The AD-B2 modular base is the medium base in our modular lineup. It is designed to accept several different risers to accommodate a variety of optics. Standalone, it is ideal for the 1.5a^'3x Power Mini ACOG Scopes, such as the TA33 and TA44. The AD-B3 modular base is the large base in our modular lineup. Standalone it is mount for for the full sized 3x Power ACOG scopes, such as the TA01, TA11 and TA31. Also fits scopes with similar ACOG Style Bases such as the Burris AR-332. Alternatively it can accept several different risers to accommodate a variety of uses. The overall length of the base is 3.5a ^2. The AD-B4 modular base is the largest base in our modular lineup. Stan Mfg: American Defense Manufacturing