Having seen a couple of excellent diorama's with the effects of the flash of a projectile (Shell) being fired as it exits the barrel, I went and bought from Diorama Shop, some of their Micro Fibre and spray colours, to try and recreate the effects. But in my old age and senility, I've had a mental blockage, and can't seem to get it right.
So I'd be grateful for a little help.

What scale are you working in, and what size projectile ie; rifle bullet, or heavy artillery? For anything small I would suggest a short length of stretched sprue (I would use a light colored sprue color, or paint it white) and a bit of cotton ball. Glue the sprue coming out of the gun barrel pointy end forward and paint just the tip black. Slide the bit of cotton ball down over the sprue to the gun barrel, and Voila!
Note: experts will argue that modern propellants are smokeless and therefore the puff of smoke is a mistake. So I guess the cotton ball would be optional.