Advancing technology shrank red dot sights small enough to be used in conjunction with scopes, providing users with magnification on distant targets as well as rapid transitions on close-range targets. This article samples some mini red dot sights and their mounting methods.
Background
Until recently, a rifleman must choose either magnifying or non-magnifying sights that are mutually exclusive. The Soviet SVD tried to meld the two by having iron sights under a quick-detach scope mount. More recent solutions include red dot / holographic sights with magnifiers and 1-X variable magnification scopes (e.g. Trijicon TR24 1-4x24mm, Vortex Razor HD II 1-6x24mm, Leupold MK8 CQBSS 1-8x24mm). However, the former options lack range finding reticles and are typically limited to 3x magnification, while the latter options have small front objectives which hinder resolving and light gathering abilities as well as limiting the “eye box”. These solutions augment a close-range oriented rifleman at distance, but leave the precision rifleman wanting.
By attaching a mini red dot sight (MRDS) in conjunction with a scope, the primary function of a precision rifleman is unhampered while enhancing close-range capabilities with a shift in head position. To this end, three MRDS candidates were evaluated: Leupold DeltaPoint Pro (Product # 119687, $6001), Vortex Razor (Product # RZR-2003, $400*), and Trijicon RMR (Product # RM07, $600*). All are offered with different size reticles, and the RMR is offered in battery powered and fiber optic / tritium powered versions. In addition, three mounting methods were also explored: 12 o’clock directly atop a scope using a Larue LT788-2 mount, slightly offset between the turrets of the scope using a Larue LT742 mount, and 45deg offset on the receiver with a Vortex RT-45 mount.
MRDS Candidates
Red Dot Sights work by projecting a reticle into the user’s eye similar to head-up-displays used in aircraft. A light emitting diode (LED) placed at the focal point projects light forward into a collimator with reflective coating in the rear, producing a parallax-free reticle at infinity. This can be verified by moving one’s eye relative to the sight while observing that the dot stays on target. All three MRDS have their collimators tilted downwards in order to place the LED outside the field-of-view.
The collimator lens represents the sole contributor to the optical distortions observed. The RMR displayed a fisheye effect which was conspicuous when panning but was inconsequential in actual use when transitioning targets. This is due to it being preferable to first shift the eyes then the firearm, instead of shifting targets while focusing on the sights2. The Razor displayed minimal distortion in the center but some fisheye near the edges. In contrast, the DeltaPoint was distortion free edge-to-edge. However, the author noted diffraction (similar to that from scratched glass) which turned the triangular reticle into a blob when the LED is adjusted unreasonably bright and one is looking into a dark background (e.g. indoors). This is not a problem when the brightness is adjusted sensibly such as dimmed for indoors or turned up for full-sun.
The coatings on the collimator affect color cast, with the RMR having a noticeably bluish-green tint throughout, the Razor having the same tint at the edges but a clear center, and the DeltaPoint having a nearly imperceptible blue tint. The bluish-green tint of the RMR decreased the target image contrast but accentuated the reticle due to it being a complementary color. The DeltaPoint faithfully rendered colors, and the Razor was in between the two. When the reticle brightness was adjusted to be barely acceptable in full sunlight, the DeltaPoint and Vortex reticles bloomed indoors from being too bright and were marginally usable while the RMR reticle remained sharp. The author surmised the red-green contrast of the RMR allowed a dimmer reticle to be usable outdoors.
The brightness of all three MRDS is manually adjustable while the RMR offers an automatic option. The author found this nearly instantaneous self-adjustment useful when transitioning indoors / outdoors and when activating a flashlight in small rooms. However, the reticle can washout if insufficient light was reflected back to the sensor such as when shining a light down a long hallway.
Mechanically, all units are solidly constructed. The RMR has the smallest view window and thickest housing; view window of the Razor is just slightly shorter than the DeltaPoint with both having similarly width. The housing still casted a faint shadow even with both eyes opened.
The RMR mounts lower than the others, reducing its sight-height-over-bore and offset at close-range (further discussed below). It achieves this by having the mount form the bottom seal, thus necessitating a dismount to replace batteries. The DeltaPoint battery door is at the top and also hosts the brightness adjustment button. The Razor features a horizontal battery tray on the side. While LEDs typically draw very little battery, DeltaPoint includes a motion sensor that turns off the illumination after five minutes of inactivity and reactivates when moved. The automatic brightness adjustment should minimize the RMR illumination when placed in dark areas such as a safe. All three MRDS can be manually turned off.
Mounting Considerations
Three popular ways to mount an MRDS in conjunction with a scope are: 12 o’clock over the elevation turret, offset between the elevation and windage turrets, and 45deg offset on the receiver rail. The 12 o’clock mount allows easy ambidextrous use albeit with a tall sight-height-over-bore. The receiver-offset mount trades ambidextrous ease for a more reasonable sight-height-over-bore. Between-turrets mount forms a compromise and allows easier ambidextrous use with turrets too tall to mount an MRDS at 12 o’clock.
Peripheral vision and situational awareness are paramount for MRDS because they are likely used at “bad breath distance”. While it is beyond the scope of this article to discuss shooting with one- versus both-eyes opened34, it is important to note that binocular vision enables the brain to merge both views and render obstructions only seen in one eye as faint shadows. The 12 o’clock mount provided the least obstruction laterally or vertically and matched the intuition to pick up one’s head when alert. The between-turrets mount had faint shadows from the turrets, but they did not detract much. However, small parts of vision were blocked when used on weak side (e.g. shooting right-handed with MRDS on left side). Receiver-offset mount proved the most distracting when the scope generated large shadows if not outright blind spots in the middle-left for right-handed users (vice-versa for southpaws). However, the exceptional view of the 12 o’clock mount is obtained at the expense of cheek weld and sight-height-over-bore.
The tall sight-height-over-bore of the 12 o’clock mount necessitated a chin weld for the author. A smaller statured tester could not even obtain that while she managed a chin weld for the between-turrets mount. In contrast, a firm cheek weld can be maintained with a receiver-offset mount. As an aside, it is interesting to note the western emphasis on cheek weld as seen with cheek-risers on scoped M-14s and G3s, while Soviet influenced small arms seemingly accept chin weld as seen on SVDs.
Mounting an MRDS either directly over the scope or between the turrets creates a large distance between the bore and the sight plane. This complicates shooting from behind barricades as well as necessitating considerable hold-over or -under. When shooting from behind cover, a tall sight-height-over-bore can lead to the rifleman seeing an unobstructed sight plane while the bore is masked. The same problem can make a 12 o’clock mounted MRDS difficult to use from an embrasure (such as that found on VTAC barricades), and force the use of scopes on close-range targets.
A tall sight-height-over-bore can complicate precision shooting at close-range. The 12 o’clock mount as tested positioned the MRDS about 4.5in over the bore, and it was estimated5 to have doubled the elevation variation when compared to the standard iron sights at approximately 2.6in over the bore. The variation is significant and changes swiftly with distance. This requires memorization as well as rapid and precise range estimation for a precision shot (e.g. hostage target).
Shooting Impressions
In order to measure the ease of sight picture acquisition and target transition, two USPSA cardboard torso targets were placed 50yds distant and 15yds apart. Each tester made a reasonable effort for a torso A-zone hit, but a hit anywhere on target was acceptable. From low-ready, the tester engaged the first target with one shot then transitioned to the second target for another shot. Each tester alternated between transitioning left-to-right and right-to-left in order to investigate if the view obstructions discussed above had any effects. The alternation was designed to minimize the inevitable effect that repetition improves performance. Time from low-ready to first shot measured sight picture acquisition, and time from first to second shot measured transition speed. Ten strings of fire comprised a data point, and each mounting method was shot from both left- and right- shoulders. The author (mostly a service rifle shooter) is left-handed and another tester (experienced cowboy action shooter) is right-handed. 12 o’clock and between-turrets mounts used an RMR while the receiver-offset mount used a Razor; this is due to mounting constraints and the author did not feel this affected the data in any way. The MRDS was mounted on the strong side of the tester for between-turrets and receiver-offset mounts.
Data is tabulated below with first number being the mean (x¯) and second number in parenthesis being the standard deviation (s). Data is also graphically presented sans error bars to reduce clutter. The difference between transitioning left-to-right and right-to-left was insignificant, and is not presented for brevity. The difference in x¯ was marginal and well within one s.
The acquisition time data matched observations made in the previous section: the 12 o’clock mount was likely the fastest to use ambidextrously. The right-handed, left-shoulder time was markedly long because the tester could not find a chin weld at all. The between-turrets mount was very likely to be just as fast as the 12 o’clock mount on the strong side, but lagged behind on the weak side. It is a good candidate if weak side use is not required because of its lower sight-height-over-bore. The receiver-offset mount took the most time likely because of the rotation involved. As expected, weak side use showed increased s thus lowered consistency. While the left-handed tester experienced less repeatable performance when progressing from the 12 o’clock mount to the receiver-offset mount, the right-handed tester had roughly equal s for all mounting methods. This is likely due to the right-handed tester being heavily right eye dominant which had to be closed when shooting from the left shoulder, and the left-handed tester being comfortably ambidextrous. The author surmised that left-handed weak side data was likely more representative of the population and inferred that it illustrated the short comings of the receiver-offset mount.
The transition time data was more difficult to interpret. While x¯ follows the trend that 12 o’clock was likely equal to the between-turrets mount while both were faster than the receiver-offset mount, the differences were less than one s and too small to draw meaningful conclusions. The s proved more interesting: both testers were more consistent with the between-turrets mount than other mounting methods. This is likely due to the better cheek weld than the 12 o’clock mount, and less rotation required than the receiver-offset mount.
This analysis showed that the 12 o’clock mount is likely the best for ambidextrous use, but the between-turrets mount may be a slightly better method if weak side use is not required. It should be noted that the sample size in this study was small, and that both testers did not have extensive experience with any of the mounting methods. Training and individual aptitude may change the results.
Conclusion
Mounting an MRDS in conjunction with a tactical scope allows a precision rifleman to have his cake (identify and engage distant targets) and eat it too (rapid transitions on close-range targets). Trijicon RMR, Leupold Deltapoint, and Vortex Razor all provide excellent sighting solutions to the close-range facet. The automatic brightness adjustment of the RMR worked brilliantly for indoor/outdoor transitions although its optical performance left something to be desired. Deltapoint excels optically if the user can accept the manual brightness adjustment. Razor is optically in-between the others, has manual brightness adjustment, but is not as costly.
Acquisition time and transition time data showed that the 12 o’clock mount is likely the fastest for ambidextrous use, and the between-turrets mount is likely to be just as fast if weak side use is not required. However, the sight-height-over-bore of the 12 o’clock and between turret mounts exaggerates the point-of-impact shift at close range and can hinder use from an embrasure.
