If you don't have a basic understanding of how a gun works, then how would you know it makes speedy things fly at you in the first place? For all you know, it could be a device to summon an amorous hippo to rape the user's assailant to death.
You see it, think "metal wand", and instantly prepare a deflection spell to deflect whatever spell the muggle is about to aim at you.
Come to think of it, according to the movies, curses act like speedy projectiles that need to be deflected, so even if it WAS a device that summoned an amorous hippo, you'd want to deflect it.