Originally Posted by Skruface
A typical airsoft bb weight 0.20 grams. A typical .177 airgun pellet/bb weighs about 0.65 grams. That's three times as much mass. Force = mass X accelleration....you do the math.
Airsoft guns have less range, less accuracy, and less impact energy.
To add, it's all about momentum. The pellet gun will have more momentum and therefore will be less likely to deviate. For example take a Smart car vs a semi trailer. Sure they may both go 50 Km/h but which is easier to stop? turn? get moving? The smart car obviously, but why? A semi trailer has a lot more mass and as such it takes more force (in an opposite direction) to slow down, start moving, turn etc.
Take that to a BB, between .20 g and .65 g the mass like Skruface said is 3x as much so you need 3x the force to get it to do something. You want it to keep straight? Well the ambient air and wind direction will affect the pellet and BB but the pellet will be affected less because it requires more force to make it deviate.
As for which is more accurate, the .177 match pellets (AKA "Wadcutters") I believe are the most accurate with the super point being the ones that offer the most penetration (ie. for pest control or w/e you want to do with it).
With metal BB's (AKA "Bullet Balls") they are just that, balls and shot through a smoothbore barrel meaning they don't get spin to keep it stable in air and as such may be slightly more affected by crosswind and other ambient factors (but still less affected than the 6mm airsoft we use which range in weights from .20 to .36+ g with .25 or .28 being most common).
EDIT: Sorry stryak, didn't know what I was thinking there, it must be an Asian thing or something (maybe a "big city" thing?) because I swear that I grew up with g and gr meaning the same thing and they can be used interchangeably. BUT now thinking about it, real steel shooters use grains as a standard measurement so yeah....