This is not a headspace problem. Your cases are too small for your chamber. The case is lying in the bottom of your chamber held firm by the bolt on fireing the case expands more to the unsupported side causeing head separation at worst. Solution 1 use a fireforming light load or add a O ring to the case to centre it in the chamber before fireing. Get better fitting cases or better quality ones. Do not mess with the bolt untill you are 100% sure it is not the ammo