and there is no Mark number shown..
Label showing non standard Smoke Tracer mixed 1 in 4 with Ball Mark Iz.
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