Sounds like your buffer tube is a smaller diameter than a mil standard buffer tube. As stated in the description, the Z-360 QD Sling Mount is designed with interlocking clamps for an extremely secure fit on mil spec and commercial buffer tubes. Installs in seconds with no weapon disassembly or modification. Fits all collapsible stock AR15 / M4 models with a mil spec (1.14 in dia) or commercial (1.17 in dia) buffer tube, and is known to be compatible with the following weapons: HK 416, Sig Sauer 716, the Law Tactical Folding Stock Adapter, Ruger AR556 pistol, and weapons equipped with a Vltor A5 buffer tube (with minor modification). The Gen 2 Z-360 is not compatible with non mil standard buffer tubes (i.e. pistol buffer tubes, Colt Canada Buffer Tubes), non mil standard receiver end plates (i.e. QD receiver end plates), ACE SOCOM stocks, the Smith & Wesson M&P15-22, the Smith & Wesson M&P15, and the HK MR762.