Can somebody explain what "flash" is?

[Subnuts]

Thank you very much; so it does indeed seem to be "flash fire" is loosely like "flash flood" - also interesting about the emphasis on gunnery competition; something the U.S. Navy overemphasized also.

Ever see that film taken during the 1950s of houses being destroyed in nuclear tests? If you look closely you'll notice the paint on the houses igniting seconds before the blast wave destroys them. I think that's what they mean by flash. Basically, a sudden intense pulse of heat. When the shell explodes, the sudden heat pulse would ignite everything flammable in the turret, including the powder.