I will share what works for me. I end up tumbling twice once before case work and again after. If brass is relitavely clean to start will I will dry tumble first with walnut. If it's range pickups that were sitting in mud I'll wet tumble without pins. I will then do all my case work resizing, trimming, chamfer, and deburr. Then they go back into the wet tumbler with pins, some woolite, and frankford arsenal brass cleaning solution with hot water. This tumble is never more than an hour, usually 30-45min is sufficient.

I think what's causing your issue is excessive time in the wet tumbler causing the necks to deform slightly. I have experienced this myself putting them in for that long.

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