Well, this "ballistics" argument has been going on in the real world as long as there have been guns and in the virtual world as long as there have been simulations, so... I wish you luck in finding any sort of standard.
A few things from the real world:
Calibre is not really *THE* determining factor in how much damage a weapon will do. In reality it's like arguing over whether you'll be hurt more if run over by a bus or a cement truck. A quick look at history shows that the military finally realized that larger rifle cartridges, such as the 7.62mm, tend to punch nice clean holes in the target while a 5.56mm slug usually gets inside them and bounces around.
In urban combat training one of the things you learn is that a 9mm round fired from a pistol will penetrate through more soft cover (wood, plaster walls, furniture, etc) than a 5.56mm round fired from a carbine, a 12ga slug fired from a standard shotgun, or a .44 mag round fired from an extended barrel revolver. On the flip side, 5.56mm rounds can slice through metal plates that the 9mm bounces off. So much for standard penetration statistics...
I guess I'm just trying to say don't sweat the small stuff, you'll never please everybody, just try to keep it reasonable (whatever that is). And whatever you do, avoid becomming a "stat monger" with ballistics information, especially if it's provided by a person who has personal preferences, opinions, and bills to pay (definately avoid anyone who says they have no opinion)... if you want some real ballistics stats, go to the nearest big city trauma center or morgue and talk to them, that way you can hear about the people who live through headshots from 30-06 hunting rifles and those who die from a .22 pistol round to the arm.
Personally, anything that performs at par (or close) to the original weapons and won't kill a Tahoe and the driver in one shot is fine with me. If you don't like having to shoot someone 3 times in the legs with the FN FAL, shoot them in the head next time. I think I even like having to hit someone several times if I screw up, it just makes it that much more gratifying when I do manage to kill that running soldier 200m away with a single shot.
Ok, I'll shut up now