Tony Williams coverd this in his article:
The Development of RAF Guns and Ammunition from World War 1 to the Present Day
http://www.quarry.nildram.co.uk/RAF%20guns.htm------------------------------------------------------------
WW2 .303 Air Combat rounds.
Sectioned .303" rounds, from the left: tracer, armour-piercing and B Mk VI incendiary (Dixon/De Wilde)
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British and German WW2 Air Combat rounds.
The three standard wartime RAF rounds: .303", .5" and 20 mm Hispano (left), compared with representative German ammunition: 7.92 mm, 13 mm MG 131, 15 mm MG 151, 20 mm (MG-FF) and 20mm MG 151
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General Internal Structure of German Compound Explosive Rounds and why they are effective.
Luftwaffe 20mm MG-FF ammunition: HE-T, Minengeschoss and API
The HE rely's on the shrapnel from it's thick wall. Minengschoss the (hexogen aluminum) is 3x more powerfull than TNT burning at 1000C. Both the HE and Mine fuzes have aluminum, copper or magnesium bodies. API has phosporus contained with an aluminum plug which ruptures on penetration contact.