Originally Posted by JonM
your not going to see any difference in a 55grain fmj. its things like surplus ss109 tracers and other special use cartridges.
bullets for the 556 has a larger allowance in how far the ogive can be down the body of the bullet at its widest point which in the diagram is the S measurement. due to the need to keep the same weight when your putting in a phosphorous charge or a steel penetrator the displaced lead has to be moved further up in the bullet body making that S measurement further out.
the reason folks say dont use 556 in a 223 is because we have no idea what ammo your using where it came from and if it was made to a lesser standard. some countries use a lead alloy or a steel jacket or some other funky stuff which can make the S measurement safe for a 556 but not for a 223
JonM: Do you know if there is a difference in volume between the 5.56 and .223 shell? Note: I only have 5.56 shells