Dumb question here.
When you feel a burst of powder from a .22 blow back from the bolt (with shooting glasses), could it be mainly because the ammo was exposed to humidity for a few months? Some of it is much older and has faded cases.
My other ammo types have never been left in the car's trunk, i.e. .223, x39, x54R.
Only two x54R rounds have suffered cracked cases and one was from a new green, oval can (440 rds.) which includes the opener. This can had been stored in the house for a few weeks since it was opened, as with all rounds except some of the old .22.