Rifle Scopes ranged shooting, optical illusion, or just me ?

[john_1182]

hi, firstly im new here, so i hope i have posted this in the correct place, i have to me what is a tricky question so i will try to explain it the best i can. (i will give a quick story of what happened then ask the question)

(i am shooting a Tikka super varmint in .308 with a nikko stirling 4>16/50 (30mm tube) scope with LRX reticule)

i was with a mate 2 days ago zeroing and shooting at 100,200,400 and 660 yards and was shooting very accurate at 100, 200 and 400 yards and same again at 660 yards and were quite pleased with our selves, untill i come home and realised that the last shots at 660 yards were taken at 8X zoom (all other shots were at 16X) and my ballistics chart i made up told me to aim 20 MOA (134 inches) high and my scope has up to 20 MOA markings so i set the scope to ZERO and just used the MOA marking for elivation and were hitting the target shot after shot (12 inch target)

so my question is, shooting at 8X zoom at 20 MOA high would that give me the same point of impact as aiming as shooting at 16X using the 20 MOA mark in the scope? its driving me crazy trying to work this out in my head, because im working it out at 8X would have been actualy aiming 40 MOA high not the 20 MOA i wanted, but if that is true, that means my round was droping twice as much as my ballisitc chart tells me it should be have been even when 100,200 and 400 yards were spot on..



im working on .308 cal 165 grain, .404 BC, 2650fps, scope 1.75 high, at sea level aprox 60deg temp..

sorry for the heap of info, but i am very confused, as im at work and just cant go pict up the rifle and look through the scope and check it

 

[hunter223]

Re: ranged shooting, optical illusion, or just me ?

no, 20moa of adjustment on the turrets is going to be the same. however, if you used a 20moa drop point on your crosshair, yes the point of aim would be different on 8x and 16x. hope that helps

 

[john_1182]

Re: ranged shooting, optical illusion, or just me ?

yeah, that is what i thought, so now that means either my charts or out or my scope or somthing is shifting or somthing,

thanks

 

[LoneWolfUSMC]

Re: ranged shooting, optical illusion, or just me ?

If your scope has an SFP reticle (which I believe it is) and you were using the 20 MOA reticle mark at 8x you would have been holding twice as high as if you were using the same mark at 16x.

Let's assume that your 20 MOA was correct at 16x. That means that if you held at the 20 MOA mark at 8x on a 660 yard target you would be hitting approximately 138" high or completely off the target.

So if you even cut paper I would bet your scope wasn't set at what you thought it was OR your marks are not at the MOA you think they are.

 

[john_1182]

Re: ranged shooting, optical illusion, or just me ?

its the LRX reticle, im thinking when i get home i will have a good look at the scope and see if it has shifted, in the past ive had big isses with the scope sliding after 5>6 rounds in the rings, but i thought i had fixed that issue, but it would not suprise me if it had done it again

ether way im looking at better rings , what brand would you reconmend as good 30mm rings med height for a picitiny rail ?

also -20 MOA would be good to or do i need to get another rail for that ?

 

[noname]

Re: ranged shooting, optical illusion, or just me ?

in general the bullet will cross the line of sight on the way up and on the way down. in sfp scopes the impact point for the bullet on the way up is usually the same at any power, its on the way down the problem starts. (and one of the reasons ffp scopes are prefered by a lot of shooters). decreasing the power of the scope will lift the poi. this is the system used by leupold in the BAS system of holdover reticles, and zeiss in the Z series of reticles. It is not like ranging with an sfp, in that it change is linear and fractional, because the ballistic profile of the bullet is not linear thus long range shooting is usually done on one power, and not necessarily the highest as it may vary with the scope. Like has been said many times on this forum you have to check the scope with live fire against the settings to find out what is really happening.

 

[gugubica]

Re: ranged shooting, optical illusion, or just me ?

The SFP reticle that Lonewolf was refering to is not a type of reticle, but rather a Second Focal Plane scope.

Quick question: Did you dial elevation for the 100, 200 and 400 yard shots and then hold over for the 660 yard shots?

If you did, then the mag setting for the closer range shots would not have mattered.

If the above answer is yes, then I am think a couple of things could have been happening. Your scope (I am not familiar with it) may be set that the retitle is correct at 8x and not 16x. Check you literature or verify on the range with a yardstick. Or, you assumed that the distance between reticle hash marks is 1 MOA when it is actually 2, or vise versa.

If you held over for all shots then that is weird (unless it is a FFP scope).

Either way, error does not usually result in rounds on target and I am hesitant to say that something is wrong with your equipment. I would verify your optical specs and go try again before I got to convinced that something is broken.

By the way, check out Burris XTR rings. They are good and priced very competatively. You can always go with Seekins, Nightforce, Badger or TPS, but they are going to be more expencive.

 

[john_1182]

Re: ranged shooting, optical illusion, or just me ?

1: i dialed the elivation for 100, 200 and 400 then dialed it back to my zero (160 yards) and held over for the 660 yard shot. (20 MOA @ 16 X) (but i was at 8 X, i used 8X to scan for a target but forget to set the power back to 16X)

2: at 16X the markings on the scope are in MOA (i checked the web site) they actualy go in 2 MOA incriments and there is 10 markings, so thats 20 MOA

3: whats is FFP/ SFP ?

4: out of a11 rounds fired, i fired 3 (1 @ 400 yards and 2 @ 660, all 3 were hits) (we were shooting at small sand stone blocks) and my friend fired 8 rounds and was hitting most of the time, so if somethign was out/ wrong, we should have been missing, as said earlier i wouldn't be suprised if the scope slipled in the rings (its done it before after a few rounds) and that it was just a coinsiadance that it shifed to 20 MOA at 8 X

 

[gugubica]

Re: ranged shooting, optical illusion, or just me ?

OK, that make sence. Since you dialed, the mag does not affect anything. When you held over, it does. According to Nikkos website

http://www.nikkostirling.com/Content/PDF...e%203%20col.pdf

the reticle is correct at 10x. I think that you were perhaps closer to 10 than 8? It is also very possible that it is not calibrated exactly at 10x.

You should take a yardstick out with you next time and mount it vertically EXACTLY 100 yards away. You can use this to determine the exact setting on your magnification that subtends MOA (or actually IPHY as according to the link that is what your scope is set up in).

 

[gugubica]

Re: ranged shooting, optical illusion, or just me ?

Oh, just realized I did not answer your question.

FFP stands for Front focal plane. In a FFP scope, the reticle remains the same size as the target. When you zoom in, the reticle gets bigger at the same rate as the target.

SFP stands for Second Focal Plane. In a SFP scope, the reticle remains the same size and the target gets bigger. Your reticle marks and ranging are only correct on one setting in a SFP scope.

There is a ton of info on the pros and cons of each on here. Do a search and you will find a wealth of info. Bookmark this page, it works better than the built in search.

http://www.google.com/coop/cse?cx=010955838166721108978:qcbx5qqy10o&hl=en

Welcome to the hide, you will soon learn that this is the best place on the web for learning this craft.

eta: oh crap, sorry again, IPHY is Inches Per Hundred Yards.

 

[noname]

Re: ranged shooting, optical illusion, or just me ?

was not referring to a reticle, but specifically to second focal plane scopes, decreasing the mag does change point of impact and the greater the magnification range of the scope the greater the effect.

 

[john_1182]

Re: ranged shooting, optical illusion, or just me ?

that could explain everything... at 16X i was dialing, but at 8X (come to think of it to could have been closer to te 9X) it gave me an accurate shot using hold over... i just always asumed at 16X that all the measurments were 1 MOA but it turns out its 10X..

as for ranging the targets i have 2 laser rangfinders one in the scope of my .223, bushnell yardage pro 4>12X40 and waht ever the handheld bushnell 1500 yard one is..

tommorow i will go out and confirm that the MOA in the scope it correct at 10X not 16X

thanks gugubica for pointing this out, when i got home after work (2 hrs) i was going to rip the mounts off and re-mount the scope, but not any more)

 

[john_1182]

Re: ranged shooting, optical illusion, or just me ?

sorry for the double post but this first/ seccond focal plane stuff has got me baffaled

tell me if im wrong with all of this

the nikko sterling night eater lrx scope when i zoom in the immage is getting bigger ( so at 5X the target is taking up say 5 MOA but when i zoom to 10X it takes up 10 MOA, then to 15X 15 MOA in the scope) so dose this mean its seccond focal plane ?

so if that is correct that would mean if im looking at a target at 100 yards these folling caculations should be correct ?

at 5X mag 1 MOA @ the target will = .5 MOA in the scope
at 10X mag 1 MOA @ the target will = 1 MOA in the scope
at 15X mag 1 MOA @ the target will = 1.5 MOA in the scope

or am i just getting all of this wrong, my brain is hurting so much trying to work all of this out

 

[gugubica]

Re: ranged shooting, optical illusion, or just me ?

yup, you got it. That does indeed mean that you have a SFP scope.

When you verify you power setting, mark you tube at that setting so you can quickly dail it back to there.

Using the ranging formula and holdover should only be done at that setting. This is unless you modify the formula or account for the difference. i.e. if you range an object at 10x and determine it is 200 yards, if you are set on 5x, you have to adjust to equation.

The same is true for your dope. It will only be accurate (for holdovers) at that setting too. You stumbled onto this inadvertantly, but it is essential information, so its all good.

 

[john_1182]

Re: ranged shooting, optical illusion, or just me ?

im glad ive worked it out (with your help) this explaines where ive been going wrong for the last 3 years (my rounds were always falling short and greater range), now all is left is to tripple check my fps's with the chrony to check that the charts are correct...

now i cant wait for bunny be gone at 500 yards

 

[john_1182]

Re: ranged shooting, optical illusion, or just me ?

im stumpted again, im tring to modify my range cards, i know all the data i have is now accurate at 10X so when i shoot at 16X do i need to multiply the data by 1.6?

(im thinking half for 5X and double for 20X so 16X should be * by 1.6)

 

[noname]

Re: ranged shooting, optical illusion, or just me ?

the relationship is not linear it is log. ranging with sfp is different than hold over. ranging is proportional between the subtension and the ratio of the power used. Calibration of the hash marks to the correspond yardages is best done on something like Exbal ballistic program, or correlation with athe outputs from something like JBM under different velocity conditions and will not work if simple ratios are used as the the ratio depends on the characteristics of the ballistic profile of the load you are using.