Cannot drill into stud
I live in an apartment that was built in the 2000s. I was told drywall, I assumed wooden studs. I bought magnetic stud finders, found the nails and then could even see them painted over when I looked closely. I went to the hardware store and bought black oxide drill bits. The store employee said they'd work for just drywall/wood studs. When I hit stud, I cannot drill any farther.
I'm hanging an Elfa shelving unit so it will need to support some weight. I guess I'll just have to call the apartment office tomorrow and ask if it's metal studs (if they even know). It's a wall that is shared with another apartment building.
Any ideas? Thank you for your time.