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Two things that occur to me, one is if the chamber has a short throat or is wrongly head-spaced so when the case expands on firing the neck hits the front of the chamber.

The second is if the die is incorrectly set or the case is too long and it's being slightly compressed during sizing. I can't see any way tumbling would give that effect.

ovenpaa wrote:The second view does appear to show a very slight lip on the outer edge of the mouth. Do you have an external chamfer tool you can just gently run around the outside to clean it up?

I have a deburring tool, like this one, which I've been using to try and remove the lip. It leaves a little bit of a chamfer on the outside edge, which I believe is sub-optimal, but probably better than having the lip there. I may trim them as well.

The deburring tool will add a chamfer which is fine as long as it is in moderation, I finish mine off with a worn out green kitchen scourer which is slightly dampened, it removes any burrs and rough edges and I much prefer it to wire wool.