Scope Over Bore: What is it and Why Does it Matter?

For most of us who shoot at longer range, we get wrapped around the ballistic trajectory of a bullet. What is the holdover for my caliber at 200 yards? What about 300 or 400 yards? If you’ve delved further into the weeds; you consider parallax, temperature, variances in ammunition used, and a plethora of other factors that affect the projectile as it travels towards its intended target. However, at shorter ranges especially, have you factored in the scope over bore offset when using your rifle?

What is Scope Over Bore?

Every firearm has some kind of offset for scope over bore. Scope over bore is the distance between the sight plane and the bore plane of the firearm. For instance, an iron-sighted bolt action rifle, which has sights less than an inch above the bore will have a small scope over bore difference. Conversely, this offset increases if outfitted with a scope. A pistol sight offset may only be half an inch. However, the firearm design can directly affect the sight to bore offset.

Scope over bore among a variety of different firearms equipped with optics.

AR-15s are notorious for a significant scope over bore offset due to their design. For comparison, we need to consider the comb of the rifle. The comb is the top of the stock and is especially important if you shoot shotguns, as this directly affects your sight picture on the rib of the shotgun. The comb on a bolt action rifle sits below the bore line of the rifle. This allows the shooter to position their head lower and more in line with the bore. It also allows them to mount the scope lower or, if equipped with iron sights, nearly parallel to the bore. The AR-15, though, is a different story.

Comb height of a Remington 700 compared to an iron sight AR-15. Note: Comb height is opposite in its orientation to the bore between the two.

An AR-15’s design makes it impossible to achieve the same cheek weld of a bolt action rifle. Before continuing, I should specify that I’m defining AR-15s in the classical context of Eugene Stoner’s design. A classic AR-15 design has a buffer tube in line with the bolt and bore that contains the action’s recoil spring and buffer. There are other weapon systems, such as the FN SCAR, that contain the action springs forward of the action, as opposed to inside the stock. This allows the stock to have a comb that drops below the bore line and allows the shooter to obtain a sight picture more in line with the bore. The primary difference between an AR-15 and most other weapon platforms is the comb sits above the bore line whereas other weapon platforms sit below the bore line. The AR-15 design has benefits for recoil, as the shooter is directly in line with the bore when shooting. However, scope over bore is an issue when performing shooting at varying distances, especially inside 50 yards.

Point-of-Aim (POA) versus Point-of-Impact (POI)

Before further discussing the issues associated with scope over bore, I should cover what Point-of-Aim (POA) and Point-of-Impact (POI) are to the shooter. Bullets don’t travel in a laser straight trajectory. This is why we sight our firearm in for a specific distance and, if shooting anywhere outside of that range, we know the trajectory of the bullet. For instance, a 5.56 AR-15 sighted in at 50 yards will shoot low inside 50 yards, high past 50 yards, and re-zero at approximately 200 yards before the cartridge drops low again. This is because the projectile follows a curved arc during its flight path as opposed to a straight line. Accordingly, your POI is almost always slightly off from your POA. This issue is worsened by a higher scope over bore offset.

The Effect of Scope Over Bore at Close Range

I utilized three different firearms to illustrate how scope over bore affects POA versus POI between 50 and 20 yards. While this doesn’t seem like enough distance to make a significant difference, the results, as you will see, are pronounced. The three firearms were a custom Alamo Precision Remington 700 in .308 Winchester outfitted with a Viper PST 6-18x44mm, Daniel Defense M4 in 5.56 outfitted with an Aimpoint T1, and a Remington 700 in 6mm outfitted with a Leupold VX-III 3.5-10x40mm. The bore to scope offset was 1.75”, 2.5”, and 1.5” while the rifles were powered by 168-grain Sierra MatchKing, 55-grain FMJ Federal 5.56, and 105-grain Nosler Partition, respectively. I recently outfitted the .308 with a new scope and only had the chance to get it on paper at 20 yards before conducting the test.

Firearms tested with different scope over bore heights and sight-in distances.

Testing was conducted from a bench rest at 50 yards with three-shot groups obtained from each rifle. The groups were MOA for the bolt guns while the AR-15 held approximately 2 MOA given the cheap ammo it was fed. The test was then conducted at 20 yards with all rifles on a bipod and sandbag.

Results of shooting targets at 50 yards (Left) and 20 yards (Right). From top to bottom: Alamo Precision .308, AR-15 5.56, and Remington 700 6mm.

From a brief glimpse of the targets, it’s apparent scope over bore is a factor. This test was designed to eliminate the drop of the cartridge over distance as much as possible. For most rifle calibers, trajectory isn’t as significant a factor unless at distances exceeding 200 yards. The rifle with the greatest scope over bore difference (AR-15 at 2.5”) had a shot height variance of 3.25” over the 30-yard change. It should be noted this rifle was sighted in 1” high at 25 yards, so the difference is closer to 2.25”. The .308, with a 1.75” scope offset, was dead-on at 20 yards but was 2.25” high at 50 yards. The 6mm, with a more classic hunting setup, was zeroed at 50 yards with shots hitting 1.25” low at 20 yards.

Results of testing scope over bore at 50 and 20 yards. Measurements in red indicate shots were below the bullseye.

If inclined towards greater scientific scrutiny, one may object to how this test was conducted – and rightfully so. Ideally, all three rifles should be sighted in at the same distance to have exactly comparable results. I agree. Regardless, the sight-in distance was unable to escape the effects of scope over bore. Scope over bore has a direct impact on POA versus POI. The higher the scope is mounted on the rifle, the more it can affect POA versus POI over changing ranges. The data confirms this observation.

Accounting for Scope Over Bore

Tools are only as effective as their user. A carpenter will know how to use a carpenter’s angle because they’re familiar with that tool and know how to use it properly and effectively. The same is true for every firearm in your collection. Depending on its use, the user should be familiar with the application and limitations of that firearm. A plinking .22 isn’t going to get the rigorous scrutiny that my patrol rifle or duty pistol will have. One is a range toy while the other requires I rely upon it to protect innocent life. I am responsible for my effectiveness with those weapons.

Previously, I touched on scope over bore offset in my review of the Vortex “Huey” holographic sight. Vortex, and some other manufacturers, have developed reticles to account for this offset at closer distances. Such reticles are a great asset for providing precise shot placement under those conditions. Scope over bore is more than just a factor in your POA and POI. It can also determine whether your rounds make it to your intended target. I have plenty of personal and secondhand accounts where shooters had a clear sight picture in their optic window and took a shot on their target – only to see no hole appear on the target. Instead, they saw holes appear at unintended locations because the scope was offset from cover but the bore wasn’t. I personally know of one instance where an officer took such a shot on an armed threat only to be surprised their rounds struck their vehicle and not the suspect.

The top image shows a clear sight picture for the shooter. Meanwhile, the bottom image indicates the shooter is about to put a hole in an obstacle. This is what is referred to as a “self-correcting” problem.

The moral of the story is scope over bore plays an important role in accurate shot placement – and whether or not your shots make it to the target (Review firearms safety rule number four for further: Know what is behind, around, and between you and the target). This consideration should be in mind when working around and behind cover and concealment. Ultimately, there’s no way to eliminate scope over bore displacement. However, testing your setup at different ranges will provide confidence in knowing where your shot will go relative to where you’re aiming. Knowing the capabilities of your optic and firearm setup is as important as knowing your own capabilities.

Tom Stilson began his firearms career in 2012 working a gun store counter. He progressed to conducting appraisals for fine and collectible firearms before working as the firearms compliance merchant for a major outdoor retailer. In 2015, he entered public service and began his law enforcement career. Tom has a range of experience working for big and small as well as urban and rural agencies. Among his qualifications, Tom is certified as a firearms instructor, field trainer, and in special weapons and tactics. If not on his backyard range, he spends his time with family or spreading his passion for firearms and law enforcement.

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