If you go to Ruger's sight and the ShopRuger.com link will take you to their store front. Then on the left side, pick scope rings, then pick model Redhawk from the drop down and then blue or stainless. I clicked blue because I didn't know what you had, but they show all the different heights and sizes available. This varies greatly depending on whether you are getting a RedDot 30mm or 25mm and the front bell housing of the scope you desire. Maybe someone familiar with the particular scope you want to use will come along and feedback on what they used.
Step 2: Select the Scope Ring Size.
Scope Tube Diameter Scope Objective Diameter Scope Height Front Scope Ring Rear Scope Ring
1 inch 42MM Medium 4B 4B
1 inch 52MM High 5B 5B
1 inch 62MM X-High 6B 6B
30MM 42MM 30MM Medium 4B30 4B30
30MM 52MM 30MM High 5B30 5B30
30MM 62MM 30MM X-High 6B30 6B30
http://shopruger.com/searchscopering.asp
