The lip is there from Starline. See the first picture. That's how it comes from the factory. I believe the mouth is rolled like that because of peening when the cases are tumbled together -- in the bin at the factory, and in the bag they package them in. Any tumbling is going to peen the mouths progressively worse. For the same reason that Starline does not package each individual case in a separate compartment and protect the mouth, I also handle my brass in bulk piles. It gets piled in Akro bins, it gets dumped in and out of buckets, and worst of all it gets tumbled in a tumbler. As I wrote earlier, I wet tumble my brass a minimal amount, typically 15 to 30 minutes at the most. That is enough to clean it without peening the mouths excessively.
While I believe bulk handling is a significant cause of mouth peening, there are other factors that cause the rolled edge of the mouth. If I ram the mouth into a bushing (or any OD sizing die) the front edge of the mouth will bend in first as the mouth is squeezed first and then the neck. Unless a mandrel is pulled back out of the neck to straighten that edge after the bushing crushes it, it will just stay rolled. To get the Redding mandrel to pull the mouth back out, the neck would need to be sized down farther than I have been squeezing it.
I get the same rolled edge on my handgun brass, for the same reason -- tumbling and outside-only forming. There is simply no process that I use that flattens that inside edge other than firing cases. Tumbling, resizing, and crimping all roll it in. Firing is the only thing that pushes it out.
The rolled edge can be cut off with an inside chamfer tool, but the more the edge is chamfered, the more it is thinned and that makes it easier to roll the edge another time.
