so i just used some pieces of wood to match the center height of the bipods. everything will clear the 30rd mag. so that leaves me with which length one to get... if i got smaller one i would probably use it extended all the time, if i got the longer one i would use it collapsed most the time. i also thought about if i got the shorter one and it was too short for whatever reason i could throw a 1/2 riser on the mount to give me an extra 1/2 inch to play with..