1 hour ago, Xtrakicking said:
Nails have never been used to upgrade, as far as I'm concerned. You can shoot zombies with the nails. Not sure why.
There is a challenge you can get from the 'Willie gang' thats apparently had their eye on you to nail chicks, you get exp for completing. Other then that so far the shooting nails is kind of useless. There are other challenges as well, you find them in loot.