As BRITZ said full length size.but while your at it trim to length,
deburr the primer pocket and weight sort the brass this way you
when you fire form you can check your chamber for concentricity
before neck sizing. and start with matched brass.
A simple way to do this is to roll the cases on a peice of glass or
mirror and you can see the run out if there is any.
Load on the light side with regular bullets to fire form but not hard
enough to shorten case life.
Now when you neck size and load you have the best case fit and
headspace for your rifle.
Hope this helps
J E CUSTOM