Originally Posted by fi3re
consistancy of FPS between shots (consistant airseal), hopup, bore of the barrel and length of the barrel to a certain point. Barrel doesnt effect as much as you think it would, but shortened to a point obviously accuracy will be effected, as well as large bore would make a bb bounce around.
This. You are almost right. Its really the consistency of the bore of the barrel. You could have a 6.08mm barrel with a very very smooth finish and as straight as an arrow and a 6.01mm barrel with an okay finishing and 'meh' quality and the 6.08mm barrel will be more accurate. Length of the barrel means very little, you really only need the first 4 inches of your barrel. The longer the barrel, the more possible inconsistencies meaning less accuracy.
The way you install your hopup and what type of hopup system you have also plays a role, as well as the bbs and hopup rubber and spacer. Also, how far your air nozzle pushes your bb forward has something to do about when the bb will make contact.
Also, the vibration of your barrel. You will notice that most guns are alot more accurate on semi auto than full auto, because when the piston is hitting the cylinder head the vibrations are sent through the mechbox shell and through your hopup and then onto the barrel. This can me avoided by 'shimming' your inner barrel to your outer barrel in 3 spots: Close to the hopup, in the middle, and at the end of the barrel.
Edit: More on bbs. These are the worst things about accuracy. Better consistency of bb the more accuracy. Don't cheap out on BB's you could get increases of accuracy by just choosing the right bb weight and brand.