BoS at level... Under 20? Eesh.

I explored the entire North hemisphere of the map before even going near Diamond City & BoS.

I refuse to use a gun which has under 1000 rounds.
Use Melee & drugs a lot more as other suggest, not a bad idea to cut legs off legendary or glowing ghouls too.

I've been in the situation where all ammo is spent, 1 enemy left, both of us on ~10% health. I died. I don't want that again heh.

Have you been to the Massachusetts house yet?