Originally Posted by Apex-Predator
Do you realize how much easier life would be for us reloaders if chambers were actually cut to spec? One magic length would work in any rifle......ahhhhh
That is a fantasy land. In the real world I just bought a new Browning so inaccurate you could aim straight down at the ground and miss, even with my best match handloads and a stable rest. You think that rifle has it's chamber properly cut to spec? No the real reason you cannot shoot 5.56 through a 223 is pressures period, the cartridges have the exact same external dimensions and there is no way you could design a throat to work with one and not the other except for pressure.
The case dimensions are identical. The design pressures appear to be close enough that they dont concern me greatly.
What does concern me is: The 5.56 NATO chamber has a longer leade, than the 223.
The shorter leade on the 223, means that rounds loaded for the 5.56 with bullets seated further out to match that chamber, will be touching the lands, and may develop extremely high pressures.
My brother and I both own 257AIs. Mine has a longer leade than his, and the rounds I load to be 0.004 off the lands are actually pushed into the lands on his, resulting in high pressures if he fires my loads in his rifle.
So, even small differences in chamber/barrel geometry can make a major difference in pressures.
I think that if I owned a 223, I'd be very tempted to have the chamber reamed to the NATO 5.56 chamber. It would still handle 223 rounds, but would eliminate the potential for high pressures from the 5.56.