Am wanting to mount a 4X ACOG on a 16" barreled M4. Am having trouble deciphering which if any of the 4X models are calibrated for a 16" barrel. Best I can tell th TA31RCO-A4 is calibrated for a 20" and TA31RCO-M4 is calibrated for 14.5". Are there any that are calibrated for 16" barrel and if not, which ones have a calibration most closely matches a 16" barrel.