The inside diameter of the pillar (hole) is = 0.260", which can accommodate up to a #14 bolt (1/4") or a M7 metric bolt freely.  
 
The outside diameter is irregular to increase adhesion.  The 1/2 pillar has a maximum diameter at 0.480", so it can fit freely in a 1/2" (0.500") diameter hole, or tightly in the same hole with bedding epoxy applied.  The 7/16 pillar has a maximum diameter of 0.430" so it can fit freely in a 7/16" (0.4375") diameter hole, or tightly in the same hole with bedding epoxy applied.
 
All our pillars are CNC machined from aluminum for the highest precision. The pillar must be cut to length (with the exception of the specific Ruger 10/22 pillar), and are long enough for all installations. In many cases, this will result in enough material for more than one installed pillar.  The roughened finish on the outside of the pillar increases the surface area the bedding epoxy can bond to, providing a more secure installation.
 
Correctly pillar bedding a rifle will improve its accuracy. Read about why on our engineering page.