Maybe I got lucky but I've froze my water extinguisher up twice and it still works fine.   Forgot it both times at the end of the season and only remembered it later.   I'm assuming it was froze solid, I know I couldn't hear water sloshing inside and it didn't operate.   After it thawed out it worked fine and still works.   There much be enough give with the air inside.   I will say I now make it a point to get it pulled before it freezes though.   I always thought they split too or at least bulge out.