Without having to go through countless threads what are the two most common ACOGs that are used on A4s?  I know the TA31RCO-A4 is the current unit but what was the one it replaced(the cheaper model)?

There have been a number of ACOGs used with the M16A4.

Originally, the TA01-NSN ACOG was used, then the TA31-F model came into play, then the TA31RCO-A4, then the TA31RCO-M150 (with certain Army units).

Thanks DiveCop

I really wish Trijicon would update their ACOG spec sheets on their website so users and potential customers could see the barrel lengths each model is calibrated for.

If you're talking about what ACOG is standard issue for USMC M16A4s, that is and only ever has been the TA31RCO-A4CP. The TA31RCO-M4CP is USMC issue for M4s. The TA31RCO-M150CP is the what the Army uses for M4s, though there is no standard issue optic in the Army. The -M150 is the same as the -M4 (for M855 out of a 14.5" 1/7 barrel), but comes with caps for both lenses. Contrary to popular misconception, the -M150 is not calibrated as a compromise between 14.5" and 20" barrels.

HunterKiller86 is right; outside of those three models, it's not immediately clear what barrel length, twist rate, and bullet weight the various models are calibrated for.

Once zeroed, most shooters will never know a difference between any model on a given weapon. These are not target scopes.

Might be true, but if I'm gonna spend the money for an ACOG, I'd like to get the one that's correct for the barrel length.

I'm in the same boat.  Trying to outfit an A4 clone and I found an m150 for $1100 to my door which seems to be a pretty good price.  Couple hundred bucks for the RCOA4 so trying to pick.  I've got an M4 clone was well I could swap between.

You can find used TA31 ACOGs on the EE for $800. Is the specific .mil version worth the extra $300?

Nope.  Thanks for the heads up I'll watch there.

As long as you plan to shoot M855 and your targets will be in meters, you'll be fine.

If you don't want to shoot that way with that ammo, then get the reticle you want.  The BDC isn't gonna match up.  You'll need to figure it out by discovery learning or a ballistics calculator and subtension data.

I understand what you're trying to convey but that isn't entirely correct.

M193 and similar rounds often track nearly identical to M855 out to 500 meters in my experience

Well, it largely depends on what you consider nearly identical.  Considering not everyone shoots the 14.5" barrel that the TA31 is designed for, stacking variables can become significant.

I used the velocity data here and Federal's Ballistic Calculator.

Consider the M855 round from a 14.5 inch barrel is about 2800fps. 
An M193 fired from a 14.5" barrel is a little faster at about 2900 fps.  At 500 yards, the difference is about half an MOA.
An M193 fired from a 16" barrel is faster at about 3100 fps.  At 500 yards, the difference is about 2.2 MOA, or 11 inches.
An M193 fired from a 20" barrel is significantly faster at about 3200 fps.  At 500 yards, the difference is about 3.7 MOA, or 18 inches.

Most ranges in the US are demarcated in yards.  That's an additional ~10% variance to consider.

I'm not saying don't buy an ACOG.  I have one and love it.  It's my go-to optic.  I use it on ranges that are measured out in yards.

I'm saying a guy should understand how the thing works before plunking down north of a thousand dollars.  It ain't magical and won't break the laws of physics to keep your rounds on target.  For most average citizen shooters, it requires some discovery learning based on a shooter's ammo, barrel length, and range.

This everytime I qualified on my A4 it was a different scope/rifle combo and each one would have different holdovers at each yardline even though they were all zeroed the same way.   I had some that would be dead on with the stadia marks and others that would be high or low.  Was never the same twice.  But again I never qualed with the same rifle/scope twice either.