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The Trijicon ACOG, or Advanced Combat Optical Gunsight, is one of the premier combat optics in the world. The ACOG is such a capable optic is has been adopted by the USMC as well as various U.S. Army units. There are several variants of the Trijicon ACOG in different sizes, magnification, and reticle assemblies.

Travis Pike

Travis Pike

Today we are going to be talking about two different models; both are almost identical. The ACOG TA31RCO-A4CP and the TA31RCO-M4CP ($1466). These optics are the two that serve with the United States Marine Corps. The only difference between the two is one is made for the M16A4, and one is for the M4. The difference between the optics comes down to how the reticle coordinates with the respective rifle's barrel length. We'll talk more on that later.

Because these models are basically identical, this review will cover both optics.


  • Magnification – 4x Fixed
  • Objective Size – 32mm
  • Weight – 15.8 ounces
  • Length – 6.7 inches (With ARD)
  • Reticle – Bullet drop compensator with illuminated chevron
  • FOV at 100 Yards – 36.8 feet
  • Eye Relief – 1.5 inches

History of Service

The United States Marine Corps adopted the optic after an evaluation in 2005. It's the standard issue optic of every cook, baker, and candlestick maker in the Corps. This is where I first encountered the optic. The USMC saw an instant rise in the success of their fighting men overseas when equipped with ACOGs. The ACOG gave Marines a decided advantage against the iron sighted AKs the Iraqi insurgents were using. The ACOG was so successful an investigation was launched into the number of headshots Marine riflemen were making in combat. The result being the conclusion that Marines are excellent rifleman and insurgents don't understand how cover works.

Since then the ACOG has remained a staple in the USMC's arsenal. So much so they went back to Trijicon to design optics for the M249 SAW, the M27 IAR, and the M240B. These optics are almost all based on the ACOG design with slightly different considerations for the purposes of a machine gun. Outside of the United States, the British Army and Royal Marines also field the ACOG.

My Time with the ACOG

The Trijicon ACOG and I met in 2009 at the Marine Corps School of Infantry, Infantry Training Battalion. From that point on all of the shooting I did with a rifle was accompanied by the Trijicon ACOG. This includes intense pre deployment training in Camp Lejeune, 29 Palms, California, and Fort Pickett, Virginia.

In 2009, I deployed to the Helmand Province, in Afghanistan carrying a weapon equipped with the Trijicon ACOG. After a long 7 months of combat patrols and dodging IEDs we returned. Almost immediately we began training for the next deployment. Again, we underwent intense pre-deployment training around the United States. All this training involved extensive firearm and ACOG use. The big difference was now we were a mechanized amphibious company and were exposed to water during almost every training evolution.

Travis Pike

Travis Pike

The next deployment took us and our ACOG's around the world with a Marine Expeditionary Unit. We trained a multitude of foreign military forces in countries like Romania, Spain, and the UAE. The spectrum of environments we trained was everything from middle eastern deserts, to beautiful Mediterranean paradises, and to old school Eastern European training grounds.

Travis Pike

Travis Pike

I wish I had an idea of how many rounds I put through an M4 or M16A4 equipped with a Trijicon ACOG. Needless to say, it was in the thousands, possibly tens of thousands of rounds downrange. With all this being said it's likely to be the most in depth review I've ever conducted with any optic.


The Trijicon ACOG is an absolute tank. It can take massive amounts of abuse. Marines can break anything and everything, and we often do. However, in all my time I've never seen a Trijicon ACOG fail in any way. I've never seen one break, or even lose zero. The ACOG is made out of forged 7075-T6 aluminum, the same material used to create AR-15 lower receivers. It's strong, durable, and designed to last and last.

The ACOG is completely shock, water, and fog proof. It's built with the ability to be submerged up to 70 meters without issue. This is an MILSPEC requirement for the SOPMOD system. This optic has remained popular for so long simply because it's so well built. This small, but tough optic is resistant to anything a Marine and the environment they serve in can toss at it.


The ACOG has a brilliant level of clarity. The glass is extremely clear, and the image the optic provides is in clear high definition. In country, the ACOG wasn't just an optic we used for aiming. The ACOG was a piece of glass we could all use to spot and scout a variety of different situations ranging from suspicious piles of trash that could be IEDs, to scouting crossings for the canals that plagued the countryside. In some cases, our lives were depending on the details we could see through the optic.

In other instances, the optic allowed us to easily determine from friend or foe, or to spot and clarify suspicious activity. The optic always gave us a clear picture to allow us to differentiate between danger and some junk on the road.

Ease of Use

The ACOG's fixed power nature, and battery free illumination makes it a simple and easy to use optic. ACOG offers a wide variety of mounts for different AR-15s. They can be mounted to picatinny rails, or even carry handles. To effectively use the ACOG you do need to know a few things. First, how to zero a rifle, this is a simple affair and can be easily learned from the owner's manual.

The ACOG is a versatile optic and to take advantage of that versatility you need to know a thing or two to effectively use the device. It does take some time, and a little instruction to perfect using the ACOG. This may be a downside to people used to the goof proof red dot optics.

The Reticle

The reticle is one of the things you must learn to master. The reticle is not the simplest on the market. However, its versatility is a strength. The center of the reticle is a red illuminated chevron, below the chevron is a series of crosses and 4 and 6 numerics. The numerics coordinate to range in meters.

The top of the chevron is for 100 meters, and under the middle of the chevron is 200 meters, the top of the first stadia is 300 meters. Below that each hash mark represents a hundred meters of distance. This bullet drop compensating reticle allows shooters to engage targets with ease at 800 meters. The reticle is highly efficient and easy to use at both close and long range encounters. The drop reticles are accurate and allow easy combat holdovers for targets at extended ranges.

Because this is a bullet drop compensator, a few factors can affect the performance of how a round flies. One of these factors is barrel length. This is why there is a specific model for the M16A4 and the M4 rifles. Another factor is projectile weight, and these optics are dialed in for the M855 62 grain projectile. Other rounds will put you close, but 62 grain M855 is the best option.

Trijicon ACOG Reticle

Trijicon ACOG Reticle

If you look closely, the hash marks start wide at the top and slowly become thinner as they descend. These lines and the chevron are designed to coordinate with the average width of a man's shoulders. Up to 200 meters the chevron is smaller than the shoulders, at 300 meters the chevrons are the same width of a man's shoulders, at the 400 meter mark the first hash mark is the average width of a man's shoulders. In a combat scenario, this makes it very easy to determine range on a man sized target.

Bindon Aiming Concept

Some people are turned off by a 4-power scope due to the perceived idea the optic is not a good option for close quarters combat. However, this simply isn't true. Utilizing the Bindon aiming concept, the ACOG can be effective as any red dot optic. In and out of close quarters combat training I've come to really appreciate the Bindon method over a red dot and magnifier combo.

You simply keep both eyes open, position yourself behind the optic, and you don't focus entirely on the reticle. Your dominant eye picks up the illuminated chevron, and your non-dominant eye captures the world in front of you. This places the red chevron over your eyesight and superimposes it on the target giving you an effective option for close quarters combat.


There aren't many cons with the ACOG optic. It's very well made, very versatile, a little complicated, but also ultra-clear. The biggest downside comes from the tritium illuminated reticle. While it does eliminate the need for batteries, it will not last forever. Trijicon warranties the tritium fiber optics for only 15 years, after that you can expect the illumination to begin to fail, and start dimming.

Looking through ACOG on the roof

Looking through ACOG on the roof

The other issue is that the reticle will grow brighter in bright environments. In Afghanistan, the days were quite bright, and the reticle will compensate for the external brightness a bit too much. It would be genuinely uncomfortable due to how bright the reticle would be. To solve this problem, you can apply duct tape around the tritium lamp to reduce the amount of light that enters into the system. This will dim the reticle and is an easy fix.

The Cog

The Trijicon ACOG is overall an amazing optic that is well suited for a variety of tasks. First and foremost, it is a go to war optic. It's also a great option for home defense, and it's a solid choice for 3 Gun style competitions. Overall the ACOG is one of the best tactical scopes made for the AR-15 series of rifles, and easily the most versatile. It's one of my favorite optics, and the proof is in the pudding when it comes to performance with the USMC.

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