"Using commercial .223 Remington cartridges in a 5.56mm NATO chambered rifle should work reliably, but generally will not be as accurate as when fired from a .223 Remington chambered gun due to the longer leade.[12]"
"NATO chambers have a long leade[1]. SAAMI chambers are tighter and have a short leade. SAAMI chambers are designed for increased accuracy,...."
I also had only .223 cases, dies and such... So I didn't really see a need for the 5.56 and then combine that with the accuracy items... easy choice for me.