The Army issued TA31 is complete poop.
I believe it's the Chevron sight with BAC.
http://www.trijicon.com/na_en/products/ ... RCO-M150CPTA31RCO (Rifle Combat Optic?) 4x32
The tritium is nice, but the sight relief is poop. It says eye relief is 1.5 but It feels closer than that. Which is why I like (in real life) the Elcan M145, which the Canadians use on their main M4 and M16 battle rifles (America seems to only use it on the M249 SAWs occasionally, and most notably/usually the M240B Machine Guns).
But the Airsoft ElcanM145 is really poop. The real one is nice, but expensive, and also very heavy (rubberized. Very sturdy)
The best optic I've found for the airsoft rifles and repros would be the SpecterDR. They have versions that go from a lower power up to 4x, or some that just stay at 4x. I really like the objective (size of lens near your eye--helps me quite a bit for target acquisition quickly, and also for ease of use) on the Elcans.
Elcan is a real company. Makes Specter DR as well as the M145. Both good scopes in real life. ACOG makes better options than the TA31 issued to troops, but I hate the small reticle size and short sight relief. But the weight is much less and helps keep a balanced rifle. I'd probably use a TA33 with doctor sight if given an option.