Really interesting game! Sadly I don't know what I picked up or what happened to my run, but I got to this point where there were numbers on my bullets and I kept shooting the boss and all it looked like was that my bullets healed him instead of dealing damage. It might also have to do with the fact that I bought some items by accident since when the round ends and im still clicking to shoot enemies, the game doesn't wait for the transition to end, so I end up clicking items in the shop before I can even see them; as for the bullets, maybe there was some mechanic I skipped over in the tutorial? (since it dragged on for so long I just skipped it, since I thought the game was rather self explanatory).
I also find it really punishing that the red / blood bullets don't even shoot out a projectile, they just damage you and help in no way. I understand that they are supposed to be bad for you, but the fact there is a timer during each round means you don't really have the time to spare to look at your chamber and spin it to miss them, and I was already taking damage from enemies hitting me since instead of looking at them I was looking at my chamber trying to dodge the bad bullets. there is an opportunity for the player to play it risky and use the bullets that actively harm them. I also think the enemies shouldn't be falling from the sky as a spawning animation, as that also threw me off a lot since they are fully visible and I thought they were already spawned in, so I ended up shooting them as well, wasting time.
Halfway through the rounds I just gave up looking at the chamber and spinning it since the damage I took from the bad bullets wasn't as high as the damage from the enemies hitting me and killing them healed me so I just put all my focus on actually shooting the enemies, and I could mostly ignore the bad bullets after a while since I bought plenty of normal ones that made the bad bullet's chance much lower than at the start.
There's a lot of potential in future updates to this game, I hope you'll keep working on it!