BBs have a long history of materials, diameter, and even weight in the long history of their use. The BB started off as your typical birdshot for a smoothbore shotgun. Made out of lead and measuring at 4.3-4.4 mm, these were mass produced, but there were always inconsistencies in the BB due to the loose restrictions on shotshells. It wasn't until Daisy started making their own 4.4 mm "round shot" that the uniformity of the BBs increased and started to turn into what we use today. Your modern standard BB is made of steel or lead and formed into its spherical 4.4mm shape and then coated in either zinc or copper to help stop corrosion and help the BB's consistent accuracy.