Brass shoulder deformation

[nijalninja]

I have cleaned up chambers with a long rod on a drill with a bit of cloth on the end. (Just make sure the rod can't bounce around and damage any part of the chamber or action).

One interesting thing however, in regards to you saying that it looks fine in there, one particular MKIII I own has a visibly pitted/marked/discoloured area on one side of the chamber, but after banging out some rounds none of the cases jammed up or even looked remotely deformed, so perhaps looks are not the best means of troubleshooting unless you have something like a bore-scope.

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[browningautorifle]

I have cleaned up chambers with a long rod on a drill with a bit of cloth on the end.

Not quite...try a brush about .45 cal size for the chamber, that's the affected area. Use oil or carb cleaner on the brush and just give it a few seconds to do the job. Then clean and examine. There's something wrong here, probably rusty chamber.