I don't see the problem with guns having insane damage compared to a bow. They DO have insane damage compared to a bow. An entire Seal team can be taken out with surprise and four well placed bullets. Wielded by children, if they're trained well enough.
Yeah, but in D&D terms, the "arrow to the eye" requires a crit or a rogue or someone with the sharpshooter feat. Or opponents with very few hit points.
So does the "bullet to the eye". In D&D terms.
The point that he's missing, or is too stubborn to acknowledge, is how firearms factor into a game with such an abstract damage and wound system. With HP, there's a fairly plausible narrative to someone "hitting" someone with 6 sword attacks but not killing him. Glancing blows and superficial cuts rule the day here -- all wounds that don't cause people to wonder how they are still alive after the fact (not strictly realistic, but *plausible*).
I think the point that you're . . . not getting, is that firearms attacks can be incorporated into such an abstract system. There is a fairly plausible narrative to someone "hitting" someone with 6 pistol shots but not killing them.
Firearms change that dynamic because there's no good way to narrate how a bullet wound doesn't result in massive damage or death within a few rounds.
Sure. A bullet wound from a powerful enough firearm that strikes the head or into a vital area on the body is going to do that.
But assuming that all firearm "hits" in the D&D system are so severe, whilst also maintaining that sword "hits" can strike superficial areas, just cause grazes etc, is a bit of a double standard.
If your group runs with a "HP are meat" basis, and PCs are capable of bouncing axes off their faces with only superficial damage and fighting an orc horde with 20+ arrows embedded in them, absorbing firearm fire probably isn't too much of a stretch.
If your group runs with a "HP aren't meat" and that HP loss from sword "hits" doesn't automatically mean actual wounds, then losing HP from barely-missed or lucky deflections of firearm fire rather than actual wounds are no more of a stretch than crossbow bolts.
This is exactly why I want them in my Curse of Innistrad campaign. I want the PCs to be able to blow away some baddies before having to resort to other weapons to finish things off. Guns have a big punch but then cost resources to use again (and do come with some risk as you say). Just like in the Napoleonic wars, the fighters lead with their pistols/carbines and then switched to swords when they no longer have time/safety to reload.
That works pretty well with firearms of that era, but the OP is talking about modern weapons, with rapid rates of fire and large capacities. D&D already plays rather fast and loose with weapon speeds in terms of bows and crossbows.
Bluntly, if a group is playing in a setting where they are actually using modern military weapons capable of 30+ shots per combat round, they arte going to need some heavy house rules, or a system more suited to those parameters.