WHAT IS COR?
It is not possible to get a COR of 1.00 (100/100) as this would mean that there was no energy loss during impact.
From a practical point of view knowing that even sound is a form of energy loss the estimated high on COR is about 0.930
COR OF A CLUB AND BALL COLLISION
It has been found that the COR of the collision between the club head, which does not exhibit any Spring Like Effect (SLE) and a standard ball is about 0.768 at a head speed of about 110 mph (see above Wooden Clubs).
If the face of the club deforms and recovers during impact, like a trampoline (see above Titanium Clubs), this takes up some of the deformation that would have otherwise been in the ball, the COR would increase as there are fewer losses in the deformation and recovery of the club face than in the ball.
Looking at the History of this rule may help:
• 1909 the rule was first introduced and required that the "club shall not contain any contrivances such springs." • In the mid 1950s the rule was changed to read that the club face shall not contain any finish for the purpose of unduly influencing the movement of the ball. • In 1984 the rule was specifically changed to read "The clubface must not be designed and manufactured to have the effect at impact of a spring… or have any other effect which would unduly influence the movement of the ball."
This rule is essentially unchanged today; however there seems to be some divergence from the literal translation of the rule and the way it is being interpreted and enforced. The rule allows for “some” spring-like effect. A decision of convenience and compromise.