First Strike Rifle Scope Design Series: Part 1, General Considerations

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. 

F**k it, I’m doing a rifle scope. I’ve got the design worked out, but due the nature of the round we shoot, I had to make some unusual design choices. Here are my first generation thoughts, with as little math-speak as possible. I’m putting them out here partly for peer review and partly to introduce the atypical design choices before the build. I’m still fiddling with the final lens arrangement, so if you spot something wrong or poorly considered, let me know and I’ll reevaluate before I put my glass order in. If you havent noticed already this post is going to be image intensive – youre reading this on the MCB FS sub-forum, so I’m assuming passing familiarity with optics in general and as they relate to FS rounds, but the concepts we’re talking about are still a lot easier to grasp visually that textually.

The Why and the What

Modern rifle scopes are designed for high velocity, flat trajectory rounds at ranges up to several football fields away. Because of this, these scope tend to have, either as a feature or something they can get away with: large magnifications, small adjustment ranges, and narrow fields of view (FoV). In direct contrast, FS rounds are extremely slow, arcing, and limited to less than two football fields.

Because of this disparity, rifle scopes on paintball guns have a very narrow “band” of usable range for any given zero angle. Players are forced to use adjustable risers for on the fly elevation changes in order to use their scopes over more than just one band of their weapons system engagement range. In the best case, a player has a scope designed for range finding and has gone through the trouble of calibrating or marking his adjustable riser for various common ranges. He must then range the target, adjust his riser, reacquire the target, and fire. Not exactly an easy or fast means of hitting players capable of moving and shooting back at you. To make matters worse, the average scope does not have rangefinders that are useful for paintball and few players have the ability or desire to calibrate their risers (if they have them at all). Check out DJMatt’s and Trinity’s posts at Tango Down for more detail on the current state-of-the-art FS sighting equipment and technique.

Crossbow scopes come the closest to the FS ideal but even they have too narrow FoV, adjustment ranges and BDC reticles that are ill suited for FS use.

Clearly, the readily available scopes are far far from ideal. We require an optic with moderate magnifications, a large range of adjustment, a large FoV, and for serious rapid/pressure shooting purposes, a dedicated FS BDC reticle. These then are the design goals for my FS rifle scope, but for reasons we’ll get into shortly, I had to get creative to reach them.

The Design Considerations

Offset reticle. Instead of having the crosshair centered on the middle of the image you see in the scope, I offset it 25% up the y-axis. It looks like this.

Image made using python and pyx. Look how much MORE working area you get with the offset.
 [spacer text]

“Why” you ask? Because it get’s us more BDC range out of a smaller eye lens. A traditionally centered scope would have required an eyepiece lens several inches in diameter (think about that, several inches in diameter – don’t look like this guy). By offsetting the reticle I was able to drastically reduce the required size and now the eyepiece is within practical and aesthetic limits. The trade-off here is minimal. Unless you’re virulently against the aesthetic of the offset, the only thing you lose is target acquisition speed when shooting at closer ranges. Since you naturally and intuitively aim with the center of the reticle, you lose a few milliseconds while your brain make the adjustment for the offset. But I reckon that if you’re using the magnified scope at close ranges, then you’re probably in a situation that demands accuracy more than speed. If you’re in a close range situation that demands speed, then one, you’ve f**ed up, and two, thats what mini red-dots are for. On the flip side, you gain target acquisition speed at medium range and long range since your brain has less distance to offset for, an increase in the usable range band of the scope, and a decrease in the needed size of the eyepiece. Thats a whole lot of benefit for not a lot of cost.

External Adjustments. The scope, instead of using the traditional internally adjusted ball-joint sleeve mechanism (seen over here) will use external adjustments.

The Elcan Spectr. A 4x externally adjusting combat rifle scope used by SOCOM.
Note the dials for windage and elevation and their placement.
 [spacer text]

The reasons for this are many, but the real driving factor is size and practicality. An internally adjusted scope’s diameter is a trigonometric function of the angular adjustment range and the distance between the ball joint and the lens. That might not make a whole lot of sense, but just know that the larger the adjustment range you need the scope to cover, the larger the scope body and objective lens diameters need to be. FS adjustment ranges would make for a monstrous objective and scope body. We could keep the internal adjustment mechanism but then we’d have to also use or build an adjustable riser for it. Instead, I figure we’ll save some design and machining complexity, in addition to some other considerable benefits, by cutting out the middle man and just going directly for full external adjustment. We also gain:

  • simplified assembly
  • radically reduced scope body and objective diameters (and the smaller profile that comes with that) without any loss in image quality
  • the scope can be easily and quickly removed with near perfect zero retention between mountings
  • customizable windage and elevation click values
  • and a system that is much more resistant to hidden zero failure

All of that seems too good to be true, I know, but it is none the less. Well briefly go into each, in no particular order.

Internal windage and elevation assemblies require lots of small, precise pieces, and special tools for both manufacture and calibration during assembly. Yeah… no, not interested. We’ve already talked about the size reduction benefit, but I want to go deeper into the image quality bit. Basically, the reason why some scope have such large objectives, is because of the internal adjustment assembly. At any given time, the scope is only using a small portion of the light that passes through the objective window, the rest is just there to give the windage/elevation assembly room to “look” around without having the image clipped by the inside of the scope body. By moving the adjustments outside the scope, we only need to use an objective big enough to admit the maximum usable light for the rest of the system, the practical benefit of which is a much smaller objective with no loss in image quality.

Left: The area of light actually used by the scope, and how it’s adjustment range is limited by the objective diameter. Right: A Swarovski rifle scope with a tiny 24mm objective – it still produces premium level optical quality. Source: Modified from a much more in-depth article at Randy Wakeman Outdoors.
[spacer text]

Since, unlike with scope rings or other mounting options, externally adjusted scopes sitting in their mounts are torqued from the same spot, with the same amount of force, and in the same way with each installation, the zero shift between removal and remounting (provided you don’t take the mounts off the gun) is practically nonexistent. This consistency in positioning, force, and torque order is much more difficult to maintain in traditional picatinny removal and re-installations.

For the actual adjustments, you can change your click values by moving the mounts closer or further apart. This alters the trigonometric relationship between the adjustment point and its pivot point, and allows you to set the mounts up for whatever value-per-click you find best.

In some scopes the internal adjustments will walk out of zero with use and abuse. Since they are internal this failure is silent and undetectable by casual observation, you’re not likely to notice it without a very thorough diagnosis of the system. Externally adjusted system have experienced a resurgence in the benchrest community precisely because any shifts or walking will be readily apparent. As an added bonus thats not particularly relevant to paintball, external mounts dont need to be lapped in order to get perfect axial alignment of the mounts to the scope.

Honestly, there are so many advantages to external mounts that I don’t know why they aren’t used in more rifle scopes. I know offset reticles aren’t used because the benefits are much smaller (if they exist at all) for firearms and don’t outway the risk of nonconformity in a very conservative marketplace but external adjustments are a whole lot of winning for not a lot of loss. They reached their height in popularity around the 60s but faded away after that. Unfortunately, the evolution of rifle scope adjustment mechanisms is a very niche topic and I haven’t been able to find anything on it.

A note on the resiliency of external adjustments. Some might say that since the adjustments are on the outside they might be susceptible to environmentally forced shifting like knocks or mud. But the design has seen several wars with the Marine Corps (the old Unertl style) and besides that, I figure if it’s good enough for combat iron sights, its good enough for combat optical sights, they both have the same environmental resistance needs.

Long, instead of short and tall. Traditional scopes are relatively short in length, combine this with the large FoV and adjustment range we’re using, and you start to run into problems with the barrel obstructing your optic and limiting the usable range band.

Note that the barrel clips the bottom of the field of view. In reality, you wouldn’t get the full area indicated by the yellow arrow. Image modified from Precision Rifle Blog.
 [spacer text]

In these situations you can go tall or long to push the FoV cone outside the barrel. Tall is a pretty common option in paintball, personally, I find it aesthetically terrible and from a performance standpoint, players can play tighter and shoot from smaller openings than they could if their scope were six inches above the bore, like a – shudder– hopper.

For some players a high scope is necessary for mask clearance, but personally for convenience and for the reasons listed above, you should be using a stock designed explicitly for paintball masks. Like the classic grip-tank stock and designs like the dogleg or the offset stocks on the DAM.

If tall is out for aesthetic and practical reasons, we’re left with the long option. What does it do for us? It keeps the scope tight to the marker, allows us to use long focal length lenses (which tend to produce better image quality than short focal lengths), and honestly looks badass as can be.

No caption necessary.
[spacer text] 

Vanilla Optical Characteristics and Closing Thoughts

Magnification and Eye Relief. Combat rifle scopes for firearms are commonly found at 4x magnification, and intended for use out to 500 yards on full man sized targets, so I figure 3x is good enough for higher precision work at 100-ish yards. For our ranges we don’t need or want too much magnification. The FoV that FS’s need, make large magnification unworkable with the other constraints.

To clear the mask three inches is a good number and consistent with Trinity’s recommendation over at Tango Down.

Second Focal Plane Reticle. First focal plane (FFP) reticles are the rage right now. They allow variable zoom scopes with BDC or mil-dots to retain their compensating values across the whole range of zoom and not just one point. But since we’re building a fixed power combat scope we’ll put the reticle in the second plane (SFP) to keep the BDC etching tolerances manageable. If the reticle were put in the first plane, you’d have to have an etching process 3x more precise, since the reticle, and the imperfections that all processes introduce, will get magnified that many times. That’s part of the reason why FFP scopes tend to be more expensive than SFP scopes with similar optical characteristics, because of the added equipment and production costs associated with that precision.

Closing thoughts. I didn’t start out to come up with a design that mimicked those super cool old-school rifle scopes. That was an unexpected but happy accident. My original vision was more short and fat like modern combat scopes, the Elcan in particular was my muse. Its also cool, and unsurprising, how the best fit design considerations produce a scope that continues to follow the historical/chronological evolution of firearms technology. Next up on the technology curve, should be radically cheaper or improved ammunition. Maybe if those RAP4 rounds are cheaper but less accurate we’ll see the first belt fed shaped projectile marker (my beating heart be still). Finally, I leave you with these badass gentlemen of American badassitude, and the Marine Corps’ Unertl-style scope of yesteryear.

Two guys who are 1000x cooler than you, using a scope design good enough for fighting nazis, imperialists, and communist insurgents across decades of American warfare. It’ll do just fine at the local paintball field.
 [spacer text]

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s