Well; I am reading this conversation and hear that case weight means a whole lot and someone else says it means nothing and there are several reasons why on both sides. I Have a theory that I haven't seen there exactly and it is how I think so I joined and am going to muddy the water some with my understanding or misunderstanding. So I am going to join your group since I see several level heads that are interested in many things that interest me.
The Rifle is Chambered in a caliber by removing some amount of metal and it has a fixed volume If I put a heavy weight case in that chamber I will have my bottle to hold the powder, sealed with a bullet, the primer holder sealed by a primer and the gasket that stops it from leaking. My usable volume of that chamber is less than if I had used a lighter weight case, More brass in the case takes up more space in the chamber. The case weight causes the effective chamber volume to vary, so I believe uniform case weight is how we get uniform chamber volume and that is what is important so we have uniform pressure at firing. We start out with a chamber that is perfect since it is used for each cartridge and we put a case in that chamber that is lighter or heavier than another one at the moment of firing. The volume is established by the size of the chamber and the case has conformed to the chamber and done it's job as a gasket by sealing to the chamber walls and released the bullet. If we resize the the case to factory spec's it can be fired in another chamber and have a different volume. As brad said if the weight is different and the volume is the same where did the extra brass go.
I hope this is clear, the more I read it the more I feel that it is not but I don't know how to make it better and hope I don't sound like a know it all. Yep this country boy is in over his head. In any size case I sort by weight more than head stamp and use about 5 grains as the limit for each batch of 308 size cases to get what I call factory equivalent loads. More demanding types of ammo get narrower limits and more things done to them. I can't see an extractor grove varring by 30 grains and every company no matter where that brass is made, In house or outside contracts will have variations from lot to lot and the range of variations will change every time there is a die replaced or sharpened or polished. Brass alloys even vary some. If any test is done to determine the changes with cases before and after cutting the extractor grove or any other action it has to be done with the same lot of brass or it is of no value. I understand that lot numbers can change from just a machine adjustment depending on what it affects and always change when dies or punches are changed. Now this is only taking into account 1 method of manufacturing (extrusion or drawing), If cases are from different methods all values must be rerun but major changes to the machines or methods still make major changes to the product.
Thanks for listening
R D
May GOD bless you and yours