...Wait, wouldn't Bubble just make you invincible then?
Also, I've realized the staggering importance of the number 4 in Gun Rounds. The standard for enemy HP at the beginning of the game is 4 HP. There are 4 worlds and 4 levels in each. The shopkeeper sells 4 items. There are 4 different enemy types in each world. You can hold up to 4 weapons at once. It's very strange.