The Lee Colette Factory Crimp die applies uniform neck tension to each round and thus removes one vexing variable from the accuracy equation. But, make no mistake, the Factory Crimp is adding neck tension. You can set a Lee Factory Crimp die so it pressure forms a cannelure where there wasn't one in your loaded bullet.
When we speak of neck tension we are speaking of the tension below the crimp on the bullet.
OK. Time for my lesson then. Why crimp at all?
The expander plug should be set so as to only expand the case mouth edge as to not shave lead when the bullet is seated. If the case mouth is expanded too much you will lose case tension below the crimp. The bullet should expand the case as it is seated, so as to see the outline of the bullet in the brass.
The crimp is not a replacement to good even neck tension, for heavy recoiling revolvers.
