The most common is c-t-c. If you measure outside to you are penalizing yourself when you use larger ammo.


Think about it if you shot .25 inch group c-t-c in in .22 LR your group would be .47 to the outside...shoot that same group with a .50 cal BMG...your group would be .75 to the outside.

As well look at how national matches and olypic shooting measure...all c-t-c....