It all comes down to the common operating systems for 223. Operating pressure for a bolt action vs an AR15 are different. The different certifying organizations also spec different pressures for the “same” cartridge.
- .223 Remington (SAAMI): 55,000 psi.
- .223 Remington (CIP - European): Up to 62,366 psi (430 MPa).
- 5.56 NATO (Mil-Spec): Generally operates around 58,000–62,000+ psi, depending on the load
The load data where Sierra lists 22gr is a 55kpsi load. Sierra has no idea if people will be shooting out of a gasser or bolt gun, and so present the safest pressure loading as their max charge.
The 23.5gr XBR load is much more in line with 60-62kpsi.
The brass doesn’t know what operating system it’s in. But the bolt lugs on an AR15 will start to have set-back into the barrel extension with persistent higher pressures.
Semi auto bolts start opening while there is still a significant level of pressure in the chamber. The closer your gas port is to your chamber, the higher these opening pressures can be. It’s one of the reasons that a rifle length gas system will shoot “smoother” than a carbine length gas system. But it also means that the bolt is very susceptible to wear and damage from higher pressure cartridges.
The lugs on a tikka 223 won’t have the same setback with higher pressure rounds. They have more material behind the lugs compared to an AR bolt. These actions are the same for 300win mag and 223rem. They just change the bolt face diameter and bolt stops.
They also effectively operate at a zero opening pressure. No matter fast you are at running the bolt, you will never be as fast as a gas operated system. And by the time you start rotating the bolt handle, the chamber pressure has already equalized with the environment.
There are also chamber geometry changes between 223/556 that can affect how pressure builds in the chamber.
At the end of the day, tikka proofs their bolts at 75kpsi. The 23.5gr/77tmk load is a pretty standard 62kpsi load. The 22.0gr/77tmk is a safe 55kpsi load. Running the 62kpsi loads in a bolt action is perfectly safe. Running those same loads in a 223 AR15 (in high enough volumes) will lead to early parts breakages.
