Which of these two barrels, 14.5" or 16", is the reticule of theTA31 calibrated for?  I'm sure (and I may be wrong) that if there is a differentiation in some Acog models, like the RCO (14.5 and 20") then I'm sure the TA31 is calibrated for a specific barrel length.

This was copied from Trijicons FAQ page on their site.

http://www.trijicon.com/Trijicon.cfm?CFID=3315636&CFTOKEN=35801954
» 03. What bullet was used to calibrate the ACOG scopes?  
   The TA01, TA11, & TA31 (.223) were designed for the 5.56mm, 20 inch barrel, 55 grain bullet, and M16/AR15 carry handle mount. The TA01B & TA11C(.308) were designed for the 7.62mm, 20 inch barrel, 168 grain bullet, and M16/AR15 carry handle mount. The TA01NSN was designed specifically for the US Special Operations Command SOPMOD M4 Carbine. To meet their requirements, the reticle bullet drop compensator and range finding stadia lines were based on the trajectory of the 62 grain bullet from a 14.5 inch barrel, flattop mount (Picatinny rail Mil.Std.1913).

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» 04. Does bullet weight and/or barrel length effect the reticle bullet drop compensator and range finding stadia lines?  
   The real issue is not the bullet weight or barrel length, but muzzle velocity, ballistic coefficient, and the distance from height above bore. The difference in trajectory from bullet to bullet or barrel to barrel is usually less then the MOA thickness of the stadia lines.

TA31F works well on all military length barrels as it is a military optic, not a traget scope.

FWIW, using the TA31RCOM4 and RCOA4 we gathered 10 riflemen with 5 using the M4 and 5 with M16A4 rifles.  They obtained a proper zero and then fired a qual course.  We then switched the optics giving the M16A4 riflemen the RCOM4 and the M4 users the RCOA4.  The zero procedure was repeated and then the qual course was refired using the same lot of ammo.  The results were damn near exact same.  Both groups fired a KD "bullseye" course and a combat "green target course" with both optics.  Calibrated for barrel length??  

Bottom line from my exposure to this experiment and that the Army uses the 31F on any and all 5.56mm weapon platform with success, I do not see how any shooter can really say that one model is cailbrated for one barrel length as the scope is not a "X" ring shooting optic, it is a combat optic made for center mass shooting.....

To answer your question as to the 14.5 or 16" "calibration".......how will you ever know for sure and why would it make a difference if you properly zero?