The throat or leade is where the projectile sits waiting to be fired, because of that area that is why when you shoot .223 bullets or bullets loaded to .223 specs you actually make less pressure. With the new 5.56 data I am finding that loads I tested with but never used because they were over the recommended charges for .223 are now within the recommended range. I have spent the last 4 days pulling around 600 bullets and reloading them to 5.56 specs because my tests showed them to be more accurate.
Stav is correct you load to magazine length unless you like a single shot AR
Here is an example of my load change...75gr Amax loaded to 2.390.. .223 data 24.9gr CFE223-- 5.56 data 26.5gr CFE223. That is a lot of difference, but will allow me to shoot further and straighter, I actually found 26.4 to be more accurate for me but that still is quite a bit of change.
Other than the extra length in the throat there is almost no difference and I noticed that they were using the same primers for both. I am hoping to go shooting tomorrow. PDogs beware, I can even stand further back.