However, it is worth noting that this caliber can be formed from 222 Remington brass (this is NOT a typo), however a small percentage of the cases are likely to crack. It's also possible to form it from 223 Remington cases, but a much higher proportion of cracked cases will occur.
It's also worth knowing that the bullets also have to be reformed from .224" bullets normally used in 223 Remington / 5.56mm. 5.45x39mm is actually .221", and at the present time there are no commercial bullets made in this size. Therefore, one must compress .224" bullets until they lengthen out a bit and end up as perfectly round .221" bullets.
-- SeanNewton - 13 Dec 2008