Bolt thrust force with the Grendel compared to 223

[LRRPF52]

Why are the pressures and forces so much less with the 223 and Grendel ?
Is it just because they're rated for semis ?

To answer this question, it's all about case capacity. The 260's case holds more powder than the Grendel case so it is going to be SAMMI pressure rated much higher. It has little to do in what type gun it's to be operated through.

Only reason I'm highlighting this is for safety and general awareness.

Case Capacity
There are larger case capacities that have way less working pressure specs, and smaller cases like 5.56 that have higher working pressure specs, so you have to look at more than just case capacity.

.260 Rem is a SAAMI MAP of 60,000psi, which is less than what 5.56 is rated for, and bolt thrust is the main reason for that pressure limitation.

Compare .260 Rem with .308 Winchester, its parent, or 7mm-08, and you will see higher working pressures for those cartridges. Why?

When you bottle-neck/overbore the case more, start pressure and bolt thrust increase even shooting lighter projectiles. Bore volume is reduced as well, so things get tight.

With less bore volume to relieve the pressure through for the same case size, the rearward pressure increases (bolt thrust).

Also, if the case has more taper like a .22-250 or 7.62x39, all other variables equal, you get increased bolt thrust due to the shaping of the detonation wanting to act like a rearward-facing shape charge.

Action types
It very much depends on the action and when it was made. Advancements in steel alloys during and after WWII really changed the working pressure ratings for rifles, and rifles like the 1896 Model 6.5x55 Swedish Mausers and Norwegian Krag-Jorgensen rifles are limited in working pressure, whereas ones made with more modern steel actions can take pressure like other modern production rifles.

Vihtavuori even publishes two different data sets for reloading old and new 6.5x55 Mauser actions, with the new being called "6.5x55 SKAN" or "6.5x55 SE", and the old being called "6.5x55 Swedish Mauser".

Another determining factor in the pressure rating for that action is the pressure containment vessel diameters, as well as the bolt lug and lock-up geometry.

For example, the lighter/smaller lever guns have limited working pressures, as do the lighter/smaller single-shot Thompson Contenders.

On the other end, you have actions like the SAKO that are extremely tough and well-engineered to handle modern higher pressures, but the OP is well-validated in asking about the smaller SAKO action chambered in .223 Remington and expanding the bolt face to a larger case head diameter. I will also point out that SAKO has chambered the smaller actions in 6mm PPC, with its 52,000 CUP pressure rating. All of the Hodgdon's pressure figures for 6mm PPC are in CUP, with none higher than 51,300 CUP.