First Strike Rifle Scope Design Series: Part 2, Externally Adjusting Mounts

This article originally appeared over at the First Strike sub-forum of M. Carter Brown. That thread can be found here. Check it out if your interest in the subject extends to the community’s reaction and inputs. I’ve broken it up here on the blog into a series since the different aspects of rifle scope design are so broad that I didnt feel comfortable adding them on to this post as they get written. Each one is really it’s own subject and deserves a more dedicated treatment. 

We have three basic styles of external mounts to choose from, we’ll call them Elcan, Stith, and Malcolm styles.

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Malcolm style mounts. These mounts consist of two rings. The front ring has a leaf spring and pushes the scope body up against two nubs on the inside of the ring. This arrangement holds the scope firmly in place while also allowing it to pivot freely in response to the the angle changes made on the other end of the scope. The second ring is where the adjustments are made, typically this is also where the scope is clamp-locked into place to keep it from freely sliding back and forth inside the mounting rings. Also note, that by clamping the scope at only one point, the scope itself is still allowed to slide back and forth in the front ring to accommodate hypotenusal changes in the triangle formed by the two mounts and the scope (brilliant). The ring is suspended in a U-shaped tower and advanced up and down by a screw mounted vertically on the side of the tower. Another screw runs horizontally through a threaded cross hole in the base of the tower and is anchored into the mount itself. Windage is adjusted by turning this screw, which causes the whole tower to move left or right as appropriate.

Elcan style mounts. They are used in one incarnation or another on the Elcan Spectr, the British SUSAT, and on some Russian optics that I don’t care enough about to go find the names of. The mechanical relationships here are complex-ish and interdependent so bear with me…. Or don’t. I originally started to write out the mechanical relationships here but they are so nested and independent that I don’t think I’d be able to adequately describe them in text. Instead if you’re interested, google pictures of the mount. It’s semi-self explanatory when you see it and think about it for a minute. If you have any questions after that about how it works, feel free to ask them. The take away is that the adjustments aren’t independent.

Stith Style Mounts. Stith style mounts are some clever clever pieces of engineering. They aren’t complex but they are very alien to the average shooter. It took me forever to figure out how these bad boys works and I’m pleased as punch now that I have.

There’s a lot going on here. On the left hand side, note:
The tab extending from the scope ring, the circles sitting on top of the tapered shapes, and the picture detailing the scope interchangeability feature.
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Basically the front and rear mounts hold two rods with opposed tapered tips. For nomenclature purposes, we’re gonna call them pins, since they are more associated with tapers than rods. The taper of the pins create a v shaped valley along which the scope body rides, and more importantly, along which the scope body is passively self-centered. The roundness of the pins also allows the scope body to pivot freely over them in response to the windage and elevation adjustments. Not only do the pins form the bearing surface along which the scope is simultaneously secure and free to move as needed, but they also proved the means by which the adjustments are made.

Each pin sits in a threaded sleeve in the mount body and has a square bar passed through its center. You can see this more plainly here. By turning this bar you force each pin to rotate in it’s threaded socket. This forced rotation inside the mount body causes each pin to simultaneously advance in accordance with it’s thread direction and rotation direction. In one mounting block both pins have the same thread direction. When the bar is rotated they advance left or right together, causing the center of the v-valley they form to move left or right with them. Boom. Windage. The other mount has the threads running in opposite directions, one left handed thread and one right handed thread. When the bar is rotated they are forced to move in opposite directions. They will both move towards the center of the bar or away from the center of the bar causing the center of the v-channel to move up or down accordingly. Boom. Elevation. The scope is secured to each mount by the compression of a leaf spring along the scope body – this is also what prevents the scope from rotating in the mounts.

Take a moment to let that brilliance sink in. Every component serves two or three additional functions. I’d call that elegant any day of the week.

Consider. Deciding between the three, we can exclude the Elcan style. The adjustments aren’t independent (changing elevation also changes the windage) and the distance between the front and back of the mounts can’t be changed to produce custom click values on the fly. So were left to decide between the the Stith style and the Malcolm style. They are both badass. Aesthetically, I think the Malcolm has a slight edge. Those tower horns strike a pretty wicked silhouette. On the slightly less wicked side, the Stith mounts are super low and offer a lot of opportunities for color palettes and stylized millings. You remember those old cars with the super exaggerated and swept back wheel wells? Imagine low swept-back mounts like that in OD green with contrasting dark earth on the minor components. Niiiiiice.

In terms of machining/assembly complexity, its kind of a toss up. I haven’t done a full step by step write up of what the design and consequent machining requirements, so this may change, but off-hand I think the Stith style would easier to machine and assemble. The Malcolm involves a lot of multi-step slot cuts that you’d have to bridge with small purpose made components during assembly. I also worry about the field-durability of the Malcolm style. While I’m sure they’re rugged, they do have much more in the way of exposed moving parts and more height/leverage for knocks to work against. On the other hand, the tower does offer a lot more vertical room. I know we only need about 1° of angle to reach our zero, but I haven’t done the math on how thick the Stith style rods would need to be in order to reach that high.

I’m leaning towards the Stith style but it’s still up in the air.

Update: In terms of mounts, I’m going to combine the stith with the malcolm to get the best of both worlds. The idea is to create V-channel blocks that can be installed into traditional ar-15 rear irons with leaf springs that slip under the blocks to pull the scope into them. That way I only have to machine two relatively simple components and the user gets all the adjustability and reliability of a combat proven design. This may or may not be the final configuration, depending on looks and usable adjustment range, but for now it’s good enough to evaluate the scope design without too much fuss on the mounting side.

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