AR-15s have two common Buffer Tube designs; Mil Spec and Commercial. When buying a new buttstock it is important to know which tube you have because a Mil Spec buttstock will only work with a Mil Spec tube and a Commercial buttstock will only work with a Commercial tube. The main differences between the two tubes styles is the buffer tube diameter. Both tubes have the same threads. The best way to determine which buffer tube you have is to measure the diameter of the tube with a set of calipers.
Mil Spec Tubes:
Mil Spec tubes are designed to military specifications hence the name. Mil Spec tubes are milled down to have a diameter of 1.148 inches. The back of the Mil Spec buffer tube is always a flat 90 degrees.
Commercial tubes are not milled down like the Mil Spec tubes are. This saves time and money in the manufacturing process. The diameter of Commercial buffer tubes is 1.168. Most (but not all) Commercial tubes will have a 5 degree angle at the end.