can some one explain this please

[Dan_Br]

Using a simple table in ChairGun I see the POI at 10 yds is ( -1.18 inch ) and the mildot is (-1.82) .

But why is at 50yds,that with a POI of (-1.06 inch) the mil dot says (-0.33)

why such a difference if the POI is about the same.

Ballistics Table

Range POI POI
(Yard) (In) (dot)

10 -1.18 -1.82
15 -0.69 -0.71
20 -0.33 -0.25
25 -0.10 -0.06
30 0.00 0.00

35 -0.04 -0.02
40 -0.23 -0.09
45 -0.57 -0.19
50 -1.06 -0.33
55 -1.71 -0.48

60 -2.54 -0.65

Here is the graph

 

[starlingassn]

looks like the second part of your post got messed up.



what are you asking?

 

[Hunt4Dinner]

If I understand your question right the -1.18 at 10yds is because of your scope to bore height since you have it zeroed at 30yds. If you had it zeroed at 10yds then it would be just that, 0. I'm assuming if you measured from the center of your optic to the center of your barrel you'd have about 1 1/2" between them. The -1.06 at 50 yds is because of the fall of the bullet caused by gravity. When you use an optic to aim you're actually adjusting the rifle at an upward angle to compensate for drop even though when looking through the optic it appears the you re looking directly at it, which your eyes are. Even though you're looking directly at it the scope or sights are situated to make the end of the barrel shoot uphill so the bullet falls at your optical sight at the given range you want. Remember projectiles start to fall as soon as the leave the barrel, we just shoot them upwards so they fall where we want them to. hopefully that's your question and hopefully this helps.

 

[Centercut]

Because the mil dots for the same distance POI vary with distance. So ONE mil dot is 10cm at 100 meters, or about 3.6 inches at 100 yards. So its half that at 50 yards, or 1.8 inches, and half again at 25 yards or 0.9 inches. Remember that with an SFP scope, the mil dots are only accurate (or calibrated at the design calibration magnification) at one "X" setting. For most scopes its the highest mag that the scope goes to, but some like Hawke have their cal. mags at 10X or 20X depending on the scope.

 

[Dan_Br]

thanks, confused but are you saying that even though the POI at 10yds and 50yds, both are about 1" low, the hold over (mil dot) is different?

dan

 

[BigTinBoat]

thanks, confused but are you saying that even though the POI at 10yds and 50yds, both are about 1" low, the hold over (mil dot) is different?

dan

Unless that's the distance you sight in for, the answer is yes. Reason is just as CC explained. At 10 yards the spacing between the mildots is .36" at 50 yds the spacing is 1.8".

Try it this way. Put the cross hairs on target at 10yds and shoot. See where your POI is on the reticle. Then shoot at 50yds while aiming the cross hairs at the target. Bet you the POI is higher up on your reticle.

Now - if you sight it in at 10yds then you will also be able to use the crosshairs to shoot dead on at 50.

 

[Guest]

For FT we zero at 25-27 yards and then we never have to hold low. For example on my TX200 zeroed at 25 yards (the range where I shoot has 25, 50 & 100 target stands) I hold dead on at 25 but at 10 yards I am holding 3.2 mil-dots up. This is with the scope set at 14X and I think the Sidewinder’s mil-dots are calibrated at 10X. If I lower the power of the scope to 7X I hold 1.6 dots up. My scope is centered 1.5” above the bore. When I first started using Chairgun I used the default distance of 35 yards and then as the OP is doing, I had to hold low for the close ranges. I was informed to zero at 25-27 yards, then I would never have to hold low.

 

[LDP]

Get a friend, wife or girlfriend to help you do this visual training aid. 

You hold a ruler in one hand at arms length. Take your other hand and open your fingers like you are bracketing 1” on the ruler.

Have your helper stand any distance 10 yds or farther away from you holding a yard stick or tape measure opened to at least 3 feet.

With your helper standing 20 yds away with a yard stick or tape measure look thru your fingers that are held 1” apart horizontally at the yardstick being held by your friend.

you will see the gap in your fingers cover a distance of 1” at arms length and a much greater distance at 20 yds. 

Thats how mil dots work in your scope.

 

[Scotchmo]

 

A milliradian is an angular measurement. An inch is a linear measurement.

 

[LDP]

A milliradian is an angular measurement. An inch is a linear measurement.

This is true. For people who dont understand reading about it a visual aid usually helps them to see whats going on. Since your vision is angular you get the same effect when you use my demonstration in my previous post. 

 

[Dan_Br]

thanks, confused but are you saying that even though the POI at 10yds and 50yds, both are about 1" low, the hold over (mil dot) is different?

dan

Unless that's the distance you sight in for, the answer is yes. Reason is just as CC explained. At 10 yards the spacing between the mildots is .36" at 50 yds the spacing is 1.8".

Try it this way. Put the cross hairs on target at 10yds and shoot. See where your POI is on the reticle. Then shoot at 50yds while aiming the cross hairs at the target. Bet you the POI is higher up on your reticle.

Now - if you sight it in at 10yds then you will also be able to use the crosshairs to shoot dead on at 50.

Thanks to all who explained this

Final question... I have 2 rifles one has a sfp scope and the other a ffp scope. Does the explanation change or is it the same for both types of optics?

dan

 

[LDP]

FFP stands for first focal plane. The distance between the reticle subtensions are always true on a ffp scope at every power. So 1 mil will always be one mil from the lowest power to the highest. 

SFP stands for second focal plane. The distance between the reticle subtensions are only true at one power. So if the scope has true mil spacing at 10x you need to be on 10x to range or have correct hold over. If you change to 5x the mil spacing is doubled. Instead of each mil dot or subtension measuring one mil it will be 2 mil. So on 5x for one mil of drop you would hold over half a mil. If you use 20x one mil dot or subtension spacing will measure half a mil instead of one mil. So on 20x for one mil of drop you would hold on the second mil. 

 

[BigTinBoat]

Thanks to all who explained this

Final question... I have 2 rifles one has a sfp scope and the other a ffp scope. Does the explanation change or is it the same for both types of optics?

dan

Yes they are the same in reference to the spacing being different at different yardages. The difference being that with a FFP scope you use the same aiming point no matter what power the scope is set on. Example - With a FFP you might use the 1st mildot at 50yds on 6x and also on 24x. With a SFP scope you might use the 1st mildot at 50yds on 6x, but you might use the 3rd mildot on 24x. A app known as STRELOK+ will help you with this on your phone.

 

[Scotchmo]

Thanks to all who explained this

Final question... I have 2 rifles one has a sfp scope and the other a ffp scope. Does the explanation change or is it the same for both types of optics?

dan

"POI (dot)" is measured at the scope. "POI (inch)" is measured at the target.

That concept is the same for both FFP and SFP, but there are some differences.

In ChairGun, the SFP reticle assumption means that when you change the value in the "Scope Mag" box, the "POI (dot)" values will change, but the "POI (inch)" values will not.

That's because on an SFP scope, changing the magnification changes the angle between the dots in the scope. But it does not change the actual trajectory.

My versions of ChairGun do not have an FFP setting, so when using FFP scopes, the "Calibration mag" is set to match the "Scope mag" (assuming correct angular spacing of the reticle stadia).

 

[LDP]

Dan_Br heres a link to a site that explains how mil dots work. You got some good answers here but you should probably read more in depth about how the system works if you want to fully understand your scope and how to use reticles with mil or moa sub tensions.

https://www.trijicon.com/na_en/support/how_to_use_mil_dot.php

 

[Scotchmo]

With a SFP scope you might use the 1st mildot at 50yds on 6x, but you might use the 3rd mildot on 24x. ...

That should not happen. With an SFP scope, the ratios will always be equal. 6x/1mil = 24x/4mil. (not 24x/3mils)

With a SFP scope, if you use the 1st mildot at 50yds on 6x, you will use the 4th mildot on 24x. That assumes that the magnification settings are correct.

 

[Dan_Br]

thanks again, I will review the link above and get Strelok for Christmas (prepare for more questions  

dan

 

[BigTinBoat]

With a SFP scope you might use the 1st mildot at 50yds on 6x, but you might use the 3rd mildot on 24x. ...

That should not happen. With an SFP scope, the ratios will always be equal. 6x/1mil = 24x/4mil. (not 24x/3mils)

With a SFP scope, if you use the 1st mildot at 50yds on 6x, you will use the 4th mildot on 24x. That assumes that the magnification settings are correct.

Exactly, if everything is PERFECT it shouldn't. How many scopes have "mil-dot" (type) reticles but aren't really "mil-dot"? How many have the 24x a true 24x? (Most seem to be in the 18x range). On a 6-24 who knows whether 12x (as marked) is actually 12x? This is why I said "might"? Guy was asking for an explanation and I was trying to simplify it best I could without actually trying to give EXACT numbers and such so that if he had a 6-24 and it really was 6-18 and he was using the 1st and 4th and it wasn't working (the 3rd would actually be the correct one to use in this situation) he didn't think it was him making an error. Just explaining the "concept" in simplest terms.

 

[LDP]

This is why I just threw the link up in my other post. Theres allot of variables and trying to explain just some of them while also not causing more confusion can be hard in a forum. I think he got a couple of his questions answered but he will benefit more by reading the rest. 

Dan strelok will help you in understanding how everything works after you read the rest of the info in the link. Read the info in the link and then play around in strelok and I bet you will understand how to use subtensions correctly. 

 

[Scotchmo]

Exactly, if everything is PERFECT it shouldn't. How many scopes have "mil-dot" (type) reticles but aren't really "mil-dot"? How many have the 24x a true 24x? (Most seem to be in the 18x range). On a 6-24 who knows whether 12x (as marked) is actually 12x? ...

That is especially true for the cheap generic scopes. I once bought a $40 6-24x50 that was really only about 15x at max magnification. On my three better scopes (>$200), the magnification is within 1% of markings. On my $100 UTG compacts, they are off by only about 3% to 5%.

I gave the correct theory and math, "That assumes that the magnification settings are correct." If the mil-dots spacing and magnification are misrepresented/mismarked, than nothing will be as expected. Best to always measure it for ourselves.