Fighting Optics for the AR-15

Shotgun News 4 February 2008

Optical sights have come of age on fighting rifles. Three examplary choices are the S&B Short Dot (top), Aimpoint M3 (middle), and Trijicon TA11 ACOG (bottom). Photo credit Jeff Carpenter.

Now that optics have come of age on fighting rifles, one of the most compelling reasons to have a longer barrel has been removed. With iron sights such as those on the M16A2, practical accuracy depends on sight radius; it is easier to shoot accurately with more distance between the front and rear sight. Optical sights have no sight radius; barrel or weapon length is now separated from the ability to obtain a precise sight picture. This change allows a 14.5-inch M4 to have the same sighting precision as a 20-inch M16A2 or even sniper rifle.

Shown here at an EAG Tactical Carbine Operators class, Type I sights, red-dot optics with no magnification, are most popular.

Short-barreled rifles are becoming standard on the battlefield. The Army is issuing the M4 on a widespread basis because of extensive vehicle-mounted use. Even shorter carbines are seeing use, such as the Mk18 Mod 0 CQBR with its 10.3-inch barrel. For close-quarters or low-profile protection work, compact carbines with ten to 12-inch barrels are common. Short-barreled rifles are also a good choice for suppressed use since a 20-inch M16 or even a 14.5-inch M4 will become a long package when a suppressor is added.

Conventional wisdom is that a long barrel is required for accuracy. More velocity does reduce wind drift and bullet drop at longer ranges, however, outside of Bench Rest games, a short barrel can be just as mechanically accurate as a long barrel. Real-world accuracy of an AR-15 is determined by barrel quality, ammunition choice, and upper assembly build quality.

The EOTech holographic red dot optic can be a viable fighting optic, however, manipulating it is more complicated than the Aimpoint.

The short yet accurate barrel has become popular recently due to demand for a compact carbine which can deliver sub-MOA accuracy at extended ranges. Rules of engagement often dictate positive target identification and shot placement. The "Recce" format originated in Naval Special Warfare (NSW) due to the need to equip snipers with a combat precision weapon which could use common ammunition. The Recce has a 16-inch stainless match barrel and free-floated railed hand-guard, and is fitted with a low-power magnified optic. The concept has been shortened even further into sub-MOA short-barreled rifles in the ten to 12-inch range. A skilled shooter can hold these short carbines to MOA at 600 yards or further.

With those facts set out, it becomes clear that carbine capability is primarily determined by the sighting system. The same weapon could be optimized for door kicking, close to mid-range engagements with target identification requirements, or can be stretched to the cartridge's ballistic limit by merely changing the optic.

The EOTech provides fast, heads-up sighting, and suits the short-barreled rifle well.

Practical competition is the petri dish in which superior gun-handling, marksmanship, and speed skills are developed. The "modern" technique of pistol shooting developed in the 1970s by IPSC shooters provides the basis of combat handgun training today. The rifle hasn't been left behind; three-gun and practical rifle matches likewise have provided an important testing and proving forum for equipment, technique, and skill advances. It's common for top practical shooters to instruct our warriors, and it's common for tools and techniques developed in practical shooting games to migrate to the tactical side, and vice versa. Outside of tactics, the shooting problem is much the same: make hits as quickly as possible, under pressure and while moving.

Unity magnification red-dot sights are easier to use when on the move than other optic types. Shown here is an Aimpoint M2.

It's no surprise, then, that common themes in sighting systems arise. I'm going to break these down into three categories: Type I, Type II, and Type III.

Type I sights are non-magnified red-dot optics. They are optimized for zero to 100 yards, and are most effective within the cartridge's point-blank distance, which is about 275 yards for 5.56. On large targets such as full silhouettes, they can make hits out to about 350 yards, but from 400 yards and further, compensating for over three feet of bullet drop is problematic. Type I optics are typically reduced-parallax or parallax-free red-dot sights with no magnification. The most common Type I optics are the Aimpoint M2, M3, or M4, and the EOTech.

The Aimpoint M2 (bottom), M3 (top), and M4 models provide battery life of 10,000, 50,000, and 80,000 hours respectively, and can truly be left on "all the time," ready for immediate use.

Unity magnification allows the fastest sight-picture acquisition and the shooter can maintain true binocular vision while shooting. At extended distances, the lack of magnification is a liability in target location, identification, and sight picture. Type I optics are appropriate for virtually all defensive rifle uses, and are the right choice for fighting with 100 yards. They excel at shooting while moving and shooting from awkward positions like rollover prone, supine, and the weak-side shoulder. Iron sights still have a place due to their dumb-as-rocks mechanical simplicity, and fit in as a Type I sighting system, although they do not enjoy many advantages of the Type I optics.

Without any magnification, the Aimpoint's maximum realistic range for making hits on silhouette-size targets is about 425 yards, using Kentucky hold-over.

I'll go out on a limb and call the Aimpoint M3 and M4 the best choices for a fighting Type I optic. The EOTech has followers and is used in harm's way, however, the Aimpoint line trumps the EOTech in several important respects: the Aimpoint's battery lasts much, much longer than the EOTech and the Aimpoint can be turned on and off using only tactile feedback. The shooter needs to be looking at the EOTech to determine if it's on. Market price for an EOTech or Aimpoint including quick-release mount ranges from about $500 up to $700.

Aimpoint offers a 3x magnifier which can be mounted behind the optic to provide some of the capabilities of the Type II sighting systems, yet can be quickly flipped away when it is not needed. Compared to natively-magnified optics, this combination-magnifier has much less field of view.

Type II sights are low-power magnified optics with reticle features providing aiming points for distant targets. The Type II optic should not have external knobs to prevent loss of zero due to the knobs being bumped. They are optimized for 75 to about 400 yards. Type II optics are generally either fixed magnification in the three to four power range, or are variable power in the one to four range. The most common Type II optic is the Trijicon ACOG, in either the 4x versions (TA31, TA01) or the 3.5x version (TA11). The US Military realizes these capabilities in the Designated Marksman Rifle (DMR) and Squad Designated Marksman Rifle (SDM-R).

The TA11 ACOG has more eye relief and requires less precise eye placement to obtain a sight picture compared to the TA31 ACOG in use by the US Military.

At very close range, Type II optics are much slower to acquire a sight picture than the Type I red dots. They can be stretched to 600 to 800 yards provided the target is large and the reticle provides sufficient bullet-drop compensation (BDC) features. These optics provide good target spotting and identification. Close-range speed can be improved by the addition of a brightly illuminated reticle center, since the brain is drawn quickly to bright objects. The TA31 and TA11 ACOGs have this feature, as does the Schmidt and Bender Short Dot.

Low-power variable-magnification Type II optics improve close-range target acquisition speed at their lowest magnification setting; the closer to true 1x the better. The goal of these variable-power scopes is to provide the speed of the Type I optic, but still the target ID and distance capabilities of the Type II optics. The price for this flexibility is paid in increased purchase cost, less durability, and more weight and bulk on the carbine.

For small but practical targets, such as a ten-inch plates, the Type II optic will limit engagement distance to 350-450 yards, where the coarse BDC, thick reticle lines, and low magnification inhibit the ability to achieve the correct point of aim and a clear sight picture.

Although the 3.5x magnification of the TA11 ACOG is slower for close-range engagements, with training and practice the difference can be slight. The author hammers a target at 20 yards while on the move.

In practical shooting competition such as three-gun, the 3.5x TA11 ACOG is one of the most common choices. It is chosen over the 4x TA31 ACOG because the TA11 provides longer eye relief (2.4-inch vs. 1.5-inch), require less precise eye positioning to obtain a sight picture, and takes up less of the shooter's natural field of view when shooting with both eyes open. Both have the fiber optic and tritium-lit reticle which aids acquisition speed. The TA11 is easier to use when on the move or when shooting from awkward and improvised positions. The US Military uses the TA31 ACOG, however, shooters would be better served with the TA11 model. Market price on the TA11 and TA31 ACOGs are around $1000.

Schmidt and Bender's 1.1-4x20 mm Short Dot provides almost true unity magnification at the low end, and useful four power magnification at the top end. Its first focal-plane reticle has hash marks every one-half mil, and a center illumination dot which is quite bright at the 11 brightness setting. The Short Dot retails for about $2200, and almost perfectly bridges the gap between Type I and II applications.

The killer feature of the S&B Short Dot is that it is almost as fast and flexible as 1x red-dot optics at close range and one the move.

Trijicon's 1.25-4x24 mm AccuPoint (TR21) shares the same fiber-optic and tritium reticle illumination as the ACOG, but can be dialed down in magnification to help speed at close range. Its reticle has no BDC features so shooting past the carbine's point-blank range will be difficult without a way to specify elevation holdover. The TR21 is a good deal at about $575.

Leupold's Mark 4 1-3x14 mm CQ/T has close to unity magnification at the low end, but only 3x at the top, less than the Short Dot, TR21, TA31, and TA11. The reticle has no BDC features and its illumination is not bright enough to stand out in daylight. The CQ/T is criticized for bulky size and narrow field of view. Market price on the CQ/T is about $700. The Leupold Mark 4 1.5-5x20mm MR/T is similar in concept to the Trijicon TR21 and the Short Dot, though its low end magnification is higher than either. It does have a reticle-based BDC for longer shots. The 1.5-5x20mm MR/T sells for about $850.

The Leupold CQ/T has been criticized for being large and it lacks bullet-drop compensation features, however, it can still be used to make hits at 300 to 400 yards using Kentucky hold-over.

The Elcan SpecterDR is a 1x or 4x optic, switchable with a cam lever on the left-hand side of the optic. Like the older Elcan, elevation and windage adjustments are external to the optic itself. The reticle is essentially identical to a TA01 ACOG, with an illuminated center. The SpecterDR is a promising concept, however, the built-in ARMS mount makes it a no-go for many would-be customers because the throw levers are known for eventually coming loose. Market price is about $1600.

In the Type II or DMR role, the Short Dot is the most versatile but also the most expensive choice, more than twice the cost of any other. The Trijicon TA11 ACOG has bomb-proof construction, a compact package, good reliability, fast target acquisition due to its super-bright reticle, and, though fixed power (3.5x), is the second best choice for intermediate engagements. It gives up a little speed at very close-range engagements.

The canonical Type III scope is the Leupold 3-9x36 mm MR/T, shown here on a SPR. Higher magnification and external turrets enable precise engagements on small and distant targets.

Finally, Type III sighting systems have the capability to take the rifle to its ballistic limit and to engage very small target at extended ranges. The 18-inch Mk12 Mod 1 Special Purpose Rifle (SPR) uses such an optic, so it makes sense to call it an SPR optic. Military-issue 5.56 loads fired from a 16-inch barrel go subsonic between 600 and 800 yards at sea level, and this defines the longest possible engagement distance. Keep in mind that the terminal energy of 5.56 at this distance is very low, however, the SPR has made kills at over 600 yards using 77-grain ammunition. The other advantage these have over the Type II or DMR optic is the ability to see and engage very small targets at intermediate and long distances. When fighting opponents who are partially behind cover or wearing armor, the ability to ID targets and place a head shot several hundred yards away can be key.

The Type III optic is generally a variable magnification scope with a maximum power of 9x to 12x. The reticle is typically some type of mil-dot or mil-hash, and has lines thin enough to discern a sight picture on 2 MOA targets. In order to compensate for bullet drop and wind drift at longer distances, the Type III optic usually has external knobs. At close distances, the scope magnification can be dialed down to somewhere between two and four power for faster target acquisition or tracking moving targets.

Although its size and weight dominate the rifle, the US Optics 3.2-17x44 mm SN3 scope makes trivial work of small targets beyond 400 yards-- although the 5.56 mm cartridge isn't always up to it.

The Type III optic is basically a long-range rifle scope with the features required for tactical or practical engagements, so it is no surprise that a lot of scopes more often seen on long-range bolt-action sniper rifles turn up on AR-15s in this role. The geometry of the AR-15 platform, however, complicates the choice. Conventional scopes have eye relief over three inches, which means the ocular lens of the scope will have to be mounted about three inches forward of the charging handle for most shooters. This requires either a cantilevered mount such as the LaRue SPR or SPR-E mounts, or a raised and extended rail such as the full-length ARMS rail on the SPR. With a scope mounted this far forward, the carbine can end up muzzle heavy, and the scope's objective can conflict with rail mounted devices such as an AN/PEQ-4. The size of the optic can also dominate the AR-15 carbine.

The canonical Type III or SPR optic is the Leupold Mark 4 3-9x36 mm MR/T with a one-MOA M1 elevation knob. Leupold developed this scope specifically for the SPR program; its small size, low weight, and slightly shorter than normal eye relief are good matches to the AR-15 platform. Leupold has recently come out with a 2.5-8x36mm MR/T in M1 (1/4 MOA), M2 (1/2 MOA), and M3 (1 MOA) knob configurations. None of these have a first focal-plane reticle configuration, so the reticle features can only be used for ranging or hold-off at the top magnification setting. These Leupold MR/T scopes cost $900 to $1100. Nightforce's 2.5-10x24 NXS fits right in with the Leupold models. Its elevation knob is capped and has 10 MOA per turn in 1/4 MOA clicks, and no zero stop. It sells for about $1200.

If you are willing to step up to a larger scope size and deal with the eye relief and mounting issues, there are considerably more choices, including the Leupold Mark 4 3-10x40mm LR/T, US Optics 1.8-10x44 mm SN3, and Schmidt and Bender 3-12x50mm PMII. The Schmidt and Bender was recently adopted as the new daylight scope for the USMC M40A3 sniper rifle and is a superlative though very expensive piece of glass. The US Optics SN3 is about $1600, while the S&B PMII is $2800.

The capabilities of a fighting AR-15 or M4 are determined in large part by the sighting system on the carbine. The warrior can choose barrel length based on the confines in which he will be working and how much ballistic reach he needs. Barrels as short as ten inches are producing sub-MOA groups and enabling unheard-of precision from such a compact assault rifle, while the 14.5-inch M4, 16-inch Recce, and 18-inch SPR have their own applications. Understanding the role in which the carbine will be employed and making the right choice of sighting system is critical.

The author engages 425-yard flash targets with a Trijicon TA11 ACOG, a compact and robust optic optimized for fast engagement of targets from 75 to about 400 yards.

With the Schmidt and Bender Short Dot's magnification set to 4x, the mil-based reticle can be used to compensate for bullet drop during fast engagements out past 400 yards.