The Talley Tikka T1x Scope Mount I would recommend for a 40mm objective is low height mount. Typically 40-42mm is a low mount/44mm is a med mount/50mm is high/and 56mm is extra high. However, It depends on what rifle its being used with and will vary from gun to gun. For example, AR15 style rifles and pistols should use a medium mount at the lowest due to the way they are designed. I would recommend emailing us at Techsales@opticsplanet.com to let us know what specific rifle you are using, if you have a picatinny mount for it, or if it has a top Pic rail already etc. That way we can accurately recommend the correct rings for your application.