.216 Zephyr

This is the .216 Zephyr cartridge, which pushes an 87-grain bullet at 2900 or 3100 f/s (depending on the loading). It's an especially well-balanced cartridge, suitable for LMGs, rifles and carbines.
EDIT: I changed the composition to include a plastic insert instead of the airspace. Why? Because it simplifies construction of the round, by allowing a very small rod to hold the lead (and the insert) while the copper jacket is put on, which makes construction more precise and quicker.
EDIT: I made the scale (and proportions) significantly more accurate. It used to be using rough scalings and measurements that may have been though multiple size increases. So now 1px=.1mm