I checked the search function, but I still have a couple of unanswered questions. I would like an ACOG, probably a TA31, so here goes...

1.) On the RCO option, it specifies a model for the 16" bbl and the 20" bbl. I assume the only difference is the scale on the bullet drop compensator. Can you use the same one for either 16" and 20" bbls, and go between the two? If so, are you better off with the 16" version or the 20" version?
2.) With the 4x ACOG, is it better suited for the 20" bbl out past 100M, as opposed to an M4 at 50M?

Thanks in advance for your help. I want to make the best choice for what I have.

This was posted recently:

Hey XXXX, Let me clarify this for you; you're on the right track! The TA31RCO-A4 is calibrated for the M16A4 20" barrel using LC M855 62 grain ammunition. The calibration was done based on firing tables provided by the USMC. The TA31RCO-M4 is calibrated for the M4A1 (flat top) 14.5" barrel using LC M855 62 grain ammunition. Again, the calibration was done based on firing tables provided by the USMC. The TA31F is calibrated for a 62 grain bullet traveling at 3050 fps. The calibration was done by our engineers on a commercial ballistics calculator. The 3050 fps. is basically an average velocity of .223 ammunition and a 14.5"-20" barrel. The TA31F provides "Combat" accuracy from a 14.5" to 20" barrel. Remember, all ACOGs are "combat" optics and are not designed for 1 shot 1 kill accuracy. Too many variables in combat dictate ballistic indifference, especially old weapons and mass produced ammunition. Others include altitude, temp., barometric pressure, etc. Snipers are trained to overcome these later obstacles and they have equipment to compensate for them. Snipers also have the time and distance advantage. Soldiers and Marines using the ACOG need a simple dual role sight (close quarters to 800m) that does not require mechanical adjustment between shots. Point (aim) and shoot! The TA31F is still the best dual illuminated ACOG for those that are going to switch the optic between weapons with different barrel lengths. I hope this helps XXXX. Keep in touch. Shaun M. Rategan, Manager Trijicon's Military and Federal Law Enforcement Division

There is a TA31F with a LaRue mount on EE for $915.  I have three for 16" and 14.5" uppers.  If I had the money I would not be posting this.

I didn't see that one. Thanks for the good info. I appreciate it.