The Meta Pro peep has a patented convex interior diameter maximizing the field of view in low light. The convex interior also provides a perfect circle as the peep begins to tilt. No more looking through an oval! The string groove is also convex creating less surface area that the string actually contacts, reducing string wear significantly. So much so that the Meta Pro has a *NO-CUT GUARANTEE!! The Meta Pro comes in 3/16" 1/4" and 5/16".