First, the Pmax for a .223 Rem SAAMI is 55,000 psi.
With a 55 grain bullet, a load of 26.4 grains of Varget is just under Pmax.
For a 69 grain bullet, a load of 24.2 is just under Pmax.
(The Sierra Manual does not list Varget as one of the powders tested with the .223 Remington.)
The Pmax for a 5.56x45 is 62,366 psi.
With a 55 grain bullet, a load of 27.3 grains of Varget is just under Pmax.
(The Sierra manual (they actually test) lists a range of 25.1 to 27.2 grs.)
For a 69 grain bullet, a load of 25.1 is just under Pmax according to QuickLoad.
(The Sierra manual lists a range of 23.3 to 26.1 grs.)
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I think this may be not quite correct in concept
Rather than transcribing,and to provide a source,this is Western Powder;s explaination.
https://blog.westernpowders.com/201...56x45mm-military-nato-cartridge-and-chambers/
Pretty much its about NATO using a different way to measure pressure than SAAMI. There is an "apples and oranges" effect. Is about transducer placement. The same cartridge and load NATO pressure will be different than SAAMI
One source I looked at said SAAMI P-max is 58.000 psi for 5.56 and 55,000 P-max for 223. The European CIP standard,per the Western Powder article,shows approx the same P-max for both 223 and 5.56.
Generally speaking (with an exception described by Western regarding a variety of thin web Federal brass) the brass is essentially the same and interchangeable. 5.56 is not "higher pressure" brass.
The issue is simply a chamber throat detail. The .223 is a 1964 tech varmint approach round. 1 in 12 twist and 55 gr max bullets. Throat details optimized for that bullet.
Its not so much about 5.56 being a higher pressure round. Its about the longer 5.56 bullets being jammed to the lands of the .223 throat. That jumps pressure above .223 or 5.56 spec.
But here is a clue. A 1 in 12 twist rifle will put 62 and 69 gr bullets through keyhole shaped holes in the target.Accuracy will be horrid.
And why would we have a 1 in 9,or 8,or 7 twist? To shoot longer,heavier bullets! So,we chamber 1 in 9 r faster twist barrels accordngly. Maybe a Wylde?
The difference IS something to pay attention to, but there is some mythology around it.