A decent flash has a critical detonable mass of between 30 and 50 grams. This means that that quantity will detonate with concussion and a shock wave when ignited in open air: no container, just loose on the ground. A cup or a shell casing does not count as loose: it still confines the flash enough to accelerate the reaction. Less flash than the critical mass will just burn violently. Compare this with black powder which has a critical mass of over 500 pounds.
http://gamekeeper.deds.nl/flash!.html

This is related to the 'failure diameter' that a homogenous brisant also demonstrates.
Too little and it just burns.

Apparently all the "secondary explosive" nitrates burn before detonating, whereas
"primary explosives" (eg lead azide) will simply detonate. Both can be shock friction
etc sensitive.