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 has 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. The have more material behind the lugs compared to a 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 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 proof 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.
