AR-15 stocks are available in two different versions: MilSpec and ComSpec. These versions vary because of different dimensions of the buffer tubes. Please measure the dimensions of your buffer tube before ordering and make sure to order the correct version.
Diameter Mil Spec: 1.148 Inch (= 2.916 cm)
Diameter Com Spec: 1.168 Inch (= 2.967 cm)
1 Inch = 2.54cm