Ammunition type – please help with the name

I am currently transcribing a taped interview I did with a Battle of Britain pilot, Bill Wells, and he is describing something he witnessed. He mentions a type of ammunition which I can’t quite pick up on the tape. The transcript is as such

“And we had that ____ ammunition, which you probably don’t know about, but it flashes whenn it hits anything, so you can see it very plainly.

The word/s he says to name the ammo type sounds hile “devortue wild”

Does anyone have any clue what he really said? Thanks.

(date is August 1940, place Hawarden No. 7 OCU, planes are Spitfires if this narrows it down)