[SlipBall]

Originally Posted by klem

OK, 'precisely what is was introduced for' was a bit narrow but my post was meant to confirm that some 303's did flash on impact.

As far as the pilot was concerned it was effectively dual purpose, causing a flash on impact as well as being incendiary. The initial activation happened on impact. Catsey's reference is the most often quoted and here's a report from a pilot using it in the far east:
http://www.aircrew-saltire.org/lib225.htm
"The golden flashes I had seen on the Japanese aircraft were undoubtedly my de Wilde ammunition exploding and I was sure that very many rounds had hit it. This de Wilde ammunition exploded on contact and an incendiary core remained so that it was highly lethal to aircraft."

And of course there are other references as in Al Deere's "Nine lives"

Here's a brief description and cutaway:
http://cartridgecollectors.org/cmo/cmo07feb.htm

I think the round was named "observer"...been flying blue lately so I could be wrong.