I use a Lee 308 collet die and a washer.
Find some washers big enough slip over the case. The washer will rest on top of the shellholder. What this washer will do is stop on the collet die from going down to far on the case before the mandrel sizes the case's neck. Works for me.
I'm not home now but it took me 3 different washers of different thickness to find the right thickness to get the sizing right. I remember it wasn't the thinnest one, maybe 1/16th in thick? It wasn't the thinnest or the thickest one I pick up at my hardware store.
An ignorant person is one who doesn't know what you have just found out.
When setting a job up for myself it must be Idiot Proof as well, as I am a bigger idiot than most people I know, and I prove it to myself everyday.