Solid idea and well-executed! There are some things that struck me that I'll mention (note after writing this review - I'm sorry it's so lengthy!):
By the very nature of how bullets are shot, players who approach weak spots will deal with fewer bullets and have an easier time, which is good! It encourages offensive play and interaction with the core mechanic, AND has the bonus effect of emphasizing the secondary mechanic, which is the bullets bouncing off walls and persisting (because the bullets likely won't hit the player as they're being shot). Truly synergistic design, and felt fun to engage with!
The highly defended enemies which were surrounded on all 6 sides were interesting because they didn't start with a weakspot - you have to create one. And doing so also leads to more bullets bouncing around. In theory, I don't dislike that - but when a player interacts with that sort of mechanic for the first time, it's a bit confusing and uncomfortable. The reason for that is the very same efforts the player is doing to open up the enemy and win are also hurting the player's chances; in that way, it's different from how missing shots for example will lead to bouncing bullets as the latter is due to the player's error.
From a purely numbers and fun perspective, I don't think that the actual mechanic needs changing - perhaps it could do with a simple disguise. Maybe when the player shoots these defenses, the bullets turn green instead of red, and the defense could make a meaty sound effect (like a crack) and show some particles to let the player know that their efforts are paying off. You likely know better than I do the best way to address this issue (if needed!), so I'll leave it at that!
Ultimately, this is the sort of game that I think would particularly benefit from clear and satisfying particle/sound effects (especially upon hitting the enemy). As for the gameplay, I think you've made something really solid! It's a unique and excellent concept turned into a fun game - great work!