It's not a bad idea, but I can think of multiple problems with this one.
First, there's the obvious balance issues..... something that's probably beyond solving this close to release. A major change like that seems very.... heavy.... for such a late point in the beta. I can guess at alot of not foreseen problems popping up with all sorts of spells because of this.
I had a lot of your same thoughts at first, but in practice it's not really that different. Because of the model for the way spells work, I'm not actually tracking literal health with them, which helps keep unintended interactions basically to zero. It's all about the damage taken from caliber shots, essentially, which is a completely different matter from any of the other healths, etc, in the game.
The main reason I jumped on a change like this is how broken the current system was with the caliber perks on and some concentration. You basically could have an invisible infinite shield in front of you, made out of your particles, against many enemies. That's just so incredibly broken. But that was really an edge case in the prior system; except in that edge case, which was only even discovered recently, things worked well.
Well, except the bit about being so uncertain all the time about what spells would do against each enemy's spells. There are so many spells and so many enemies that was too much to ask anyone to remember. The new system ties that up pretty neatly without getting too complex.
But the other problem is more of how it would work compared to how the current system works. In the current system, let's say, it's me VS a monster that fires out a steady stream of shots at me, whichever monster that might be. In the current system, if I have a higher caliber, I can pop the thing's shots, and bam, it's done. But with an HP system.... it ends up being more like "stand there and keep firing until they EVENTUALLY get through", because you'd have to lower the HP of each shot before they'd break. The challenge here doesnt really increase because of this, it simply takes longer. A system like this is also likely to be completely useless against any enemy that moves too much; you can count on it for enemies that DONT move much, because you know where their bullet stream is going to be, but lots of enemies DO move alot, and often rather erratically... against these foes, there's no point in worrying about caliber if it cant break through in one go, as with a slower system, by the time you break the shots, they're already gone (as in, the enemy has moved to a new position, requiring you to change your aim now). It's simpler just to dodge and hit the enemy directly. Even simple wall crawlers would cause this to sorta break down. I'm not going to try to use shots defensively against enemy bullets either if it takes more than a single strike to pop the shots, either, because that means more time spent firing at the enemy bullets JUST to break them, while other things are still going on and potentially attacking me....
Part of this is as always my own general absolute lack of patience, so keep that in mind, but those are the problems I see with that idea. I would honestly probably just ignore the whole system after a change like that. Doing anything that involves any randomness also is one that seems like it'd be a mistake.
Check out the release notes, I think you'll like what you see. In general the system is the same as the old, except better organized and more consistent. Then when you get to the perks and the concentration system, it gets slightly more complex, but we're talking about 2-6 shots to break through one enemy shot. Given the rate of some enemy shots, and the rate of some player shots, this means that you can pound through really quickly -- half a second in some cases -- and it's not a big deal. I structured it specifically to avoid the grind that you're worried about.
A lot of this really comes down to the details of the math, and Gemzo really laid out some excellent goals for the math, I thought. I was initially really unsure about whether there was any math that would actually balance it properly, but after about an hour of hashing it through with Josh (who was really reluctant to make any caliber system changes, unlike me), we came up with math that made a lot of sense on paper, and it turned out to work well in practice in my testing, too.
All in all it's not a grand sweeping change in terms of how the game feels. But it makes it easier for new players to get into the caliber system, and prevents some exploits on the other end of the spectrum. All the middle cases are pretty much just shifts in which spells are balanced against which other spells, which I think we all were agreeing needed some tuning lately anyhow. So there's nothing apocalyptic here, don't worry.
