For a rifle chambered in 5.56 with a 16" barrel I recommend using the H1 or H2 model of the Geissele Premium AR-15/M4 Mil-Spec Buffer Tube Assembly w/ Super 42 Spring. If you plan on shooting suppressed I recommend using the H2 model of the Geissele Premium AR-15/M4 Mil-Spec Buffer Tube Assembly w/ Super 42 Spring.