At the end of this comic page one character says: "Grab your nails". At the next pages the second character found a chest with a gun and bullets inside. Could "nails" refer to bullets in this context?
It isn't an expression as far as I know. The idiom "fight tooth and nail" refers to animals that fight with their teeth and claws (in fact, "tooth and claw" is also said) so it seems unlikely it is a derivation of this - one does not need to collect their fingernails.
At the next pages the second character found a chest with a gun and bullets inside. Could "nails" refer to bullets in this context?
It could, as 'nail guns' do exist, although they are a tool, not a weapon. Still, this is a comic, so anything is possible. However, as the house explodes in the very next panel after the character says "get in the house and grab your nails before...", this character never manages to retrieve any 'nails' so they are unlikely to be his.
I would expect that earlier panels are more likely to give you a clue as to what these 'nails' are.