By John Bibby
Yes, there is math. No, it isn’t difficult math.
If you are someone who routinely shoots past 200 yards with a rifle, it would be quite helpful to know how to build your own Ballistics/Dope Chart. By doing so, you can adjust your scope to have the crosshair line up with the point of impact at any given distance. In a pinch if you have a mil-dot reticle, you can use the mil-dot lines to adjust the aim. The first choice is more accurate, if done correctly. The second option is a lot quicker, but a much less precise method.
To build the chart, you need some tools and some information.
Tools:
Fairly Accurate Rifle and Scope
Fairly Consistent Ammo
A Computer and Internet Access
Targets at Known Distances
Chronograph (optional)
Information:
Bullet Weight
Bullet Velocity
Ballistic Coefficient of the Bullet
Desired Distance Ranges
The place to start is with information provided by the factory. This does not matter if you are shooting reloads or factory ammo. There will be some slight differences depending on which course you take there, but the factory information is always the starting point. Some factories even provide very solid ballistics calculators and they should be leaned on heavily.
An example:
I shoot a lot of .270 Win as hunting ammo. As such, I have a ballistic chart for that round when shot from my rifle, with handloads. In order to build this chart, I need to know the ballistic coefficient of the bullet, the velocity my rifle shoots the bullets at, the weight of the projectile and how consistent the velocity is.
Bullet – Hornady 140 gr SST #27352 – 140 grain weight, G1 BC .495
Powder – H414 – at just over 49 grains of powder, I have a strong accuracy node despite being far from maximum pressure or velocity. When I make precision loads my ES (Extreme Spread) is about 10 fps, confirmed with my Magneto chronograph. From my rifle, this translates to 2780-2790 fps. If you are using factory ammo, (and do not have a chronograph) they give their best guess on the side of the box. You will need to determine the length of barrel used in their test and calculate approximate gain or loss from your barrel length. When that rough number is obtained, a basic chart can be made to test versus known distance targets.
Pick your ballistics calculator of choice. I like both options from Hornady, found here. https://www.hornady.com/team-hornady/ballistic-calculators/#!/ note the 4DOF is designed for longer distance and to provide more advanced calculations with input from a Kestrel. Well beyond the scope of this article.
I also like the Berger Bullet calculator, found here. https://bergerbullets.com/ballistics-calculator/
Key information to input:
Maximum range – 500 yards. With this rifle, I won’t normally harvest deer past 500 yards.
Interval – I set this at 50 yards because it makes things more precise and I am only looking at a few more data points.
Ballistic coefficient – Factory G1 value of .495 – for longer distances G7 values tend to work better.
Velocity – as I know my ES is 10 or less, I split the difference with 2785 fps.
Bullet weight – factory value of 140 grains.
Zero – I zero my rifle at 200 yards. This choice provides no need to adjust on a deer out to 250 yards as my rise or drop is never more than 3.1” within than range.
Optic height – measure this yourself but 1.5” is standard unless you have a scope riser or a optic larger than 50 mm.
My chart
As mentioned above, I do not use the chart for anything shorter than 250 yards. Considering this, I abbreviate that portion to make a more compact chart. Past 250 yards each 50-yard mark is notated for easy extrapolation from known listed distances. I include inches of drop and Mils for several reasons. Inches are intuitive and if I need to use the mildot markings or Kentucky windage, I have the information. If I have the time, my scope is in Milrads and 1.5 Milrads is 15 clicks. The velocity chart is a reminder for expansion velocity. Hornady states minimum reliable expansion on this bullet is 2000 fps. This makes a 500 yard shot questionable for expansion; and thus, hunting.
The charts I use also incorporate wind drift, but that is a whole other topic. For this article, I focused solely on drop.
If you do not have a chronometer, the above, would be the initial stage of your chart. The next step would be to do live fire confirmation, from a solid rest. If your rifle is zeroed at 200, then your bullets should be 1.8” high on a target at 100 yards and 7.7 inches low at 300 yards. Shoot a 3- or 5-shot group. The group average being plus or minus ½” is easily written off as user bobble or inconsistency of bullet velocity. If your impacts differ significantly more than that, go back to the ballistics calculator. Fiddle with the velocity numbers to see if you can make the observed numbers work. If the factory velocity is listed as 2900 fps and your rifle has a shorter barrel, it might give 2750-2650 fps. With a 200-yard zero, this will be noticed as higher impacts at 100 yards and lower impacts at 300 yards. By dropping the velocity values by 50 fps a few times, the numbers should get closer to observed data. As the data gets closer, fine tune with smaller velocity adjustments, then reprint your chart based on what matches real world observations.
Congrats, you now have a custom drop/dope chart for your bullet/rifle combination.
