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 mount. So, what is a cantilever mount, and why does it work on rifles like that?
A cantilever (meaning an overhang/free-hanging part, or a part that is fixed only at one end) 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. Now, to explain why this is important, we first need to talk about why normal rings are not the best option for the AR-15 style 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.
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.
- 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.
- 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.
- There are also some other advantages of the cantilever mount when putting optics onto an AR. 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.
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.
In conclusion, there are, of course, several ways to mount scopes or other optical devices on an AR platform, and there are ways to do it with normal rings as well. There are rifles that offer other solutions even in the AR family. And even with the described advantages, there are also some things that speak 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 sport shooting, but it works for tactical applications just as well. It was made for this purpose, and it simply works.
Source and author: Blaž Spruk, www.optics-trade.eu