Originally Posted by
MistWolf
The military specified that the leade of the 5.56 was to be longer than that of the 223 Remington in order to get more velocity while keeping pressure about the same. When the spec was set, the bullet to be used was the 55 ge FMJBT which isn't a very long bullet, or they might have specified an even longer leade. Service Rifle competitors do the same thing to allow them to load the longer 90 gr bullets with increased velocities. Roy Weatherby used the same trick to get more velocity from his line of proprietary calibers.
I don't know what the practical limit is, but the bottom line is it's proven to work