So, I started with a Trijicon TR24 1-4x24, moved on to a K16i with the SI1 reticle, then onto a K16i with the SM1 reticle (I now have 2 of these), added a K18i and recently got a K18i-2.
On the reticles, the K16i SM1 and SI1 are brighter than the K16i 3GR. I've looked through a K16i with the 3GR but have never owned one.
The K18i 3GR is daylight bright. The difference between the K18i and the K16i is that when you turn on the K16i it's MUCH brighter initially. The lower half of the K18i and K18i-2s illumination dial are NV compatible, so they appear dimmer as you go through the settings. When they're on their max setting they're very bright. They're not as bright as a Razor, which is ridiculously bright.
I've never looked through the IPSC reticle, but I expect that an IPSC reticle is useless for anyone not doing IPSC. The 2nd Dot on the K18i 3GR reticle never bothered me. My eye always went to the center dot since it's bigger. The lower dot broke up the tree, but the same effect is achieved with just having an unilluminated dot in the 18i-2. The 3GR on the 18i-2 has a couple of extra mil stadia, so if you were putting it on an AR10 that would be something to make note of.
The rifles that I have the K16is on are an 11.5" in 5.56 and a 14.5" in 5.56. (My wife's) I wanted these to be good to ~400 yds. The bottom of the "halo"/"donut" on the SM1 reticle is 2.5 mils. The wide line above it is 2, the wide below it is 3. The first wide line below the dot is 1, so there's a fairly quick 3 mil tree in that scope.
The K18i was on my 16" AR, and in using it, the big things that I found were that sometimes I'd want to be back on ~6x. 1x between the K16i and K18i are identical, and fantastic, so there was never an issue there, but I used the ballistic turret on the K18i and never felt like I wanted one on the K16i. If I was taking a shot that needed an accurate correction in the reticle, I was always on max power anyway with the K16i. Between ~300 and 400 yds, when taking shots on small targets that were very widely separated, being on 8x felt constrictive. Spending more time training with it, I'd probably work getting more consistent about pulling mag back and zooming back in smoothly, but that's a consideration that a FFP scope doesn't have. I was able to address the issue mostly just using the BT for close distances and holding from there zoomed in on long ones. Also worth noting, is that the K18i and K18i-2 can have their throw levers index anywhere on the band that you wish. I set mine up so that the throw lever is straight up and down when the scope is on 4 power. It makes the 1 mil reticle holds be 2 mil reticle holds and is fairly easy to recognize / index.
The K18i was moved to a Mk14 EBR clone when I bought the K18i-2 because it eliminates the feeling at 8x of "I have too much mag", and the EBR just doesn't see the kind of shooting the ARs do.
The field of view on these scopes at max power is what drives your desire to be at max power, or not. If max mag doesn't give you enough field of view, you'll want to pull the power back so that you can get on target and take your shot. In a SFP scope, this limits you, because you're either fighting your field of view or spending time zooming in and out. You can get around this with a dialing turret (many low/mid power hunting scopes do this), but it doesn't change the underlying problem. FFP scopes solve this by keeping their hold values consistent no matter what power you're on, so you can bring the power back if needed to up your FOV.
You can see why this is necessary looking at the field of view of the "high end" LPVOs when they're at top mag. (all values at 100 yds):
Scope..................................................Linear FOV............................. FOV Area
K16i 1-6x24......................................20.1 ft.......................................317.31 sq ft
Razor Gen II 1-6x24......................20.5 ft.......................................330.06 sq ft
Nightforce ATACR 1-8x24..........13.1 ft.......................................134.78 sq ft
Nightforce NX8 1-8x24...............13.2 ft.......................................136.85 sq ft
S&B ShortDot 1-8x24..................14.7 ft.......................................169.72 sq ft
Kahles K18i 1-8x24.......................15.9 ft.......................................198.56 sq ft
Kahles K18i-2 1-8x24...................18.6 ft.......................................271.72 sq ft
Razor Gen III 1-10x24..................11.7 ft.......................................107.51 sq ft
The standouts are the K16i and Razor GII, which always feel "right" at 6x. There's enough field of view to smoothly transition your targets, so it's easy to use until you start getting shots long enough that you start wanting more mag.
You can see why the Razor Gen III is frequently cited as only the 1-8 being useful. 10x is a lot of mag for a 24mm objective, and the field of view of the scope is just really limited zoomed in that much.
The NX8 and ATACR at 8x have a similar situation as far as magnification and field of view are concerned. 8x is great for precise shots with them, but 6x
feels better for a lot of the shots the rifles these scopes are on need to take.
The ShortDot's mag/FOV relationship mirrors the Nightforce scopes, but with a wider FOV I expect you spend less time dialing the mag back.
The K18i (Gen 1) has a fairly wide field of view, and is much more forgiving in terms of how much you can see through it at once than the other 8x scopes, but there are times when it still feels "tight" as noted above. This brings us to the K18i-2.
The K18i-2 has a linear FOV and area FOV up around the K16i / Razor G2, and it shows. The scope is just much, much faster at 8x, sufficiently that it points and shoots like a 6x. It breaks the traditional "over 6x needs to be FFP" paradigm because the FOV is so wide.
On the K18i, the turret feels almost like a crutch for overcoming the scope being SFP. It's still a great scope, but it feels like if it had been FFP it would be a clear favorite. On the 18i-2, the turret is basically dialing fine corrections on very small targets, and isn't about shooting fast.
Hopefully that helps to understand the "high mag" end of the spectrum. Low mag is a completely different animal. Basically, the a wide FOV at 1x paired with a bright dot in the middle shoots like a red dot, which is good. The wider the field of view, the more red dot-like it becomes. There's no feeling like you wish you had more or less mag, just how much is the scope "in the way".
Scope..................................................Linear FOV
K16i 1-6x24......................................126.9 ft
Razor Gen II 1-6x24......................115.2 ft
Nightforce ATACR 1-8x24..........96.1 ft
Nightforce NX8 1-8x24...............106.0 ft
S&B ShortDot 1-8x24..................105.9 ft
Kahles K18i 1-8x24.......................127.5 ft
Kahles K18i-2 1-8x24...................150 ft
Razor Gen III 1-10x24..................116 ft
One of the things that you hear about the NX8 is that it has a tight eyebox, and this is relieved quite a bit with the ATACR. I have never heard an eyebox complaint on any of the other scopes at 1x.
For every scope on this list, you can think of them like a red dot with a housing around it that obscures more or less of the view in front of you when you're on the rifle. You can think of the ATACR like a red dot with a small aperture and a big housing, with the housing getting smaller and the aperture getting bigger until you get to the K18i (gen 1) which feels really wide and open.
The K18i-2 is different from every other scope on this list at 1x, because the field of view is so wide at 1x that you see around the housing of the scope.
You can see through these (lighting is not great and you can see a little scope shadow in the 18i, these aren't pro pictures, was just getting the FOV) how much wider the 18i-2 is, and the bottles in the globe bar give you an idea of how close that wall is. (The 16i and 18i have the same view at 1x)
K18i
K18i-2
As far as the functionality of the image, the 18i-2 is wildly superior.
As far as length, the 16i (10.9"), 18i (11.8"), and 18i-2 (11.8") are pretty similar overall. The ocular on the 16i and 18i is basically the same at 46mm, the 18i is a bigger ocular at 52mm. The 18i-2 is a 34 mm tube while the others are 30mm tubes, and the weights are 16.9, 19.6, and 22.5 oz.
The weight on the 18i-2 is in the bigger ocular bell, so I haven't noticed it.
If I was taking my current knowledge into the market freshly, I would pick the 18i-2 over the 18i 100 times out of 100. The 16i is a great scope for applications where being a little lighter is appreciated and 6x zoom is adequate. Hopefully that helps!
