Historical background courtesy of Tony Edwards:
In the late 1930s, with war approaching, the British wanted a new .303 inch incendiary for air service to replace the phosphorous filled B.Mark IV that
was based on the WWI Buckingham design. A base fused design was under development as the B.Mark V but this ultimately proved unsuccessful.
In 1938 a Mr. De Wilde, a Belgian based in Switzerland, offered the War Office new designs of incendiary and explosive bullets. A trial was held at
RSAF Enfield with de Wilde’s bullets loaded in 7.5mm Schmidt Rubin cases and the results were sufficiently encouraging for further trials to be held and
experimental .303 inch bullets to be manufactured in the UK. A modified version of the De Wilde design utilising a small steel ball in the nose
was provisionally approved for manufacture in early 1939 when problems occurred.
One of the requirements of the new design was that it could be fired in a continuous 300 round burst, since it was intended to load one gun in the new
eight gun fighters exclusively with the incendiary round. The de Wilde failed this test and prematured in a hot barrel causing bulged or burst
barrels. Despite extensive testing the hot barrel” problem could not be solved and the De Wilde design was written off as a failure.
A new design was urgently developed, mainly by Major Dixon at Woolwich and this had some similarities to the De Wilde, including the small steel ball
in the nose to initiate the incendiary composition. This bullet, to Design DD/L/10091, was approved as the B.Mark VI in December 1939.
It was officially acknowledged that whilst the new bullet owed ten percent to De Wilde’s work, it was in fact a new design. Whilst erroneous, the use
of the name “De Wilde” was encouraged as it was hoped the Germans, who had tested the original bullets, would continue to believe the British had
perfected the design