I'd put hollowpoints as Bludgeoning damage, myself, sort of like blunt arrows.
As for the simple-vs-exotic? While it didn't require the long practice to aim well, an early muzzleloader required plenty of practice if you wanted to reload at a decent speed. The powder had to be measured out (pre-measured cartridges came much later), the shot had to be loaded correctly, and so on. Now, once you get to the breechloaders that used cartridges (basically what we do in the modern day), it's definitely not a complex weapon.
IMC, as I've mentioned before, we treat Exotic weapons as being simple or martial weapons that gain extra bonuses when you take the right EWP. So in this case, Firearms would be a Simple weapon with a 1-minute reload time, but that with the EWP the reload time drops to, say, 3 rounds.
I'm sure you could write up something similar that would fit into the core D&D rules. Take the Bastard Sword as a model; without the EWP it can be wielded as a martial weapon, just in a more limiting way.
And on the shotgun discussion: people ALWAYS overestimate the "cone" spread of a good shotgun. Unless you've sawed off the barrel and opened the choke the whole way, at the gun's extreme range, you're looking at MAYBE a spread of 10', and if it's spread out THAT much, your target's not going to be hit by many individual BBs, so at the maximum effective range the radius is even smaller. (And if you do saw it off, your range sucks.) I hunt pheasant, and if they're at anything other than close range, you're just not going to hit them enough to take them down; hitting with one or two BBs out of 300 isn't enough to stop them from flying away, usually.
The point is, that's not nearly enough to worry about as a "cone". What it does do, though, is improve your odds of hitting (in D&D terms, a flat attack bonus) as well as mitigating the effects of partial armor (some exposed spot WILL get hit), while being totally ineffective against full armor. D&D doesn't model this last bit well, so I'd just stick with a flat attack bonus, although I'd also say that anyone along the line of sight (including beyond the target) has a chance of taking damage as well. BBs ricochet really easily.
(Also: "slower to reload"? No. Modern shotguns have an internal magazine of 4-7 rounds. I personally prefer a pump action, which only takes a second or so to reload; my father uses a bolt action, which takes about the same; my grandfather preferred a semi-automatic. Yes, 4-7 rounds isn't a whole lot, but you can also reload quickly if you know what you're doing.)