Scope magnification and point of impact

C

cdoc42

New member
First and Second plane reticles
I have a second plane scope reticle on all scopes; the reticle doesn’t change in size as you increase magnification.

What I didn’t realize is if you sight in at 100 yards with 9 power, then shoot at 200 yards with 12 power, the point of impact may not be the same had you shot 200 yards at 9 power. Now I may have this all wrong, so I’m open to corrections and suggestions.

Example: I have a Nikon M-308 800BDC scope on my 6.5 Creedmoor. It has circles and hash marks on the entire lower vertical reticle for holdover at various ranges. I went to the Nikon Ballistic website and entered data for a Sierra 140gr SPBT at 2610 fps.

If I sight in at 100 yards using 9 power, the first circle(hash mark)should put the bullet at 209 yards, which means that Point Of Impact should be high. If I use 12 power to shoot that first circle, the POI is 186 yds so the POI should be low.

Now if I sight in at 200 yards at 9 power, the first circle should put the bullet at 282 yards; if I shoot at 12 power, the reported POI is 262 yards.
In each case the point of impact is lower as you increase the magnification.
If I look at trajectory tables, I will be able to discern bullet drop at each yardage based on velocity. But it does not tell me what scope magnification will match that trajectory.

Nikon instructions relate that their ballistic tables and scope hash marks are based on maximum magnification. So it seems if you are going to use the scope hash marks for bullet drop at various ranges, you should sight in at max and shoot at max at all ranges. What’s the point of having a variable magnification?

With a second focal plane variable scope, what’s the best way to sight in and use hash marks for various ranges?
 
The ballistic trajectory of your cartridge does not change with a change in magnification. What is changing with a second focal plane scope as you change magnification is how much distance on the target is subtended by any given distance on the scope reticle.

With a second focal plane scope, as you increase magnification, the image of your target gets bigger and the scope reticle stays the same size. So any given distance on your reticle, say the distance from the crossing of the cross hairs to the center of that first hold over circle on the reticle, is going to subtend a lesser distance on your target than it does at lower magnification. So if you are using that distance to estimate projectile drop, as with a bullet drop compensation reticle, that distance will amount to a lesser bullet drop with increasing magnification, ergo that circle will correspond to the expected POI at a shorter range.

If you are centering the cross hairs on your target, the POI should not change any significant amount with a change in magnification at any given range with any given zero.

As to how to use a BDC reticle on a variable power, second focal plane scope, personally I don't. But I am not a hunter, and am using the scope for target shooting. I dope the scope for different known ranges and adjust the turrets, using only the cross hairs for POA. But I can see how the BDC reticles might be useful for hunters who are able to accurately estimate range and need to make quick adjustments in hold overs to hit at various ranges.

The BDC scopes are really only good for providing estimates for your POI. There are too many variables affecting muzzle velocity and air density to allow them to be exact. If you want to use the BDC built into the reticle, I would make up cheat sheets for several different magnifications based on the ranges you expect to shoot at. The POI that corresponds to the circles and hash marks will be different for every different magnification.

You could just leave the scope at maximum magnification, but that would afford you a very limited field of view at shorter ranges. I have a couple of Nikon scopes with BDC reticles. I haven't really tried working with them enough to make a valid judgement regarding their utility. But when I have tried to use the BDC feature, I find that there are just two many things to try to remember, and too many ways to screw up.

You need to make sure that you have adjusted the scope to the correct magnification, you need to remember the ranges that those circles and hash mark hold overs represent, and you have to remember to use the correct circle or hash mark for your range estimate. I also find that trying to hold aim on the center of a circle is less precise than holding aim on cross hairs.
 
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variables

The advantages of a variable power scope, with or without hold off/BDC aiming points, is what it always has been, one can increase or decrease the magnification level to suit the circumstances, typically less magnification up close and more as one increases range. Also, usually to some degree, lower magnification tends to allow viewing clarity when light is poor.

Regardless of what distances one zero's the rifle initially (let's say ON at 100yds) the rifle will be ON at 100, across the range of magnification. Only when using the hold off points will POA and POI change with magnification. The usual practice is to zero the rifle at one magnification setting, and use that same magnification setting if one intends to use the hold off points at longer distances. Sometimes a manufacturer recommends a power setting. On modest variables, say 3-9x, typically the top end (9x) is recommended.

Having just read a commentary on the M308 BDC 800, (4-16X) the author indicates that the BDC hold off's points are calibrated for max magnification (16X......?) and the 168 gr HPBT bullet at factory velocities. I have no idea if that internet commentary is correct. But....were I to have your scope, that is how I would use it, one load and max magnification at distance.

And I would shoot it a considerable degree to see just how close the POA/POI correspond.....
 
True!

It isn't unusual for point of impact to change some as magnification is changed. It doesn't matter if it is 2nd FP, or 1st FP. But the difference is rarely enough to matter with decent quality scopes. This is more of an issue with cheap blister pack scopes selling for under $100 though.

The way I deal with it is to always zero my scope at the most available magnification then confirm POI at the lowest magnification. Any error won't show up at shorter ranges where I'll be using that magnification. Most of my scopes are 3-9X, or 1-4X although I've used 2-7X in the past. I hunt almost exclusively on the lowest setting and have killed deer at 200 yards with 2X.

If a shot longer than 200 yards comes up then I'm more likely to have the time to change the magnification up, than move it down if a 20 yard shot presents its self. And I simply never fool with any of the other settings. I know it is perfectly zeroed at either 9X or 4X depending on the scope and at 200+ yards any magnification in between has no advantage. Plus with a 2nd FP scope the other aiming points are only calibrated on the greatest magnification.

Where 1st FP scopes come in useful, and POI changes are a problem, is with much more powerful scopes in the 5-20X or greater range. There are times where 5X isn't enough, and 20X is too much. And the other aiming points are calibrated at all ranges. If you're into long range target shooting and need a lot of magnification then 1st FP scopes come in handy. They aren't really any advantage for most hunters.
 
This, I THINK, might explain what I have experienced. Ballistics tables tell me with my given load, sighted in at 200 yards, I should be 1.5 inches high at 100. So I sight in at 100 yards at 9 power and set the scope to deliver a POI at 1.5 inches high. Now I should be dead on at 200. I shoot at 200 with 12 power because of my aged eyes. It is low because the real POI is 209 yards at 12 power.

So it seems to answer is to sight in at 12 power at 200 yards, then go back to 100 yrds at 9 power to see where the POI is. Then, when I hunt, I use 9 power under 200 yards, keeping in mind how low my crosshairs should be (but how many scopes have hash marks above the center of the reticle?) At 200 yards I use crosshairs. I then need to shoot at 300 yards at 12 power to see where the POI is, regardless of ballistic tables.It might be the first circle of the BDC or a tad below, near the next hash mark.

Sound reasonable?
 
The center of the crosshair should be in the same place regardless of power setting. The scale of the BDC will change with power, but the center should not move, at all.

It could also be that your ballistics tables are not accurate for your rifle load combination.

What I would do in your situation is sight it in at 200 yards at max power, and then shoot at 100, still at max power and see where it is hitting in real life, taking the change in scope magnification out of the equation.

This could be different from your table for a several reasons, sight height, altitude, temp, velocity doesn't match, BC is not accurate, etc.

Once that is established, then drop the power down to 9, and see if the POI changes. If it does, the scope is defective and should go back to the manufacturer.
 
I have two Nikon scopes with BDC reticles, although they are BDC 600 rather than BDC 800 reticles, and I am pretty familiar with the Nikon spot-on ballistic calculator.

I really can't make any sense out of some of what you said. First, from the things you have said, it sounds as if the maximum power of your scope is 12X. Nikon made a M-308 4-16x42 BDC 800 which has now been replaced by the M-Tactical 308 4-16x42. Obviously, both of those scopes have a maximum magnification of 16X.

Nikon also made a P-308 4-12x40 BDC 800 which has now been replaced by the P-Tactical 308 4-12x40 BDC 800. Both of those scopes have a maximum magnification of 12X.

Is your scope one of these models? If so, which?

Secondly, I am not sure that you have entered all of the pertinent data correctly into that app. If I select the P-308 4-12X40 BDC 800 scope (which is what I am guessing you actually have) and select the 6.5 Creedmore cartridge with Sierra Spitzer boat tail projectile (default ballistic coefficient 0.495), enter a muzzle velocity of 2610 fps, and a zero of 100 yards, I can only get the application to make the first holdover circle correspond to 186 yards at 12X and 209 yards at 9X if I accept the default sight height of 1.5 inches.

Did you accurately input the height of your sight axis (center line of scope tube) above bore? A sight height of 1.5" above bore is very low and would probably only apply to iron sights. For example, if you have an AR platform and a typical scope mount, the height above bore will probably be more like 2.65-2.75". An incorrect height above bore setting will make a big difference in your holdover ranges. You have to be careful with that Nikon ballistic app. Even if you entered the correct sight height, if you then change something, the app may default back to the 1.5" sight height without you noticing the change.

I agree with emcon5 that if you use the scope cross hairs for your point of aim at any given range, your point of impact should vary very little to not at all regardless of magnification, unless your scope is defective. Perhaps there could be some POI shift with a crappy scope, but Nikons are reasonably well-made. The ballistic drop compensation reticles will only give you approximate ranges for your hold overs. Even if you have chronographed your ammunition in your rifle, the muzzle velocity will vary with air density which is dependent on temperature, elevation, barometric pressure fluctuations due to weather, and humidity. The ballistic coefficients provided by the ballistic calculators are also not always entirely accurate. So take those numbers with a healthy grain of salt.

Ballistic drop compensation reticles are usually most helpful when the rifle is zeroed at or near the maximum ordinate of the projectile path. That way, compensation for any other range requires a holdover, no hold-unders are involved. Assuming a sight height above bore of 2.75" the maximum ordinate of your cartridge should be very close to 100 yards. If the sight height above bore is lower, the max ord will be a little closer in. So if you wanted to use the BDC feature of your reticle, I would zero at 100 yards.

Whenever you zero at any range, you want to use the cross hairs for your POA, not a hash mark or holdover dot or circle. And ideally you should zero at actual range. If that is not possible, I would zero at the farthest range you can closest to your intended zero point, and use a ballistic calculator to predict where the POI should be for your intended zero.

But whenever I have zeroed at a shorter range than my intended zero using a ballistic calculator, I expect and have found that some additional correction was required when confirming zero at actual distance. Unless you can chronograph your load at the given place on the given day, have a precise determination of your local density altitude, and have a close approximation to the actual ballistic coefficient of the projectile, or can calculate it, there are always going to be variables that cause the actual projectile path to deviate from that predicted by the ballistic calculator.

With a 200 yard zero I would expect your POI using the cross hairs to be in the vicinity of 1.5-1.6" high at 100 yards. You will have a near zero around 50 yards and your maximum ordinate will probably be around 125 yards, where your POI will be about 1.75 - 2" high. You should hit within about 3" of your POA out to around 250 yards using a "point blank" sight picture with the cross hairs. That might be a better approach than messing around with the holdovers on the BDC reticle for a hunter.
 
Put a collimator in the muzzle then adjust scope W and E to center/boresight.

Zoom power from limit to limit. Watch the semi-figure 8 loop the scope reticle makes around the collimator reference.

That's caused by the erector/zoom lens mount clearance tolerances to the guide tube pins moving them.
 
I'm going out shortly to shoot my .25-06 and .270 rifles. Both have Burris Ballistic Plex reticles. I'll share the details of my plan and results later.

PS: I tried to access The Firing Line yesterday but kept getting a block that said the site's security code was invalid. Did anyone else have that problem?
 
I have 2 Redfield Revenge, and 2 Vortex Crossfire II scopes with BDC.
All 4 scopes were zeroed at 200 yards on max power.
I've foung with 7mm-08, 280 Rem using 140gr bullets with BC of aprox 0.485 G1, that the hashmarks align closely at further distances. Aka out to 600 yards.
Same with my 257 Roberts shooting +P with 100 gr Ballistic Tips.
My 250 Savage needs to be set at 11 power to get ballistic drop to match.

My newer scopes are 2 MOA based hashmarks at max power. So i have to do some math when not at max power.

Lucid uses MOA based hashmarks. While having exposed turrets, they don't understand having the negative feedback on the quality of their turrets feel/sound, and in accuracy doing a box test.
They want you to zero it, then not touch the turrets again, relying on the reticle for any adjustments/hold overs.

I've been getting same message for several days.
 
I'm sorry for the drop-off on this thread; I never got to the range as planned, then family duties completely rearranged my schedule. First, let me thank everyone for their advice and comments. I apologize to pblanc for the apparent confusion in my description. I do have a Nikon M-308 4-16x42 BDC 800, but I was not using the 16 power, just the 9 and 12 in my post. In retrospect, I think I got stuck in mud when I researched this subject and tried to raise a question in the post.

Let me start over. With a Burris "Ballistic PLex" reticle and the Nikon M-308 I typically sight in at 200 yards with 12 power (due to age-related vision changes) then shoot at 100 with 9 powder to see how high the Point of Impact is. Then I shoot at 300 yards with 12 or anything that appears clear to 16) to see if the first or second hash mark will give me the desired POI.


For whatever reason, I retrieved the Nikon instructions that came with the M-308 and it suggested zeroing in at maximum power (16) and referred me to the APP that really screwed me up when I started playing with the entry data that resulted in my impression that I posted above: "Now if I sight in at 200 yards at 9 power, the first circle should put the bullet at 282 yards; if I shoot at 12 power, the reported POI is 262 yards. In each case the point of impact is lower as you increase the magnification."

I think what I was failing to understand was I could not use the APP results in conjunction with the technique I was using to sight in at various ranges. I'm really sorry if this was all a waste of everyone's time, but be assured I appreciate your input and I did, in fact, learn something.
 
In some scopes, cheaper ones, poi changes with changing power. It is not supposed to happen, but it commonly does. It doesn't have to be up and down. It can be right, left, or diagonal. Nothing to do with 1 vs 2 focal plane. It's the result of lens moving slightly crooked as power is changed. Pre Leupold built Redfields are notorious about it.
 
reynolds357 said:
In some scopes, cheaper ones, poi changes with changing power. It is not supposed to happen, but it commonly does. It doesn't have to be up and down. It can be right, left, or diagonal. Nothing to do with 1 vs 2 focal plane. It's the result of lens moving slightly crooked as power is changed. Pre Leupold built Redfields are notorious about it.
Click to expand...
I've seen this happen with several top brand made scopes.

The popular fix is to set power to one limit then leave it there. Or trade the variable in for a fixed power scope.

Put a collimator in your barrel then check your scope.
 
In a properly functioning variable power scope, changing the magnification will not shift the center reticle crosshair aiming point. The use of other aiming points on the reticle (such as BDC aiming points) becomes more complicated with second focal plane reticle scopes because the aiming points shift as the magnification is changed. First focal plane reticle scopes have no shift in reticle aiming points as the magnification is changed (essentially a requirement when using those reticles with numerous aiming points).
 
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