I'm in the already-living-in-the-woods and building-a-small-farm stage of prepping so speaking not from the armchair but from the workbench:

You know what I've run out of more times than I can count, while working on a project? Nails and screws. You know what I've never run out of because they go a long way? Bullets.

Of course, stock up on bullets. I just bought a new firearm with a new caliber to me so I immediately bought 500 rounds, and I'll probably buy another 500 before I even shoot 1/10 of what I already bought.

But unless you're ONLY prepping for end-of-the-world, anarchy-in-the-streets, shoot-your-neighbour style collapse, you probably need more nails and screws. I see people talking about how they have a box of screws and that they've never run out. Uh, have you ever built a new subwall? A chicken coop? A wood shed? Screws and nails go fucking fast (as do studs, I have never brought home the correct number and always need to make a second trip.) If you're prepping for any and all service disruptions (like, oh, a pandemic when you might not want to hit the hardware store three times in one day?) then having building supplies on hand is *key*, unless you don't do anything in a crisis except sitting at home cleaning your guns.