If the powder does not burn before it reaches the end of the barrel there will be flash.
The very fast powders can burn 100% before the end of the barrel with either flake or ball type powders in pistols.

I have not noticed too much flash from my medium flake and ball powders but I usually am done shooting before the sun goes down.

Would like to know how the CFE turns out since it works in my 9mm's.