Pop Rivet Mandrels
I threw away the nails from the getgo - not enough pulling force (IMO) before the head popped off. So I have used exclusively the steel mandrels out of pop rivets that best fit the holes in the dies & skin. These mandrels are designed with a "frangible" stem - it breaks at an engineered axial load, and is (within manufacturing limits) repeatable. The breaking load is much higher than any nail I could ever find (maybe some hardened ring shank nails would work - just never tried them), and there was always a ready supply as close as my rivet assortment. On -4 holes I opened up the holes in the dies to accept a larger mandrel in order to get more "pulling power" - in some instances you don't need to break the stem to get an acceptable result, and you can keep using the stem/die combination over and over again until fatigue takes over and it finally breaks.
I "harvest" a selection of mandrels ahead of time and keep them with the die sets for ready access when needed.
YMMV - but I'll bet that if you have "gone over to the dark side" and used these mandrels for this purpose, you won't go back to nails anytime soon!
HFS