Marrakia,
I had a 500 nitro built by Ron Webb .The load he recommended (80gns 3031) was a woefully low pressure load
about 1700fps with the 570 woodleigh.The cases all had a similiar problem to the subject at hand.It was my impression that ,in my case,the problem was inadequate pressure not producing a good seal at the neck allowing gas "back to the body of the case"and causing a dent.
There was some powder burn effect in association with the
dent.