...now we will hear from the "bbbbut it's not marked!" contingent.
</snark>
Critical thinking time-
- Shooting 5.56 ammo in a true SAAMI spec chamber can result in pressures reaching as high as 70,000 psi. Especially if the leade is cut to minimum length.
- If the leade is lengthened, pressure peak is reduced. That's why a 223 Wylde (and similar chambers) spec a longer leade. To fire 5.56 ammo safely.
- Not all 5.56 ammo is loaded to maximum pressure.
- 5.56 chambers have a reputation for being less precise than 223 chambers.
- Bolt action shooters look primarily for precision.
Gun makers are well aware of the above facts. You can bet they don't cut their 223 barrels with a shorter leade as specified by SAAMI because someone is going to run 5.56 ammo through the rifle. You can bet they won't mark their bolt action barrels as being chambered for 5.56 because shooters looking for precision will stay away in droves.
Just as marking a barrel 5.56 doesn't guarantee the chamber is cut to actual 5.56 dimensions, marking a barrel 223 doesn't mean it's cut to SAAMI dimensions. Doesn't mean it isn't.
Taking all the facts together, is it wise for a shooter to just start banging away with 5.56 ammo in a barrel marked 223? What if there's a chance that there's a perfect storm of minimum leade and maximum pressure? There's a reason serious reloaders measure the leade before getting started on any caliber.
Sorry - what I have always understood is this:
The .223s are loaded to lower pressures and velocities compared to 5.56mm. Due to its lower pressure, you can safely fire .223 Rem. ammunition in a 5.56mm chambered gun; however, the same cannot be said in reverse.
223 ammo isn't loaded to less pressure. It's loaded to make full pressure in a chamber with a shorter leade. When fired in chambers with longer leades, pressure drops.
SAAMI & NATO use different methods to measure pressure and the two cannot be directly compared.