Can a nail safely be used as a cotter pin, it's harder, stronger than a cotter pin, but some would argue its dangerous cuz it's not a factory style cotter pin.
sure it can. and i'd have no qualms about doing so if you're broke down at the end of a dirt road in some third country **** hole.
but if i see that on a car it's a sign and it tells me that i need to be on the look out for other half *** hack-a-saurus wrecks work or boondoggle engineering. if you're not willing to use the right part for something as cheap and trivial as a FN cotter pin, where else did you cut a corner and how else have you played Cheapjack McGee.
one day it's a nail as a cotter pin, the next it's speaker wire to install an ECU upgrade. slippery slope my friends.