I had heard that most of the ACOG 5.56mm models have the BDC calibrated for the 20" barrel. I asked about it on the optics forum and really never got a solid answer as to which models are calibrated for the 16" barrel. So I sent an inquiry to ACOG asking which models are have the BDC calibrated for the 16" barrel.
Here is Trijicon's reply:
Thank you for your interest in Trijicon products. Models TA01-NSN and TA31RCO-M4 are most closely matched to your application. Both are for flat-top calibrated for the M4 platform (14.5" barrel.)