[ThunderCactus]

That's a completely different bird.
Paintballs explode and spread that 14j over a significantly larger area on impact.
Airsoft BBs deform only a little, but apply all their impact force over just 10 square mm or less.
1.64j with a 6mm BB is 24PSI or higher
Whereas a paintball is just under 13PSI
And consider the fact a BB also holds it's shape, making it infinitely more likely to penetrate skin compared to what is essentially a paint filled water balloon.

As little as 350fps can penetrate skin point blank, that's why we have MED's for sniper rifles.
And as far as the science of it, it's the velocity and shape of the projectile that's more likely to penetrate, not necessarily the joules of force, as your skin doesn't have time to deform to the projectile to absorb the impact.

You can get hit in the arm with a hammer at 200j but it's not likely to break skin.
There are many cases of people chipping finger bones with BBs (I've actually fractured Amos' finger with my sniper rifle), and many more of them having BBs shot into skin and even a tongue. And those are all at field allowed joule limits.