Normally, when the primer is struck by the firing pin, the primer detonates and ignites the powder in the case, propelling the projectile down the barrel.
There are times, though, when the primer is struck, but it doesn’t detonate. Instead, is smolders ever so slightly until a short time later — up to a minute! — the powder ignites.
It’s called a hangfire.
The right way to handle it, of course, is to always keep the firearm pointed in a safe direction. Never point the muzzle at anything you don’t want to destroy.
Linked here is a good example of the timing of a hangfire… and an even better example of how NEVER to handle one.
Honestly, I don’t know what line of reasoning could ever possess somebody to look down the barrel of a firearm after they suspect something is wrong with it. Maybe that’s how they saw it in those old Bugs Bunny cartoons.