Provided one is willing to accept a degree of abstraction, I disagree. One attack roll does not have to equate to one bullet (or burst).One attack roll equating to a flurry of bullets (most of which miss) is entirely plausible and more realistic (in real world gun fights most shots miss).
I've just never seen a d20 system with rising HP make this remotely plausible. As a teenager I did a lot of fencing and shot a lot of guns (of various kinds, in various situations, kind of unusual for the UK, but anyway), and I've just never seen anything in d20 with rising HP which didn't feel incredibly stupid and immersion-disintegrating compared to my actual experience of firearms and so on. With melee I can totally buy you might well only get one good chance to connect in six seconds. It's completely plausible to me. With archery or the like? Sure. Draw and firing arrows is not necessarily a speedy process. Same with throwing weapons.
With a semi-automatic or multi-barrelled gun? You have to be kidding. Someone with L1-equivalent training could certainly fire multiple shots in six seconds (potentially empty the clip - people often do), and if at close range, many of them might hit.
And HP, which works okay conceptually in other situations, just doesn't do well with this whole idea, both because its less plausible (you're constantly having bullets either slightly clip people, or hit armour plates or whatever and bruise), and because of the way HP steeply rise in D&D, which just makes everything even more questionable.
AC also doesn't really work well with modern firearms.
I know people love to say "I SOLVED THE PROBLEM!!!!" and I'm sure I've done it, but this isn't one problem, and you're not the first to tackle it. It's a constellation of problems that combine to create a situation that grinds immersion into dust. It's not like I haven't played tons of games which thought that they "solved it". I have - obviously including d20 Modern and Spycraft. They just don't end up plausible, not even as action movies.
Whereas loads of other RPGs have fundamentally different mechanics that handle this sort of situation both gracefully and plausibly. Using 5E for a modern-day type game is like deciding to use a hammer to cut wood. The only times it can work, in my experience, are far-future, Buck Rogers-esque fantasy, where ray-guns and the like don't necessarily work like RL guns, nor does the "space armour" you're wearing, or in Buffy-esque games where guns just simply make themselves scarce 90% of the time, and aren't good tools for the job most of the rest.
I do think you could do a d20-based system that loosely drew from 5E mechanics to make this work, but I think you'd need to rework HP and AC massively, and likely make it so you had multiple attacks/round as a matter of course and at some point it's probably just going to cease being 5E in any meaningful way.
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