Sounds like the gun is properly designed...at least you now KNOW the hammer can't hit the primer until the bolt is fully home. You also have checked that the projectile isn't engaging the rifling when chambering...yes? I had a short chambered Browning Hi-Power that just wouldn't take ammo that chambered easily in all 9 other guns at the time and if it did manage to get one chambered it would have very much higher pressure.
But super high pressure doesn't show on the primers (though that's a very imprecise way to gauge it) so early bolt movement coupled with perhaps some lack of ductility in the brass seems to be a good enough explanation for what happened. A more ductile case most likely would have just swelled up as shown in the examples posted by others and given a more gentle reminder that something is slightly amiss with the rifle.
What to do now? First question: what kind of hammer spring are you running? Unless you can add weight to the bolt/buffer assembly perhaps a heavier hammer or spring would add enough resistance to retard the blow-back a little longer? If I were you....I'd be doing something along those lines. You NEED to keep that bolt closed a wee bit longer methinks.
