The thing people don’t really get about “bullet proof armour” is that it’s job is to stop the bullet going into you and messing up your fragile internals - but Newton still wins. The force still has to go somewhere.

Imagine someone held a stake to your chest then someone else smacked it with a sledgehammer - this would be a Very Bad Time for you, what with all the bleeding and internal trauma. If instead someone held you down with a steel plate and that was sledgehammered with the same force - it would hurt like hell, but probably not do the same amount of internal damage because the force is distributed over a wider area. There is of course a limit - at some point the force is still going to be too high and cause fatal damage.

Helmets work the same way - the internals of your head are very fragile, so keeping the bullet out is pretty important, but the same problem exists. The force has to go somewhere, and while getting whacked in the face with a sledgehammer is better than having a stake driven into your forehead it not that much better