All BB's are pretty much the same size, the difference however is the quality.
Take crappy BB's for example, they have seams and will chew up your hop up and barrel. So it's a given to use high quality BB brands such as BBBastard, Mad Bull, Airsoft Elite/Metal Tech, etc.
What will give you the best bang for buck is probably a good hop up unit/rubber. You could have an extremely short barrel (eg. pistol) and still get accuracy comparable to a gun with a longer barrel and a mediocre hop up.
Tightbores (smaller bore on the inside of the barrel) do help however I was told not to go tighter than 6.04 or 6.03mm. Stock most rifles come with 6.08mm barrels. NOTE: What matters here is the consistency of the construction of the barrel. You could have a 6.04 TBB (Tight Bore Barrel) that has crappy inside lining and end up with worse accuracy than stock.
Note that when someone says 6mm BB it's rounded up. Most BB's will have a diameter of between 5.94-5.97mm with a tolerance of +/- .02mm
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