Neck tension

[Klem]

At .004 mine still move.. maybe .005 is the ticket. Crimp die ruins bullets before solving anyrhing in my case.

Blute,

Maybe, although it could also be a lack of elasticity (ductility) in the brass. If the neck is not elastic enough then it doesn't matter how narrow the neck is for seating. Bullets will expand the neck as they are seated but will be held with less friction than needed to stop it moving. Annealing should restore much of the original elasticity. If not, case-hardened brass that has been fired multiple times will hold the bullet less than needed, regardless of the neck diameter.

Obviously the amount of shank in contact is also important, as is the weight of bullet (inertia).