Bruce Winker – Optics Reports Publishing
Shooters who use the same rifle year-round for precision target shooting, but also take it on a long range hunting trip later in the year, need a solid dual-purpose rifle scope. This is the type of long range shooter who favors a first focal plane scope, but is also looking for a good hunting optic that’s not too heavy and can be used in lieu of a spotting scope if necessary. The Kahles K624i 6-24×56 fits this description very well. It is nearly ideal for precision rifle use, and gives up very little as a hunting optic as well. Shooters who favor an illuminated reticle in low light will find the K624i a good value.
The K624i scope comes with five reticle options, all located in the first focal plane (FFP). Customers can choose between one MOA and a variety of mrad reticles (with matching turrets). I tested the configuration with the SKMR2 reticle and mrad turrets. As one of the oldest rifle scope companies, Kahles enforces high optical engineering standards on their products. This is quite evident in the near-perfect turret accuracy, high transmission and excellent resolution of the K624i.
Readers should be mindful that there is no perfect rifle scope. Every scope on the market was born out of a process of skilled engineering and compromise, and therefore has strengths and weaknesses. I diligently scrutinize details of scope design, construction and performance, and expose shortcomings, no matter how small. Use your own judgment in weighing the pros and cons (see my summary at the end of the review). An explanation of my transmission, resolution, veiling glare and turret measurement methods can be found in a sticky in the optics forum (Rifle Scope Reviews) and additional details can be found at www.opticsreports.com.
The Kahles K624i test sample used in this review was a brand new scope provided by High Power Optics (www.highpoweroptics.com), an Authorized Kahles Dealer.
What’s in the Box: Scope; bikini style lens covers; Allen wrench for setting the elevation turret zero; lens cleaning cloth; documentation in three languages, warranty registration card.
Look and Feel: The first things I noticed about this scope when I first handled it were the large size, wide elevation turret and the location of the focus ring just below the elevation turret. The illumination control is located where the target focus knob is usually found.
The K624i is 15.9” long, which is ½” longer than both the Nightforce ATACR 5-25×56 FFP and BEAST 5-25×56 FFP scopes (15.4”). The K624i scope weighs 33.5 ounces, which is actually light for a 34 mm tube 24X scope. It compares to 38 oz for the ATACR, and 41 oz for the BEAST. The K624i has a 1/3 – 1/2 pound advantage over the closest Nightforce competitors, which is a significant savings for hunters. The K624i also has a lower elevation adjustment range than these Nightforce scopes, so this is not quite an apples-to-apples comparison.
The engineering, fit and finish of the K624i scope is flawless. The scope is large, but does not overwhelm my rifle with a 26” barrel. A photo of the scope mounted on my Winchester 70 rifle (7 WSM) is shown below. The front and rear lens surfaces appear to have no hydrophobic properties that would make rain drops roll off the surface.
Kahles K624i rifle scope mounted on a Winchester 70 Coyote Light rifle.
Ergonomics: This scope has good ergonomics and the mechanical design works very well, with one minor exception – setting the exact elevation requires a close look at times because of the small clicks on the turret (see below). The scope adjustments are well executed, with smooth parallax, magnification and illumination adjustments. The elevation, windage and illumination adjustment knobs have identical grip surfaces that are wide and easy to grip and manipulate. The magnification values printed on the zoom ring are difficult to see unless the shooter raises his eye high above the scope, but then it is usually unnecessary to know the exact magnification on an FFP scope. I just rotate the knob until I get the right compromise between field of view and magnification.
Locating the focus ring just below the elevation turret offers the benefit of being able to operate the zoom, elevation and focus knobs easily with either hand. Since these controls are the ones most frequently used in precision shooting, this design offers a significant ergonomic benefit over the traditional location of the focus knob. The focus ring includes distance values that provide a good starting point for removing parallax. Kahles now offers the K624i in a left-hand windage model for optimum use by right-eye dominant shooters. This scope has the windage adjustment and illumination control locations swapped. In this configuration the windage turret can be operated easily with the left hand, and the windage value can be observed with the left eye without breaking a cheek weld.
Rear and side views of Kahles K624i 6-24×56 rifle scope.
Elevation and Windage Turrets: The elevation and windage clicks are audible and tactile, and are spaced very close to each other. The windage turret adjustment range is 108 clicks – less than 1 complete turn. The elevation turret rotates a maximum range of 260 clicks – less than two turns at 140 clicks/rev, and it has two rows of elevation markings. The elevation turret has a small red button on the top that is lowered flush with the top of the knob during the first rotation, and raises about 1.5 mm during the second rotation. This button gives the shooter a tactile and visual indication of the exact elevation – read the lower row of marks if the button is down and the upper row if the button is up.
On the MOAK reticle model, the turret has three rows of numbers. The turret button works similarly: flush during the first rotation, up 1 mm during the second (button is red), and up 2 mm during the third (exposing a white band on bottom half of the button).
The zero stop actually stops the turret a few clicks past zero, allowing you to adjust the point of aim for a slightly shorter range. Zeroing the rifle and adjusting the zero stop is easy to do using a small Allen wrench. The user manual has detailed instructions. The combination of an illuminated FFP reticle and the elevation turret design make this scope easy to use in near total darkness by counting clicks from the zero stop. High Power Optics sells custom turret labels that work with this scope (www.highpoweroptics.com).
The small click marks and spacing can make it challenging at times to set the elevation to a specific angle. The elevation knob fits 140 clicks into one revolution, which puts the 0.1 mrad click marks at 0.8 mm spacing on the 1.44” diameter knob. The spacing between marks varies up to 0.4 click as the knob is rotated, which requires you to look closely to know which mark to use. This is a minor flaw that becomes even less noticeable with practice.
Reticles: Kahles offers five reticle options with this model scope, all of which are illuminated. My favorites are the SKMR2 (mrad with elevation and windage holdoffs), and the MOAK (1 MOA milling raticle). The MOAK reticle comes with ¼ MOA turrets. In the MOAK, SKMR and SKM2 reticles all the fine crosshair features are illuminated, whereas in the MSRK and AMR reticles only the center portion of the crosshair is illuminated. The crosshair line width is only 1.3” at 1000 yds, which is about right for long range shooting. For dedicated hunting use, I would prefer either of the MOAK or MSRK reticles, which have the thick reticle bars closer to the crosshair, and only the center of the crosshair is illuminated. The MSRK reticle has extra milling and range finding features that most shooters are likely to either love or hate.
Turret Range, Accuracy and Stability. The K624i 34 mm scope tube provides 26 mrad of elevation adjustment. The bullet up range was 13.5 mrad from the optical center. I downgraded the total range to an effective 24 mrad because of exit pupil vignetting (see below). An effective elevation range of 24 mrad is still quite large, however. If the K624i scope were mounted on a 30 MOA rail (adding 8.7 mrad of bullet up from boresight), it would have about 20 mrad of bullet up adjustment from a 100 yd zero (without any exit pupil or other vignetting effects), which is enough to dial out to about 1750 yards (~1 mile) at sea level using a 300 gr SMK in a 338 Lapua magnum rifle. At 5,000 ft elevation the maximum target range grows to about 1900 yds. Holding off on the reticle adds at least another 8 mrad of elevation, which further extends the range to about 2250 yds.
Shooters must use caution when comparing published elevation range specs because vignetting issues are quite common and yet rarely acknowledged or discussed. Manufacturers know that a large elevation range spec sells scopes, and shooters may never discover eyebox or field of view vignetting at these limits. It is safe to assume that any scope that has an unusually large elevation range probably also has some problem that restricts the scope’s utility near the limits of elevation adjustment.
I performed thermal and shock tests to confirm point of aim (POA) stability, and found that the K624i turrets are exceptionally accurate and stable. Each test was done by first placing the K624i on V-blocks and aligning the reticle to a target while rotating the turrets in the bullet up (weaker erector spring) direction. I performed the thermal or shock test and then placed the scope back on the V-block to measure the POA shift that was induced by the test. There was no internal fogging at low temperatures (<32°F), and the average POA shift after two full cycles between 30 minutes at 0°F and 100°F was .025 mrad , or a ¼ of a click. This amount of POA shift is very small and only slightly above the detection threshold of this test equipment (.010 mrad). I also subjected the scope to ten ~500G shock impulses using a pneumatic shock testing machine (see photo below). There was no detectable POA shift, which is astonishing because most scopes show some reticle movement after this test. Incidentally, I didn’t notice any POA shift when changing magnification or focus either.
Kahles K624i mounted on the Optics Reports shock impulse test equipment.
I measured the elevation turret accuracy from 0 to +10 mrad of adjustment, for -5, 0 and 5 mrad windage angles. The average turret error was 0.39% (0.39 click at 10 mrad) over this range. On average, an indicated elevation on the turret of 10.04 mrad was actually 10.00 mrad. This is an unusually small error (1.4” at 1,000 yds) that most shooters will simply ignore. I noticed a very small <.02 mrad hysteresis (less than ¼ of a click) in the reticle movement. Nonetheless, the POA always returned to zero if rotated from the same direction, regardless of how far they were adjusted. The level of turret accuracy, repeatability and POA stability that I observed is exceptional and reflects an extremely good mechanical design process at Kahles.
Sight Picture and Eyebox: The sight picture on the K624i is quite good, especially for a 24X magnification scope. At all magnification values the reticle features are sharp from one field of view edge to the other. The eyebox is forgiving and the sight picture is easy to acquire under most conditions. The image fades smoothly across the field of view when the eye moves past the edge of the exit pupil. The visually obstructed area surrounding the eyepiece field of view is narrow, indicating good eyepiece design. Very minor tunneling occurs at the lowest magnification (between 6.0 and 6.4X). This amount of tunneling is barely noticeable and represents a very small blemish on the optical design.
The optical design is not quite up to the large turret range, however. I noticed blurring of the edge of the field of view and a reduction in the exit pupil at high magnification and at elevation values greater than about +/-10 mrad from the optical center. At +/- 12 mrad, or +/- 10 mrad with the windage set to +/- 5 mrad, the shrinkage of the exit pupil makes the sight picture difficult to hold. For example, at 13.5 mrad bullet up, the eyebox is so small that I have to focus more attention on eye placement than aiming. The top of the sight picture is blurred and any minor up-down movement of the eye causes the sight picture to fade. Because of these issues, I downgraded the effective elevation range from 26 mrad to +/-12 mrad (24 mrad total).
Resolution: The resolution of this scope at high magnification is excellent. I compared the K624i side-by-side with a Swarovski Z6 scope and could tell no difference. I was able to pick out the same minute image details in both scopes. Optical aberrations at all magnifications are hardly noticeable. Chromatic aberration is minimal and only noticed on very high contrast objects. There is no apparent field curvature in the objective lens, which means that the target focus does not need to be re-adjusted when the elevation is changed. There is a very small amount of field curvature in the erector optics, which means that the diopter ring may need to be adjusted when dialing to large elevations. The K624i comes as close in performance to a good 24X spotting scope as any rifle scope I’ve seen.
Transmission and Low-Light Performance: I measured the visible transmission spectrum for the central 13 mm of the entrance aperture (see the figure below). My measurement gives a photopic (daytime) transmission of 90.5%. Kahles product literature claims a transmission of 95% for this scope using their AMV lens coatings, which is so exceptionally high that I suspected a misprint or typo. My measurement of spectral transmission does not exceed 92% at any wavelength. Frankly, the difference between 92% and 95% transmission is hardly even detectable by the eye and has no adverse effect on daytime use.
The scotopic (low light, dark adapted eyes) transmission is more important than photopic transmission for gauging low-light performance. I calculated a scotopic transmission of 89.4%. Both of these transmission values are good for a scope like the K624i that has additional lenses to manage optical aberrations. The relatively flat curve between 450 and 700 nm verifies that this scope produces accurate image colors. These transmission characteristics are excellent for a hunting scope. In low light conditions, I would set the magnification to 9-10X, producing a 6-7 mm exit pupil that is easy to acquire and larger than my middle-age pupils. To the naked eye the image would be as bright as any scope on the market.
Spectral transmission of the Kahles K624i rifle scope (AMV lens coatings).
In general, the available reticle options are better suited to daytime use than low light use. Reduced reticle visibility at low magnification is the primary drawback with a FFP scope, and the K624i is no exception. I tested the scope at low magnification up to ½ hour after sunset on a moonless, partly cloudy night. At 10X the reticle hold-offs were still visible until about 15 minutes after sunset. After then my ability to aim using the reticle without illumination diminished rapidly. While the target image was visible, I found that either the non-illuminated reticle was too thin or the illuminated reticle was too bright. Reducing the illumination intensity range to about 20% of the current range would improve low light aiming.
At 25 minutes after sunset I found that turning illumination on the lowest setting improved my ability to aim at <10X. At 10X I was able to aim just as well using the crosshair without illumination. At >10X aiming became more difficult in general because image brightness decreased rapidly, just as expected from the decreasing exit pupil (all variable 56 mm scopes would behave this way). With some effort I was able to aim at ½ hour after sunset, but a thicker SFP reticle would have made aiming easier.
For frequent hunting use, the MOAK and MSRK reticles are probably the best choices because the thick reticle bars are closer to the center of the crosshair. For someone who prefers to hunt with an illuminated reticle in low light, I would recommend the MSRK and AMR reticles because only the central part of the crosshair is illuminated, and the illumination would probably not overwhelm the target image with glare.
Glare and Image Contrast. Veiling glare comes from light that originates outside the field of view, bounces off internal surfaces in the scope, and falls on the image inside the field of view. It can severely degrade image contrast in bright daylight, under overhead clouds or when facing a setting sun. Glare Contrast (=1/Glare Index) measurements indicate how susceptible a scope is to glare. I measured Glare Contrast values of 5.5-6.6 at magnifications between 15X and 24X, and 11.1 at 10X magnification (see the figure below). The reduction in Glare Contrast with increasing magnification is very common.
Veiling glare measurements for Kahles K624i rifle scope.
These Glare Contrast values are not bad for a 24X scope with a 56mm objective, and are more than adequate for target shooting. When Glare Contrast is below about 8, however, subtle differences between shades of gray (antlers vs branches) and tan (fur vs dry vegetation) can be lost under bright daylight conditions. The K624i is not in the same league as Swarovski Z6 scopes, for example, which typically have Glare Contrast values >10 even at the highest magnification. Nevertheless, Glare Contrast values >5 make this scope a verygood candidate for a dual-purpose long range target and hunting rifle. Use of a sun shield ($60 accessory) may improve image contrast if the shooter experiences glare problems when facing a setting sun.
Summary: An excellent precision, long range rifle scope. A strong candidate for a dual-purpose target/hunting rifle scope, especially if hunting is the secondary rather than primary use. The MOAK, MSRK and AMR reticles are probably the best options for hunting. Good value.
Excellent turret accuracy, stability and repeatability.
Excellent optical transmission, brightness and resolution, especially at high magnification.
Primary control knobs can be operated easily with either hand.
Relatively lightweight for a 34 mm tube scope.
Good selection of reticles for daytime use, with matching mrad or moa turrets.
Cons: (all relatively minor)
Large scope, but comparable in size to closest competitors.
Exit pupil vignetting limits effective elevation range to +/- 12 mrad, though this scope is still capable of >1 mile shots.
Adequate image contrast for hunting, but not nearly as high as some hunting scopes.
The reticle visibility is acceptable but not great in low light; the lowest illumination setting is too bright.
Read more about the lab testing here.
Read the field review of the Kahles K624i by Justin Hyer here.