First off, I agree with Oogaphobia. Also, you can fix the self harm thing by putting the player on a collision layer that doesn't interact with the player bullets layer.
I would have made the colliders on the ammo boxes a bit bigger. I wanted to aim at the sky as I pick them up so my first few bullets don't go into the ground, but I was having to look straight at the ammo box to not miss them.
I like the bullet hell feel of the game with hundreds of projectiles flying around as the enemy closes in. The 3rd level in particular feels like an epic standoff. I did find there was something invisible about 2 sections up the tower stopping me from re-climbing to the top where all the ammo was so once I fell off it was pretty much over.
Anyway, it was a pretty interesting game, and I had fun, so thanks for making it.