Hi rnshooter and welcome to TFF. For an explanation of what roll sizing does, see the following:
All brass is not the same. Each manufacturer has dies that make the brass to his dimensions. Case wall thickness and web area are the most important due to the fact that this controls the volume of the explosion upon detonation. With thicker case walls, you have less volume and therefore more pressure when detonation occurs. Same with the case head and web area. The smaller the area the more pressure develops with the same amount of powder charge.
Military once fired brass is usually the thickest case walls and web area and will cause the most problems in this area.
A way to measure this is to take several cases from different manufacturers and fill them to the very top with water. Then weigh and record each measurement and you will get an idea of the volume of the case. You can even take the same manufacturer and measure different lots (made at different times) of brass and see a difference also. That is why a lot of people who reload, make sure that all their brass has the same volume prior to loading. It can make a big difference on the range and in your weapon. Sometimes, in cases of maximim loads, can be down right dangerous.
Once fired brass is fine if you know it all came from the same lot and manufacturer, or if you are only loading reduced loads for enjoyment.
Serious shooters in competitions will most usually know the exact lot numbers for the brass and manufacturer that they are using.
Hope this helps.