The main benefit is that the Roll Sizer works along 100% of the case height, to remove bulges left by unsupported chambers, or case head expansion experienced by cartridges fired in "race guns". These and other
out-of-spec case abnormalities happen where traditional Sizing Dies simply can't reach due to the presence of a Shell Holder.
Typical case fired in an unsupported chamber...
The issue for the high volume reloader and handgun competitor has always been
the ability to check each case before reloading. The problem being that the need to
stop and clear a bad round during competition can be the difference between winning and loosing a match. Competitors on the leading edge go though mountains of ammo each month as they train and compete. Checking each individual round to insure that there are zero case defects would take an enormous effort.
During roll sizing, the cases are individually placed between 2 opposing steel blocks that are machined to precise tolerances. Typically the interior plate is rotating which makes the cartridge case roll between the 2 opposing blocks. The precision and strength of the steel blocks effectively removes all abnormalities in the same way that a "steam roller" flattens out new pavement.... except that God is holding the paving equipment stationary, while the Earth turns under it.
The down side is that the cases must be washed clean before going through the roller. Dirt brought into the case roller would definitely work to destroy the precision surfaces and clearance, just as it does with standard Sizing Dies. And then, the case must still go though the normal Sizing Die as the first step of reloading.
And too, it only corrects the case. A shooter could still experience 1 or 2 bad bullets when buying 10 or 20,000. So it doesn't eliminate all cartridge inspection, just the most likely source of problems.
This site has a lot more info and videos...
https://www.rollsizer.com/Hope this helps.
