Ok, here's the new stuff, first part is cutting open three BBs of each weight to take a pic.
Notice that the lighter BBs have a larger bubble (or two) inside them, and the heavier ones the bubble either gets smaller or dissipates into dozens of tiny ones. Is because more material is packed into the BB. Bubbles (large ones anyways) are bad to have in BBs, mainly because they often rarely sit in the middle, causing a slight wobbling of the BB in flight as it spins (hop up). This is one reason, besides the effects of the variable air density which will cause yawing (moving the BB to the side), or causing the lift generated by hop up to cause a rolling action to one side to occur on the BB (pilots can understand that, if one wing generates more lift than the other, the plane will roll towards the side that has less lift), and because of that 0.20g BBs should never be used outdoors.