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I assume that a .308 bolt action will be OK with case head separation? Am I wrong? What happens if a semi-auto blows a case?
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Cartridge cases act as a gas seal. If the chamber design provides a lot of support, then it is likely the edges of separated case head won't be exposed.
Some semi auto designs, like the Garand, provide good case support. Some bolt gun designs, like the SAKO finnbear, have a lot of unsupported case head.
If the edges are not supported and the combustion gas is free to flow, what happens afterward all depends on the action.
Garand designed his action so the gases go out the top and down the magazine well. The receiver heel is tight to the stock, but he put gaps on either side of the receiver heel as gas vents. This action was designed to protect the shooter in the event of a gas leak.
The 03 Springfiield provides almost no shooter protection. Gas will flow down the receiver rails and firing pin right into the shooters eyes.
Good post war designs like the Savage M110 provide gas baffles and ways to keep gas away from the shooter.