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Why Use a Cantilever Scope Mount on AR rifles?

Picture this: you are trying to mount a scope on an AR-15 or AR-10 platform, or perhaps something similar. You Google what the best mount is, and the result comes out – the cantilever scope mount. So, what is a cantilever mount, and why does it work on rifles like that?

What is a cantilever scope mount?

A cantilever scope mount (meaning an overhang/free-hanging part, or a part that is fixed only at one end) is a single piece scope mount, usually with a Picatinny fixing point that has the rings moved at a certain distance forward from the mounting base. Now, to explain why this is important, we first need to talk about why normal rings are not the best option for AR-15 style rifles.

Why Use a Cantilever Mount on AR rifles?

Most cantilever mounts are made up of a single piece, usually with a Picatinny mounting system, as do most AR designs. The single-piece construction makes them sturdy and rigid, offering a more stable and reliable base for the scope. But the truth is that this does put them on the heavier side, compared to two-piece mounting rings. As usual, each design has some advantages of their own.

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Cantilever Mounts

Mounting of Cantilever scope mount

Now, to the predicament at hand:

  • The optics on a rifle needs to be mounted forward enough to get the right eye relief, but the stock on an AR, even when fully extended, is usually shorter than most hunting/sporting/sniper rifle stocks, therefore the shooters head is tilted more forward, and usually too close to the optic to offer comfortable eye relief and field of view.

Why Use a Cantilever Mount on AR rifles?

  • The solution would be, of course, to simply move the scope and mount forward, as most modern AR rifles have a Picatinny rail all along the top. But that brings in a new set of problems. The rail is usually made up of two parts, the rail on top of the receiver, and the rail on top of the handguard. Now the receiver (and the barrel, consequently) is at a quite fixed position, but the handguards are usually free-floating from the barrel, fixed only at one point. That makes them move or float around when shooting. Now the movements are, of course, tiny, but that can still play a big role. Firstly, if the mounting base of the scope is placed entirely on the handguard, it can quickly lose zero, or the zero moves a bit, making the rifle inaccurate. If it is moved forward for just a fraction so that the back ring is on the receiver, and the forward ring on the handguard, we have two fixing points, one of which is rigid and fixed and the second movable, putting a lot of stress on a sensitive piece of equipment that an optic is. Also, with many rifles, there is a slight notch, or stair if you will, between the two pieces of rail, as they are seldomly perfectly aligned. Therefore, both these options are far from ideal.

Why Use a Cantilever Mount on AR rifles?

  • The actual solution lies in the fact that that the base of the scope is placed entirely on the receiver, making it fixed and steady, but the scope is pushed forward to offer the desired eye relief. Hence, the cantilever type of mount comes into play. The overhang or outreaching protrusion of the rings forward from the mounting base is exactly what this type of mount is made for, and it works perfectly.

Advantages of Cantilever scope mount

There are also some other advantages of the cantilever mount when putting optics onto an AR.

  1. Firstly, the fact that the cantilever mount pushes the optics a bit forward not only offers a better eye relief and field of view, but also clears a bit of space on the rail in the back of the rifle, making it possible to mount something else in that spot, for example, magnifiers or folding back-up sights, and so on.
  2. Secondly, a scope, especially a full-sized scope that one would mount on an AR-10 or other DMR type rifles, is a rather heavy piece of equipment. The further you push it on your rifle, the greater the lever you create, making the rifle feel heavy and unstable. Bringing the point of weight backward towards yourself makes the rifle much easier to handle.

Why Use a Cantilever Mount on AR rifles?

Conclusion on Cantilever mounts

Against cantilever mounts, namely the weight, and the fact that it can take quite a bit of extra space (although, the length of the scope is usually the space used up, no matter the mounting option). But, everything considered, it still is the most used and probably the best option to correctly place a shooting scope on an AR, at the very least in sports shooting, but it works for tactical applications just as well. It was made for this purpose, and it simply works.

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Why Use a Cantilever Mount on AR rifles?
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Why Use a Cantilever Mount on AR rifles?
Description
A cantilever mount is a single piece scope mount, usually with a Picatinny fixing point that has the rings moved at a certain distance forward from the mounting base.
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Optics Trade Blog
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4 Comments
  • I can see your point but I do disagree on a long scope mounted on the upper receiver not being far enough forward on an AR built for long-range shooting.

    If the shooter intends to use the AR for long range shooting, (and was serious about doing so), he most likely would have also gone with a Magpul adjustable stock which is considerably longer than a standard AR stock, (and of course adjustable), in order to get the proper arm relief needed for competition shooting.

    I recently built a Tac Driver AR and along with a lot of precision parts designed to help make it a Tac Driver, I installed the Magpul PRS® GEN3 Precision-Adjustable Stock because it allowed me to adjust the distance from the crook of my elbow to the pistol grip/trigger and it also provided me with an adjustable cheek weld as well.
    I have plenty of eye relief while using the scope, (even when firing from the prone), with rings mounted entirely on the upper receiver’s Picatinny rail. I also have a much better spot under the Magpul stock to use my “squeeze bag” with my off-hand to adjust elevation with. If any part of the above seems confusing to you, just get with any Sniper and he, (or she), will show you firsthand what I’m talking about!

    If you think about it, a scope is for accuracy shooting out to distance. While it is not true of all scopes, generally the longer the scope, the longer the desired range the person intends to shoot. If the shooter is only looking to get a little better sight picture then their scope will likely be a lot shorter than your average sniper setup and eye relief will not be a problem even with the standard AR stock and rings mounted on the upper receiver.

    I actually know a few people that swear by a Red Dot and 3x magnifyer for shots out to 500 yards.

    While a standard AR rear stock is fine for iron sight shooting, it is not even close to optimal for scoped accuracy. So if you intend to put a long scope on your AR build, (so you can reach out and touch someone), rather than go for the extra forward weight of a Cantilever Mount, just set the rifle up properly with an adjustable rear stock and mount the scope in the proper balanced position on the upper receiver. The cost will likely be about the same either way but the gun, (and you), will very likely shoot a lot better.

    Too bad there is not a way to post pictures here or I’d show you exactly what I’m talking about.

  • On todays ARs, forget it, not going to beat around the bush. TODAY’S scopes, designed for the AR platform, are already designed to be flush mounted on a pic rail such that center of the optic (l/r and u/d) adjustments are relatively close to the chamber (less than 1 MOA at 100 meters). The point of a cantilever mount is to move the optics back to provide better eye relief while maintaining the mounting position. Short of it it being a riser where backup sights would be of use which would add to the U/D MOA (raise the optics and you have to adjust the sights increasing the MOA over distance).

    Your post which seems to be from 2/21 isn’t so far back to be outdated, it’s just just doesn’t apply to AR specific scopes. The mount should be as close to the rifles chamber as possible such that the center of the scope and mount are as close to the chamber as possible for better accuracy.

    That said, few people buy ARs or scopes to shoot more than 100 meters, so the issue never presents itself. If you want to shoot 500m with an AR, the optics (scope adjustments, tuning knobs, turrets or whatever else you want to call them) need to be as close to the chamber as possible. The pictures on this page show exactly what I’m saying but it’s just not the case when mounting scopes that are not specifically designed for the AR platform.

    The mounting position on a pic rail also plays an important role in the game to find center mass on a target (bulls eye). The distance to center of the scope mount adds to (or subtracts from) the distance from center of the optics, turn it around and you negate much more of the wobble and/or wiggle of the mount and shooter. The ultimate goal is to have the mount and adjustments as close to center over the projectile with the mounts firmly on the receiver (unlikey but the closer the better). 1 inch is much better than 2 by no stretch of the imagination. A mount 2 inches from center with a scope being 1 inch forward of the center of the mount is less desirable than a mount being 1 inch from center with the scope being 2 inches from center of the mount. These are two different ratios. Center of rotation for the scope is center of the mount, center of rotation of the mount is realitive to it’s position on the receiver.

    As stated previously, the pictures provided show optimal scope placement (optics adjustments as close as possible to the chamber). If you want to move the scope backwards for better eye relief then you need to consider the position of both the mount and scope (and desired accuracy at varous ranges) if it will move the scope adjustments away from center. It’s not as simple as “use this mount for better eye relief” period.

    You can always find the bulls eye if you know your weapon and understand windage.

    The sad truth, anything short of a $1000+ scope probably uses 3.142 for Pi vs 3.1416. The difference adds up over distance which greatly affects low quality variable magnification scopes as much as the adjustments resetting to factory zero with a moderate bump.

    Just saying,
    J

    Bump, yeah, it’s a year old web post. The more you know!

  • John says:
    Your comment is awaiting moderation. This is a preview; your comment will be visible after it has been approved.
    Is the cantilever scope ring a better choice than single scope rings for a Ruger .22 Long range target rifle holding the bushnell rim fire 9×40 scope?
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