Apparently this is a problem that I was not aware of and there appears to be several myths around it and neck sizing only. I’ve added three videos advocating FL resizing that include the why’s from credible sources to illustrate the point. I have included my own simple tutorial below that.
https://youtu.be/ECUcIeak7CM
https://youtu.be/lLG2kSrD40g
Start this one at 11:00 minutes.
https://youtu.be/ZrTKhVPPi34
Here is how to do it with simple tools, a factory chamber, and an I care more than zip attitude.
Here is 308 brass fired in some unknown chamber that I’m going to resize for my sons Kimber Montana 7-08.
A standard FL die. Screw die down until it touches the shell holder and back off one full turn.
Remove your firing pin assembly so you feel no resistance when closing the bolt. I have smoked the brass for demonstration purposes. This works on a rifle with a fixed blade ejector and will not with one that has a plunger type. Resistance and feel works for both. If the firing pin assembly has been removed.
Move die down until the resistance when chambering the round just goes away. This shows about two thousandths bump. Notice the slight ring.
Or do the above process with this $40 tool. This one is from Sinclair.
This expander ball gives between three a four thousandths neck tension which is plenty, but will do.
You can fire these Winchester cases, unsponsored, in a factory chamber, using standard dies 50 times and they will not separate. AND you do not have all the baggage associated with neck sizing only.