• Post author:
  • Post last modified:June 12, 2020

Trijicon ACOG 3.5X vs 4X comparison review, which one is the best for you and does their differences matter to you?

In this article, we gonna explain why Trijicon came out with a 3.5X magnification ACOG and how different it is from the 4X.

Right off the bat, the Trijicon ACOG 3.5X has a better eye relief distance than the 4X version, but there are more.

In the end, you should know what you're looking for.

Why Use ACOG ScopeUSMC shooting HK M27 with ACOG 3.5X

Image Referenced USMC 

The ACOG has been an outstanding combat magnified rifle optic served well with the U.S. military. It's a super rugged optic made with forged 7075-T6 aluminum, designed with combat in mind.

For the young U.S. Marines who are rough on their equipment, the Trijicon ACOG can literally be physically beaten, set on fire, run over by tanks, and would still work.

Trijicon ACOG 3.5X vs 4X Comparison Chart

Invalid table id.

Magnification 4X vs 3.5X – Which One Is For You?

The 4X and 3.5X look very similar with just 0.5X off surprisingly, and the shooter can still pick up the 3.5X ACOG and feels just the same as they picked up a 4X ACOG with better eye relief distance.

3.5X Has Huge Eye Relief Improvements

One biggest change the ACOG 3.5X has is the eye relief distance. About 1″ more than the 4X model. This tweak is geared toward military users who wear helmets without bumping into the scope.

The shooter can still see the sight picture clearly with a great eye box to work with.

The ACOG 4X has a very short eye relief distance, which requires the shooter's eye to be right in front of the lens, which is not optimal for shooting 7.62mm round.

It's perfect for anyone shooting without wearing a helmet or a head-mounted action camera.

The Ones Used In The Military

Minot AFB security forces M4 with ACOG 4X scope

Image Referenced 5th Minot AFB ICBM Security Forces USAF

The most popular ACOG model in the U.S. military is the 4X model for the M4 and M16 weapon systems, also known as the M150.

Recently the USMC adopted the 3.5X ACOG with Trijicon RMR for the HK M27 and the Squad Day Optic version for the M249.

While a large volume of 4X ACOG is still used in the Army, Air Force, Navy, and Marines. They're all beat up, but still works.

Lens Clarity Matters


Image Referenced AR15.com

Whether you are using the 4X or the 3.5X in low light condition or bright daylight, the large objective lens 32mm or 35mm capture a large amount of light to brighten the image when you look through the scope.

The glass is made of high strength material and brilliantly clear, you will not disappoint with either one.

Which ACOG Reticle Should I Use?

The ACOG series optic offers so many different reticle options based on caliber, bullet weight, and barrel length. So it's kinda hard to know what you're looking for.

I have organized all of them onto the chart below, and you can see which one makes more sense to your personal use.

ACOG scope reticle BDC Chart

Image Referenced USMC

Reticle Styles Comparison

Trijicon ACOG Reticle styles basics

…So which one should you pick and why?

Each reticle style has its own purpose and tactical reasons

  • Chevron – Upside down V, also known as “RCO” in the military for 100 meters zero at the tip. The base of the V is about 5.53 MOA, which is about 19 inches at 300 meter. In another, that's a width of a human-size target at 300 meters. Perfect for simple range estimation.
  • Triangle – Similar to the Chevron reticle, but prefer the Chevron for not masking the target down range
  • Donut – Perfect for competition shooters, also known as the TA11 with that circular reticle, the shooter can place the circle over the steel plates and take the shot with accuracy up to 800 meters.
  • Horseshoe/dot – This reticle is similar to the Chevron reticle concept with the bottom the opening circle represents 19 inches at 300 meters, and the circle itself basically circle around the target with a center dot for 100 meters zero. In addition, the large circle is so visible and helps fast target acquisition.
  • Crosshair – Just a regular old school crosshair, we recommend other reticles.
  • Machine Gun Reticle – Only available for the 3.5X ACOG Squad Day Optic. Calibrated for M249 with built-in bullet drift reticle.

What's ACOG's Effective Range

The scope is designed for medium to long-range engagement and positive target identification up to 800 meters for most of the ACOG models

Close Quarter Shooting

CQB with M4 with ACOG scope

Image Referenced USAF

Both models can accept the piggyback offset reflex sight mount for the Trijicon RMR. In fact, the Trijicon RMR and ACOG combo is the official HK M27 optic configuration for the USMC (basically an MR556). For the purpose of enabling close-quarter shooting while using a magnified 3.5X ACOG scope.

M27 with ACOG 3.5X and Trijicon RMR

Image Referenced SHOTTIMES.NET

Some argue that the piggyback offset setup shifts the holdover way too much for close-quarter, and suggest to use offset angled mount, you can read more on piggyback red dot setup vs angled offset red dot setup here.

Apply Bindon Aiming Concept & Shoot With Both Eyes Open

The ACOG scope integrates the Bindon Aiming Concept, which was created by the Trijicon founder Glyn Bindon. In layman's term, it lets the shooter aim and shoot with two eyes open.

Both 4X and 3.5X ACOG scopes have reticles that have sufficient contrast to the target background, which allows a clear image when shooting with both eyes open. Your dominant eye should focus on aiming through the scope, and your nondominant eye should observe the field in front of you for maximum situational awareness.

The reticle will be superimposed over your vision and your brain combines the magnified image and unmagnified image together.

It's recommended to have 20/20 vision or wear corrected lens to make this work. Once you're comfortable with it, you can almost shoot close range target like a red dot.

Construction and Weight

ACOG 3.5X SDO on SCAR 17

The ACOG scopes are all built with hammer-forged 7075-T6 high strength aluminum. The strongest type of material in making gun parts. All ACOG scopes are built for combat in mind.

Weighing average around 16 oz with the mount it's definitely heavier than a red dot sight but definitely lighter than a variable power optic.

Scope Mounts

Geissele super precision ACOG scope mount

The 3.5X and 4X ACOG are compatible with the same scope mount. There are many options out there, even though the ACOG itself can handle heavy recoil and retain zero perfectly, but we highly recommend replacing the thumbscrew mount with something like the Geissele super precision mount or the LaRue Tactical QD lever mount in order to stay on the rifle tightly.

Fiber Optics vs LED Illumination

Do you need any one of these features on your ACOG?

The answer is YES! here is why


Trijicon ACOG is an awesome combat optic that doesn't require a battery, however, there are LED model introduced to the market for adjustable reticle illumination on command.

This feature also allows the ACOG to be used at nighttime.

Fiber Optic

The fiber optic strip on the top of the scope provides battery-free reticle illumination during daylight, but the problem arises when it's too bright outside and the reticle also gets too bright, which can be distracting at times.

Marines in the field would place tape over the strip to dim the reticle just a little bit to make aiming more comfortable.

You don't have to tape over the whole thing, you can just cover up parts of it to adjust the brightness until its comfortable for you.

Final Thoughts – ACOG 4X vs 3.5X

Trijicon ACOG is the world's toughest battle optic used widely in the World especially the military. The 4X and 3.5X fixed magnification scopes are the 2 most popular ones on the market.

Overall, they're both indestructible optics, but the 3.5X has the best eye relief distance optimized for helmet wearing shooters, and still feels like 4X with only 0.5X less.

Related Content: ACOG VS ELCAN