Hello, all!!! Not to dispute SqaureNail, but I'm quite sure that that is a 5.56mm (.223 caliber,civilian) military cartridge to be fired in training exercises. The cartridge is crimped on the end to keep the powder in, of course, and the muzzle of the M16 is blocked off with a red cap (to force the gases, when fired, from the powder round, or blank cartridge, up through the front sight to a gas tube), which in turn causes the bolt to unlock and the empty, or "spent", casing to be ejected, at which time the magazine forces another cartridge in front of the bolt, starting the cycle all over again. If you use a micrometer on the outside of the casing, just back where the bullet would be, I'm sure it will read (guessing) around .258-.260. It is really just a juiced up .22 caliber, as it sure seems, compared to the penny. I apologize for all the explanation, just thought it would help make what I was trying to say understandable! By the way, fire several mags of that round, and you'll spend the next 6 hours cleaning up all the ball round foul from your weapon!! Great hunting and good luck all!! Regards, Richard.