Optic of the week: Trijicon TA31RCO-A4


I can’t believe I haven’t written about the TA31 series of ACOGs yet.  These were my favorite optic for a long time.  My first experiece with them was in the Marine Corps using the TA31RCO-A4 on the M16A4.

The 4x magnification of the TA31 series of ACOGs will leave a shadow of the front sight base in the sight picture.

I believe that the first of the TA31 models of scope came with a donut reticle.  I had one of these back in 2007ish and while it worked fine, I didn’t not care for the donut as it did not give a clear aiming point for 100 meters.

Reticle picture taken from Trijicon’s website.

The 31 series of ACOGs started with a Bullet Drop Chart(BDC) that went out to 800m.  Longer than the 600m that the TA01 series reticles were set up for.  Later the 4x ACOGs would get reticles that would go out to 1000m.

The TA31F used a blended BDC that was suppose to be close enough for the 20 inch rifle or the 14.5 inch barreled carbine.  Numbers that have been provided make me think that the BDC on it was based off a 16 inch barrel.

The TA31F had a chevron where the tip was used for 100m and the inside point of the chevron was 200m.  The top of the BDC post is for 300m.  The Chevron is 19 inches wide at 100m, approximately the width of a man’s shoulders.  The USMC adopted and issued the TA31F, but then had two models made to replace it.  The TA31RCO-A4, and TA31RCO-M4 with bullet drop charts for the respective weapon systems.  Marine Corps reported that the BDCs were close enough that for the Corps purposes the optics were interchangable.  There were rumors that Trijicon used the same BDC for both optics, but does not appear to be true from testing.

The RCO reticles added 10 mil hash marks on either side of the BDC to allow troops a tool to help them adjust indirect fire assets.  Occasionally I have seen misinformed people say that the lines are for leading running targets, but when I ask those people how much they are suppose to hold over for a moving target none of them have ever answered me.

An unreliable source (Marine Corps Times, slightly less reliable than the National Enquirer) claims that when the Corps is sending ACOG scopes back for refurbishment that they are moving to the reticle used in the TA11SDO ACOGs.  That looks like this:

The Horseshoe dot reticle is suppose to cover less of the target up close and the modified BDC is ment to be easier to  use and useful out to a longer distance.  It looks like the mil hash marks have been expanded and given markings to help avoid confusion.

ACOG reticles can also be had in green or amber.  People don’t seem to care for the yellow amber reticles, but green has steadily been growing in popularity.  Note that due to the prisim in the ACOG the far right side of the mil hashmarks will be bury.

Older ACOGs came with capped 1/3 MOA adjustments that required a tool or coin to turn.  Those were replaced with the capped 1/2 MOA finger adjustable turrets.  On the RCO models these caps are tethered to the scope.  Some of the newest models have switched to a 1/10 mil turret, which would be about .36 MOA.

I still love the ACOG, and I argue that with practice a person can be very fast with one.  But ultimately up close it is going to be slower than a proper reflex sight.  While the ACOG excels are helping you put rounds on man sized targets from say 1-500m, it is not a good choice should you want to do precision shooting.  You would want a finer reticle and better adjustment.  ACOG scopes are notorious for having squishy clicks in the turrets and the scopes not tracking nicely or smoothly.

Now the ACOG is falling out of favor due to the new options of 1-6x and 1-8x scopes.  While many of those are a good deal larger, heavier, and more expensive than the ACOG, the variable power scopes are improving all the time.  The variable power is appears that it will be the future of combat optics, and the ACOG is the stagnating past.


    • I do love how simple using the ACOG is. It seems to me that almost every rifle I have used a mildot scope on the various round number distances (200, 300, etc) ended up being stuff like .4 mils, 2.6 mils, etc. All stuff that didn’t line up nicely with the reticle.

  1. For shear utility, size and ruggedness there isn’t a variable scope out there currently that can beat the ACOG. For a general issue optic I’d posit there isn’t anything better. Powerful magic indeed.
    Alas, that won’t always be the case though. The Elcan Specter is bigger and a little more fragile but showed a unique fix to the magnification or no magnification argument. The 1-4, 6, & 8 optics are getting smaller and handier too. Eventually someone will come out with a blended solution that won’t necessarily be a game changer but will set a new standard. I bet Trijicon is probably working out something.

    • I think there are some 1-4/6/8 optics out there that would meet the ruggedness requirements. Problem is that they are priced beyond the reach of most people who aren’t spending tax dollars to buy them.

      For the rest of us, I really do think the ACOG family (I really like the TA-11 and TA-33 over the TA-31) represents a quality option that provides significant benefit to the user in any “field” situation. RDS is still king inside 50 meters, though.

    • I personally agree with you that the ACOG is the best general issue all purpose optic out there. when it comes to something tough and simple to use and understand for people who don’t even care to learn much about it once its zeroed, it was a great solution


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