If he's getting enough 'spark' to light the powder under the cap, he's getting enough spark to light the powder in the chamber.
It's like the pan-powder in a flintlock. The object isn't to fill the flashhole with powder, that'd mess-up the ignition, one wants the hole completely unobstructed for the -flash- to be transmitted through.
"Those who have long enjoyed privileges as we enjoy, forget, in time, that men have died to win them."
Franklin D. Roosvelt