It's stupid and immersion-breaking if you are familiar with edged weapons too. A knife through the heart will make you just as dead as a bullet. 285 people where killed with knives in the UK in 2017/18, around ten times as many who where killed with guns.
HP is just stupid and unrealistic however you swing it, but since it makes for a fun game we just roll with it.
Sure, but that's only
one of the problems. HP don't represent being "stabbed through the heart", anyway, which is part of the issue.
With a knife, it's very plausible to say you got a shallow cut or gash for that 4 HP you took, or they stabbed your chain repeatedly and couldn't get through but you're going to have bruises and are a bit winded. With a 7.62mm rifle bullet? Um. You basically end up getting "Tintin'd" constantly "Oh the bullet just grazed me. Oh the bullet just grazed me. Oh the bullet just grazed me." and so on, and it's a lot inherently sillier than melee weapons which are easier to explain. If they're wearing armour, it's like the plates end up being bullet-magnets as suddenly they keep being hit and causing bruises.
Modern firearms just an absolute ton of problems
on top of the existing problems D&D has with immersion-breaking stuff. And whilst YMMV, for my money, they smash that immersion to pieces.
There are a lot of options. If you want action-movie logic I think D&D would work fine. Real world logic? D&D doesn't even do that very well without firearms.
Have you actually tried action movies (not SF movies or Star Wars, note) in a version of D&D, or even a close relative? Because I have. I've tried a bunch of systems. And the results are awful. This is because of a few things:
1) AC derived from DEX and armour and so on doesn't work in action movies.
2) D&D doesn't, by default, have any kind of "that didn't happen" mechanism to prevent one-shots and so on (whereas most good action movie RPGs do).
3) Rising HP with levels makes little sense in an action movie or action-TV scenario. PCs should start with a lot of HP-equivalent.
4) HP which assume you get hit make zero sense in an action-movie scenario in most cases. VP/WP can work though it has a whole bunch of problems of its own.
5) Guns firing a single time (even if it is supposedly multiple times) per attack just raises tons of questions and feels very un-action-movie-esque.
6) Guns doing more damage than melee doesn't make any sense at all in an action-movie context, because melee is extremely effective in virtually all action movies.
Can you fix all this? Maybe, but you're building something pretty far from 5E, as I said.
AC could derive from being a protagonist and so on, rather than armour.
You could add a whole subsystem which let players say "Ok I didn't get shot in the head!".
You could start with like 30+ HP and not have them go up much, if at all (instead the subsystem above could increase).
You can replace HP with something like VP/WP (a system with problems of it's own), but history relates that this is not an easy thing to get right. At the very least you probably need rules equivalent to CDGs for kill-shots and the like. Which may involve a whole separate subsystem and economy of "kill points" or whatever.
You could make all firearms have multiple attacks but whatever way you do this its going to be tricky to make work with 5E's action economy, and it's likely to mean you end up simulating on specific subset of action movies, rather than "most" action movies (which is fine, but you need to be clear about it).
You can make guns not do more damage than melee, but the whole notion of damage gets pretty fraught and complicated even when compared to D&D (which already has enough problems with "meat points" and so on).
And maybe you end up with a game that has a Proficiency Bonus, Advantage/Disadvantage, six stats, and uses d20 rolls + stat + proficiency as the main mechanic, but it's sure not going to be 5E or remotely 5E compatible.
And some game which didn't even try to do all this nonsense, but had a fundamentally different mechanic is just going to laugh at this house of cards you've built, because why not use the right tool for the job?