Question: how big are bb pellets?
Answer: Modern BB guns usually have a barrel with a bore caliber of 4.5 mm (0.177 in), though many other varieties are available. These guns usually use steel BBs, either zinc- or copper-plated to resist corrosion, that measure 4.3 to 4.4 mm (0.171 to 0.173 in) in diameter and 0.33 to 0.35 g (5.1 to 5.4 gr) in weight.