Discontinued Ball Bearing Plug
On smaller sizes like 8g, 6g, 4g, and 2g the bearings really don't slide around much at all.
8g has LARGE 4g flares
These have ball bearings in them that rotate. They will move if you touch them with your fingers, but aren't so loose that they move around on their own.
Jewelry is measured through a gauge system (see chart below). The higher the gauge number the smaller the wire is. A standard "earring" is usually 20 gauge. Jewelry gauges/sizes can vary between manufacturers. There is no universal regulation on companies to make a standard size in body jewelry. A big part of this is that some jewelry is made overseas and measured in millimeters. And in the USA jewelry is measured in inches/gauges. So many times jewelry can be anywhere from .5mm to 1mm different.