Trijicon does not recommend mounting TA31s on a flattop, but what is your experience? have you ever NOT been able to zero your TA31 at 100yds? is the TA31s 15 MOA maximum reticle adjustment enough? Trijicon is able to replace the reticle (for some $) in your TA31, but would a TA31F reticle offset the CH / flattop difference, or would internal prisms also need to be modified? please advise

There's no problem mounting a TA31 to a flat top as far as zeroing the scope goes; people do it all the time.  There's no need to have Trijicon replace the reticle either - hell, why not just buy an ACOG designed for mounting to a flat top instead??

If you're interested in the adjustments to the reticle calibrations when you go from carry handle to flat top mounting, check out this thread.

Quoted: Trijicon does not recommend mounting TA31s on a flattop, but what is your experience?

My experience is that it works great

Get a Larue, ARMS 19S or even the Trijicon TA51 mount and you are good to go.

Mine has enough adjustment to zero.  Also if you use m193 and a 16"bbl try this zero method out for size:

Zero top of doughnut at 40 yrds

then you'll be:
3/4 of a doughnut high at 100m
1/2 of a doughnut high at 200m
middle of the doughnut at 300m
top of the first stadia line at 400m
dead on the 2nd stadia line at 500m
dead on the 3rd at 600m
bottom of the 4th at 700m
about two line thickness under the 5th stadia line at 800m

I know some would argue that being 3 inches above the top of the doughnut at 100m is not preciese enough but for COM shots it realy is not that far off and with little practice you learn to hold just a little low.

The fact of the matter is that it really doesn't matter all that much which ACOG BDC reticle you go with or whether you mount it on the carry handle or a flat top. You still have to shoot the gun under the prevailing environmental conditions and with the ammo you will be using and learn to compensate for the imperfections of a fixed BDC arrangement.

If you went with one of the 55 grain/carry handle models, the BDC would only be exactly correct for M193 ammunition (which pretty well limits you to XM-193) and then only under standard temperature/pressure conditions. If you came out here to Phoenix to shoot, on most days you would have to hold 6.5" low at 500 meters to get an X ring.

Bear in mind that if you want to consistently hit at 500 yards or greater, you are probably going to need to shoot something besides M-193.

I use a TA-11 on a flat top with 77 grain bullets and have learned to compensate from the BDC indications at various distances. You are going to have to do the same in any event. So don't worry yourself to death over carry/handle vs. flat top, etc. Buy want you want in an ACOG. Find some ammo the gun likes. And head for the range and be prepared to learn and to remember.

Luck,

SD