The revolutionary, patent-pending Drivelock design, uses three independent wedges that interface with the angular grooves in our proprietary barrel nut. This system clamps the rail onto the steel barrel nut with extreme force and precision, while also driving the handguard rearward towards the upper receiver. The slightly larger ID of that extrusion gives us more real estate around the gas block, even more rigidity than the M76, and takes longer to heat.We are not discontinuing the M76. That will remain in full production. We are just adding the M89 to the lineup. M89 Drive Lock Rail Available Length: 8", 9.5", 10.5", 13", 15" and 16.75" ID: 1.4" T-Marks: Engraved T-MarksFinish: Mil-Spec MIL-A-8625F, Type III, Class 2, Hard Anodized Coating, Non-Reflective, NiAc Seal (Anodized to match SOLGW receivers)What's included: Rail, Barrel Nut, Lock-Up Components, and Instructions for Installation Weight 4 lbs Dimensions 3 x 24 x 4 in