No, a bullet hitting a helmet will not break someone's neck
Bullet resistant vests are impact rated, and designed to disperse the kinetic energy
^ that. the energy required to separate a vertebra, much less dislocate a disk, with a head shot is more than any normal shoulder-fired rifle has. (dont be a smartass and start including .50bmg or big-game sized rifles)
first and foremost, there is no such thing as 'bulletproof'
there are many instances of direct impacts of 7.62x54 at medium range on current-issue helmets where the wearer received little more than a concussion. 7.62x52 is one of the most common and hardest-hitting shoulder fired rifle calibers, more 'powerful' than 7.62x39 and the 5.56xcrap
vest? if you shoot someone in the chest and theyre wearing a vest the vest will get a hole in it. if you meant body armor, the effect of the kinetic energy of the bullet is dependent on what class of armor (III? IV? more? less?) and if theyre wearing any type of hard-plates as reinforcement. the distance between the shooter and the target factors in as well.
slugs from a 12ga are the same as any other bullet when it comes to energy dispersion equations and are no better or worse. lead slugs tend to be softer than the harder jacketed rifle bullets so deform more readily.
again...you get an 'a' for effort but your application sucks a festering niggers penis