Armor is a bit of an odd duck, I guess.
Armor as Damage REDUCTION has never really flown well with me, even for modern settings. If I walk up to you and shoot you half a dozen times in the chest with a handgun, even if you're wearing a vest, I'm pretty sure you'll know. But if that vest offers DR 5 ... well, you might notice, but not much.
At the same time, having that armor mean that, say, if I miss you by 5 or less that the armor totally absorbed the gunshot and you don't notice ... eh. Even worse!
Spell-rays (where we get d20-system rays) usually mean something that IGNORES the damage-mitigation of armor. You take Elemental damage, for instance, which usually ignores armor in D&D entirely. Or you get Disentegrated, or Enfeebled.
So, for D&D style armor, if we are shooting a Ray Gun that causes elemental damage (Fire, for instance, for a Heat Ray) ... then yes, I suppose that it should function as a Ranged Touch Attack.
BUT ... if we have Future Style Armor that is supposed to protect from Ray Guns ... then I would say NO. Which may be conceptually odd, but the reasoning is based on some fundamental assumptions they made when putting together D&D which all d20 is then based on ... for them, Elemental (Energy) damage ignored the damage mitigation of armor that made Armor Bonus functional.
Likewise we could say that Modern Armor wouldn't allow Modern Guns to fire as Ranged Touch Attacks because they're designed to protect vs. ballistic weaponry, so ...
While conceptually interesting, I think it's just too fine-grained to really bother with.

You could do something like Archaic Armor functions as Armor Bonus to B/S/P damage, while Modern Armor functions as Armor Bonus to Ballistic ... but Archaic Armor offers no bonus to guns and Modern Armor offers no bonus to Piercing ... and Future Armor functions as Armor Bonus against lasers, but Archaic and Modern armors DON'T offer Armor Bonus ... etc etc.
There can be Too Much Of A Good Thing.
For my part, something I've started doing is Damage Conversion ... where armor ALSO offers its bonus as a number that converts Lethal to Nonlethal damage. Not going to say it's any more or less realistic than Armor Class vs. Damage Reduction, but it's sort of the best of both worlds and my players really liked it. I might eventually go to a system that does Damage Conversion with no Armor Bonus. Dunno.
Price to Value. In the end it is a game. You start adding too much realism to your game, it loses the game aspect.
--fje