I have some Federal .223 cases with loose primer pockets, I use my Lee Universal decapping and depriming die when the primer feels loose when seat the primers. I place the case in the Lee die while holding the die in my hands, if by just using finger pressure the primer pops out then the case is tossed in the junk bucket.

If you let a loose primer in your reloaded case it can leak high pressure gas that will etch and erode the bolt face and damage the bolt. Buying new cases is cheaper then having the bolt replaced.

Below is a AR15 bolt face that has been damaged by loose primers and high pressure high temp gas leakage. And this is why I carefully check my primer pockets if they feel loose.



The type rifle you have and the amount of headspace along with a loose primer pocket will determine if you blow a primer.

Below watch the primer move as it is being fired, gas pressure forces the primer to the rear and as pressure increases the case stretches to meet the bolt face.



A primer can move by the same amount of head clearance or "air space" you have behind the case when fired.