This is a neutral post!
-----------------
Well lets think about it for a second. If you've ever taken Physics in High school you should learned that wind and air on a bullet or BB is called a force. More specifically it's a force called friction. Now when a bullet or BB is lauched from the gun a force is applyed to make it move. If you multiply it velocity times it's mass and divide by two you get the kinetic energy. So with that said, if two bullets are launched with the same mass, but different diameters, they will have the same kinetic energy, but the one with a smaller diameter will experience less friction from air due to it's smaller surface area. But if the bullet or BB with the larger diameter had a larger mass this would give it a greater weight- or in terms of Newtons: Force. So in order to move the bullet/BB it would require a greater amount of friction from the air.
So, in this case, we can say that the horizontal momentum would have a direct relationship to the diameter of the bullet; and horizontal momentum would have an inverse relationship with the mass. Meaning, the bigger the diameter gets the more the momentum will be effected. And the larger the mass gets, the less the horizontal momentun would be effected.
So in conclusion, it's obvious that the direction of the forces would be different with many different combonations of mass and diameter. So trying to determine- physics wise- whether and an 8mm is more accurate than a 6mm, or vise versa, isn't enough.
There, not trying to a smart allec but I'm just trying to help solve a disspute, and so I had to lay the facts out in order to prove it.
Once again this post was completly neutral, so no critisism is allowed, thank you.