Need info for understanding ranges and magnifications on a SFP scope

[Stone111]

I recently bought a Kahles K18i V1 1x8 with the 3GR reticle and have done a lot of searching and reading on SFP scopes but still have a small handful of questions. FWIW its going on a 12.5” 5.56 SBR and I will be zeroing it at 50 yards since for what I will be using it for, that works best for me, banging steel on my carbine course, whacking beavers and eventually deer hunting. My go to zeroing, SD and hunting round will be Speer 75g gold dots.

This is my first real scope besides a 3x9 that’s on a 10/22 so I am trying to figure it out as I go along, go easy on me... Most of the videos and articles and threads I have read seem to go on and on about the differences between FFP and SFP, kind of beat to death but never really get into the specifics needed in understanding hands on use of SFP scopes. Kahles says that the sub tensions are 1 Mil at 8X, and from what I have read it would be best to zero the scope at 8x, correct me if I am wrong. What I am not sure of is, are the sub tensions still 1 Mil at different ranges? I thought I read that at 100 yards its 1 Mil at 8X which I believe is one centimeter, but is it still 1 Mil at other distances like 25/50/200 on 8X? I do understand holdovers come into play at certain distances but that’s not what I am referring to.

I also partially understand that the sub tensions WILL change when you change magnifications (or better said the distance or value between them change) so for simplicity and doing the math on the fly in my head until it comes natural, I will stick to either 1X, 4X or 8X.
I bought the scope used and its basically brand new but he had it mounted to a rifle so I decided to reset it to its mechanical zero and then bore sighted it today to get it real close to my initial zero and will be headed to the range tomorrow.

The second thing I don’t understand is what do the sub tensions change to at different magnifications. What happens at 4X and 1X? Let’s go with 4X, is it half of what it is at 8X? half a Mil? I hope all that makes sense... TIA! Oh, and please don’t say “you should have bought a FFP scope” I like a challenge, I am just having a hard time finding clarity for my quest...

 

[oldrifleman]

The subtendions will appear smaller at low magnification, but they are the same size relative to everything you see through the scope( dog is bigger at 8x than at 1x, same for subtentions). The purpose is that by keeping everything in proper perspective it is easier to range the target.

 

[dirtybores]

I'll explain it to you. But my brain can't handle that much typing. PM me and we'll get on the phone.

 

[Trigger Monkey]

I recently bought a Kahles K18i V1 1x8 with the 3GR reticle and have done a lot of searching and reading on SFP scopes but still have a small handful of questions. FWIW its going on a 12.5” 5.56 SBR and I will be zeroing it at 50 yards since for what I will be using it for, that works best for me, banging steel on my carbine course, whacking beavers and eventually deer hunting. My go to zeroing, SD and hunting round will be Speer 75g gold dots.

This is my first real scope besides a 3x9 that’s on a 10/22 so I am trying to figure it out as I go along, go easy on me... Most of the videos and articles and threads I have read seem to go on and on about the differences between FFP and SFP, kind of beat to death but never really get into the specifics needed in understanding hands on use of SFP scopes. Kahles says that the sub tensions are 1 Mil at 8X, and from what I have read it would be best to zero the scope at 8x, correct me if I am wrong. What I am not sure of is, are the sub tensions still 1 Mil at different ranges? I thought I read that at 100 yards its 1 Mil at 8X which I believe is one centimeter, but is it still 1 Mil at other distances like 25/50/200 on 8X? I do understand holdovers come into play at certain distances but that’s not what I am referring to.

I also partially understand that the sub tensions WILL change when you change magnifications (or better said the distance or value between them change) so for simplicity and doing the math on the fly in my head until it comes natural, I will stick to either 1X, 4X or 8X.
I bought the scope used and its basically brand new but he had it mounted to a rifle so I decided to reset it to its mechanical zero and then bore sighted it today to get it real close to my initial zero and will be headed to the range tomorrow.

The second thing I don’t understand is what do the sub tensions change to at different magnifications. What happens at 4X and 1X? Let’s go with 4X, is it half of what it is at 8X? half a Mil? I hope all that makes sense... TIA! Oh, and please don’t say “you should have bought a FFP scope” I like a challenge, I am just having a hard time finding clarity for my quest...

Typically, SFP mil based reticles will equal 1 mil between the hash marks or dots at the highest mag and it will not change based on the range. If you're looking at a target at 100 yards it's 1 mil from the crosshair to the hashmark, if you're looking at 200 yards, it's still 1 mil. What that 1 mil equals on target will change based on the range, for example 1 mil equals 3.6" on the target at 100 yards, 7.2" at 200 yards, at 50 yards 1 mil equals 1.8" on the target. What Khales is saying is that all that is true only on 8X.

If you change the magnification, going to 4X for example you do change the subtension value because the distance from the cross hair to the hash marks is no longer 1 mil but 2 mils. The image got smaller but the reticle stayed the same size relative to the shooter's eye so the distance between the hashmarks grew. So if you were at 4X and looking at a target at 100 yards, the distance on the target from the crosshair to the first hash mark would be 7.2" / 2 mils.

 

[Old Man with Gun]

For LPVO on an SBR it doesn't matter much. Your center aiming point will be the same at 50yds regardless of magnification. The hash marks below are only good for holding over to shoot at distance or estimating range with some very confusing math.
At 1x your next hash mark is impact beyond any practical range of that setup. You will likely never shoot past 100yds and not crank up the magnification. Even at 8x your next hash mark is 250-300 yards away depending on ammo choice. This is for a 60g V-max at 2,750.

 

[Old Man with Gun]

50 yard zero is a good choice, you will be within a couple of inches of point of aim out to 200 yds, although I would zero at 200 to trust that statement.

 

[Tictacticaltimmy]

If you really want to be able to visualize this and play around with holds at different magnifications, get the Strelok Pro app. Its the app that is screenshotted above.

I would highly recommend only using the reticle on 8x. That is the practical limitation of an SFP scope. US snipers used to memorize holds at different magnifications, but that was their damn job so they were ok practicing hours upon hours to make sure they never got that wrong. Just ain't worth it with the scope options we have today.

For visualizing Mils, just remember 1mil= 1/1000 distance to target. Target at 100 meters? Divide that by 1000, its .1meter or 10cm is 1mil. Then each click is .1 of that, 1cm.

PS I think you'll find that a 100Y/M zero is best if you are actually going to be using the reticle. 50Y/M is great for red dots when you don't have a reticle for holds.

 

[Ikeman83]

1. Spend the money and get this exposed dialing turret for the scope: https://www.eurooptic.com/Kahles-K18i-2-K18i-K16i-Ballistic-Turret-30210.aspx
2. Learn your holds in mils. It's worth taking the time to chronograph your rifle and set it up properly in a ballistic calculator to have this information.
3. At 8x, every line is 1 mil from the next line, and 1 mil is 10 cm at 100m, or 3.6" at 100 yds. Don't worry about what a mil is too much. At any other power, every line is 8/(Current Power) mils from the next line. So at 4 power, every line is 8/4=2 Mils from the next line.
4. I index my power ring to be vertical at 4x so that I know that if my ring is stopped right I'm at 1, left I'm at 8, and vertical I'm at 4 without having to look. 4x is great for fast shooting on larger targets that are closer but you need more mag on.

All of the above applies to the K18i and K18i-2

 

[Stone111]

Thanks guys! A lot of great intel, it will take a little while to digest...

 

[Stone111]

I really wrestled between the 100Y and 50Y zero, but on my land which is pretty heavily wooded most shots will be betwwen 75Y and 25Y with the majority right at 50Y. Quick hypothetical question for you guys: I am zeroed at 50Y, I take a shot to 50Y on 8x (center dot, center hash marks as POA) and it hits dead center. I now pull the target back to 25Y and drop to 4x, but for understanding purposes I still use the center dot/center hash marks as my POA at the same point I did at 50Y so no hold, where will the bullet impact? If I am tracking correctly and since my sight over bore at the muzzle is 3" my shot should impact 1.5" over the bullseye which normally would be my holdover. Is the drop to 4x going to change the equation enough to realy have to factor it in?

 

[Trigger Monkey]

You'll get an impact about an inch or so low. Changing your magnification won't change the fact that your zero, on the center dot, is at 50 yards or the physics that a closer than zero distance shot will impact lower than the point of aim.

 

[Stone111]

Your right, brain fart... I meant to say 1.5" lower not higher since I didnt hold over. Thanks... What about the same equation but dropping the magnification to 1X? In all reality that will be the vast majority of my shooting situations...

 

[Ikeman83]

Your right, brain fart... I meant to say 1.5" lower not higher since I didnt hold over. Thanks... What about the same equation but dropping the magnification to 1X?

Magnification has absolutely zero impact on point of aim/point of impact relationships when you are shooting at your zero.

Your zero distance also has nothing to do with where your shots are, it should be purely determined by the capabilities of your scope to correct for distance, and the ballistic trajectory of your chosen bullet relative to your sight line.

The reason to use a 100 yd zero is that it is:
-unaffected by elevation
-has extremely minimal deflection between 25-150 yds
-ensures that you never need hold unders, only hold overs no matter how far you're shooting.

The reason 200 (or 300) yard zeros are used for hunting rifles with no ability to correct for distance, as well as aimpoints/eotechs/etc is that a 200 yd zero will have you within 3" on an AR from the muzzle to ~225-250 yds.

 

[Tictacticaltimmy]

The center is always the center. What is changing is the angular distance of all the other lines/marks on the reticle from the center.