This ^^.
Easy to have .002"-.004" variance in neck thickness. The full length die uniforms the o/s of the case, pushing any variance to the inside. The mandrel die pushes the variance back to the outside.
After thinking about what I am seeing this is the next thing to check. With Laupa brass I see .002 after mandrel neck sizing. With RP I am seeing .005. I am thinking neck turning to get uniform neck thickness is in order.
