Holding for Elevation, Part II

Yesterday’s post examined using a mil reticle for holdovers.  What if you don’t have a super cool calibrated reticle?  That’s the question I find myself asking, because although I’d like a mil/mil scope on #1, I don’t.  I have a Leupold Vari-X III, 3.5-10×42, circa 2002.  It has a duplex reticle in the second focal plane.  More on focal plane later.


One way to holdover with a duplex would be to learn your trajectory in inches and estimate holding over the correct amount.  This could work for someone who can accurately estimate distance through their scope.  If you know about how big your target is as a reference then this may be doable.  This method is going to depend a lot on luck.


The duplex reticle is not made to use as a precise tool for holdovers.  That doesn’t mean that it can’t be done.  This is where we put the “art” into the rifle shooting.  The duplex reticle has an intersecting pair of thin lines that turn into thicker lines just outside the center of the sight picture.  These distinct features of the reticle give us a measuring stick we can use for holds.  The first task is to determine the amount that our stick measures.


To get a good read on the reticle, we’re actually going to have to measure the distance that it spreads over a known distance.  This can’t be just close, we have to physically tape off the yardage to the target first.  Don’t trust a marker at the range; tape it off at exactly 100 yards (or meters if you’re stuck on the metric system).  Next, you’ll need a target that will allow you to know how big your reticle measures.  What units to use?  You could just go in inches, or you could use what your knob is calibrated in, which I think would make a lot of sense.  You don’t want two different types of data competing for your brain power.  Also if you used inches, the measurement is only valid at the distance you did your measuring at.  MOA or mils, being angular measurements, mean what they mean regardless of distance.


I checked my Leupold Duplex reticle at 100 yards at 3.5 and 10 power magnifications.  I measured the total distance from the bottom of the upper thick stadia line to the top of the lower thick stadia line.  At 10x, the distance was 5.375”.  At 3.5x, the distance was 16.75”.  In MOA this comes out to 5.13 and 16 respectively.  Remember that inches per 100 yards don’t exactly equal minutes of angle that your ballistic software will give you.  MOA equals 1.047, etcetera, etcetera, decimal points ad nauseum… per 100 yards.  Small difference, but why not get it right?  This means that from the intersection of the crosshairs to any thick stadia line, there is 2.57 and 8 MOA distance respectively.


A couple things about the scope…  It says it’s a 3.5-10.  I suspect some rounding of numbers was involved in that labeling decision.  Scopes usually have a round number as the divisor of the magnification range.  For example, 3 (2-6x, 3-9x, 4-12x), 4 (2-8x, 4-16x, 5-20x), 5 (3-18x, 5-25x).  I would guess that my scope is actually something on the order of a 3.333-9.999x.  The other thing is the focal plane, which in this case is the second focal plane.


Second focal plane scopes have been the norm in the U.S. since forever.  American hunters like them because the reticle always appears the same size to the eye, and the reticle’s dimensions are always at (or at least appear to be) the “optimum” size as far as line width.  Because the size of the reticle always appears the same to the shooter, regardless of the zoom, this means that the distance that it subtends changes with the magnification.  


First focal plane scopes have reticles that always subtend the same amount.  This means that if you have, for example, a mildot reticle, a mil equals 1 mils at 10x and at 5x, and for any power the scope can dial, for that matter.  If you watch the reticle while you zoom the scope, you’ll see its size change in accordance with the image size.  The disadvantage is that the reticle can appear very thin and small at low magnification, and quite thick at the maximum magnification setting.  Manufacturers are getting pretty good at designing first focal plane reticles that are effective and usable throughout their entire range, but it’s not something you can take for granted until you try it. 


What I did was plugged my load data into the Berger Bullets Ballistics Program and looked at the data.  I ran out of the 185 grain bullets I’d been using, so I’m in the process of developing a load for the 155 grain Nosler Custom Competition bullet that I had a bunch of lying around.  I wasn’t completely done with load development, but I know that I’m going to end up with a muzzle velocity somewhere around the mid 2900’s.  I plugged in the data as 155 @ 2960 with a G7 BC of .213.  I tweaked the zero range to play with the options for using the reticle.


For my rifle and load, the only zero distance that I would not have to worry about holding under would be the 100 yard zero.  The bullet is actually a tenth of an inch high at 80 yards, but in practical terms that’s insignificant.  The disadvantage of a 100 yard zero is that the bullet is dropping a LOT after about 400 yards.  Also an 100 yard zero for a 30-06 doesn’t take advantage of its trajectory.


What I figured out is that, because the thick to thin line transition is 2.57 MOA at 10x, I might be able to take advantage of this by setting my 100 yard zero at the top thick/thin intersection at that magnification setting.  This puts me at a 253 yard zero with the crosshairs, which is quite good for a general 30-06 zero. 

What about when I’m using the lowest magnification setting?  In that case I would be significantly low if I used the same top thick/thin intersection.  If I just used the crosshair, I would be within a point blank range trajectory range of about 310 yards for an 8” target, which is not too bad if I had to take a quick shot at something.


Still using the 10x setting, the lower thick/thin intersection would correlate to a 360 yard zero.  If I want to stretch out the distance at which I have a definite point of aim at a particular distance, I have to dial my magnification back down, which is counterintuitive and unfortunate, but not completely unreasonable considering what one can actually accomplish with a magnification of 3 or 4.  At 3.5 (or whatever it actually is), the bottom thick/thin intersection is equal to about a 545 yards zero.  I could actually make a small quick reference card for the data book or to attach to the rifle with pictures of the zeros at various range.

I’ve been thinking about making a target with 2 horizontal lines 11.06” apart.  If I was exactly 100 yards from the target, and adjusted the zoom until the top and bottom thick/thin intersections bracketed the times, this would be the exact middle of my scopes adjustment range.  This would give me a third, “medium” option.  If it gets too busy to remember, I can at least write it down for reference.


Let’s say you wanted to know an exact point to aim for a specific distance, say 400 yards.  Check your trajectory table for your drop, in MOA, from your zero to 400 yards.  Mine’s estimated at 4.53.  We can figure this from the 100 yard line.  First we need to convert the MOA number to inches at 100 yards, which is close, but not precisely the same: 4.53 x 1.047 = 4.74.  I just made all the math nerds mad by being inconsistent with my decimal places.  Make a target with horizontal lines that distance apart.  Place the target at 100 yards and put the crosshairs on the top line.  Adjust your zoom until the bottom thick/thin intersection lines up with your bottom line.  Mark your zoom adjustment so you’ll have a reference.  There you go.  


So what I ended up with can be thought of as an enhanced point blank zero.  If nothing else, I still have a workable point blank zero if I forget my holdovers and holdunders.  But in addition, I now have reference points to enhance my precision and extend my usable range to about as far as I should consider firing on game.  This is good.


What’s not so good is that I’m only making use of about half of my cartridge’s maximum effective range.  A second potential problem is remembering to hold over/under.  About the first 21 rounds I fired with this type of zero were a little off because I forgot to hold under.  With a little time and dry fire, I’m starting to think of the reticle as more of a “continuum”.

I have only guesses as to how to compensate for the part of the cartridges trajectory that really demands the most precision (farther than 600 yards).  There’s a lot of fun and challenge in stretching our cartridge’s legs a bit.  If only from a training standpoint, long range shooting has the potential for a lot of benefit.  I’m still planning for a mil/mil scope sometime in the future.  Stay tuned.

One thought on “Holding for Elevation, Part II

  1. i have been doing this very thing for a long time with my straight 4x on my 22, but you have brought on a new light with dialing up and down in magnification which I wondered if that would work. Additionally you have put it all into words which are easily communicated to us, thanks for a job well done. I have already put some of this into action on my deer rifle and plan on refining it until next season rolls around. Carry always, shoot often, Steve

Leave a Reply

Your email address will not be published. Required fields are marked *