Machined from steel and are easily installed.
If included in description, the fractional dimension is the outside diameter of the metal aperture.
The decimal dimension is the size of the hole drilled in the aperture which is actually looked through by the shooter.
Apertures are standard thread size and will fit Williams, Lyman and Redfield receiver sights.
The twilight aperture is specifically designed for low-light conditions.
Big Game aperture is designed to mount on the FP or 5D receiver sight and has an extremely large opening, similar to a ghost ring type of sight.