What most people don't realise is that you will see just as much variances between different brass, ammo and rifle chambers in any other chambering. Often a Remington 270 chamber will vary slightly from a Winchester chamber. You will find that at times Federal 270 ammo is loaded to very differnt pressures than Hornady 270 ammo and with slightly different brass specs.
This is my point exactly . You have one country making a slightly longer 5.56 round and a gun maker making a slightly smaller 223 chamber/lead/throat . This is when you get the pressure spike