Ok, seriusly, most weapons in D&D are bullshit.
A light, or worse, a hand crossbow that you can reload with one hand is a completely useless weapon that will deliver 0 penetration. An early pre-perfect-steel-grade heavy crossbow is a 10-20lb monstrosity that can - in its heavier variations - only be reliably fired on a bi- or monopod, the same as early high-power muskets. (Especially seeing that with the slow reload, you do want that one shot to hit.) On the other hand its power is basically arbitrary. In D&D terms, with stupid physical and mental stats, and magic, it should be possible to construct basically any kind of draw weight.
Check out these modern-made historical crossbows:
http://www.wikingerschmiede.de/Armbrust__Jagd-Armbrust_Kampf-/body_armbrust__jagd-armbrust_kampf-.htmlNote that the hunting crossbows have standard draws of 200-250lbs (up to 600lbs optional), and weigh around 10lbs, but even the hand-crossbow at the bottom has a 100lb draw. That's not something you can pull just like that, so even the hand crossbow has the stirrup affair at the front. The winch crossbow has up to 1000lb draw, longbows can just go home now.
But once we're there there is just nothing wrong with just making firearms just like crossbows, with a one full-round-action reload for muskets, or one move action for breech loaders. Any damage bonus or special characteristics come simply from class features. Or you could give them a boost and make the damage like the crossbow one category larger.
(Once we include the stupid mental stats that can be attained in the game, it's also not too far-fetched to even include modern, cartridge loading weapons. In fact, this is D&D, even a full auto heavy machine gun is hardly game breaking, it'll just ruin the challenge rating of encounters.
For example:
Muzzle loader:
2d8, complex action reload.
Breech loader/bolt action
2d8, move action reload
Semi-auto-rifle
2d8, can do full-attacks,
Automatic weapons get weird, I simply suggest volleys:
M16
2d6, when using 3rd-burst mode fires a volley of three shots with each attack for a -2 penalty on all attacks
MG3
2d8, when using full-auto mode fires a volley of three shots with each attack for a -2 penalty on all attacks
50cal HMG (LARGE weapon)
3d8, when using full-auto mode fires a volley of two shots with each attack for a -2 penalty on all attacks.