The Salmon River Solutions Universal Picatinny Rail is designed for flexible mounting with 10-32 and M Lok fasteners. It can be used with a standard sling stud and has an integrated flush cup for QD options. The 3.5" version weighs .6 oz. and has bolt spacing of 1.25"-2.5". The 4.25" version weighs .65 oz. and has bolt spacing of 2.5"-3.25". Each rail comes with 10-32 screws and M-Lok mounting hardware, to provide compatibilty with a wide range of forends.