So it's a nice bright hot sunny day. You sit down for a long range session. You dial
in for wind and do everything the book tells you too. The first shot is a flyer. Ok
it's just the coffee I had on the way to the range. You look and make a quick
adjustment. The second shot is off by an inch or so. You dial down the correct
clicks in your scope and start shooting groups. As you look at your scope you start
to reverse engineer how you got to where you are, your more than puzzled. The more
you look at the numbers the more puzzled you get. The book says I should be using
this dope for this range and wind. I'm shooting great but no where near what the book
says. Is the book wrong? Is there something wrong with my scope? Chances are no on
both counts. Welcome to external ballistics. External ballistics deals mainly with
the other factors that affect a snipers shot. Heat, humidity, air pressure Ect.
You know, the other thing you blame your bad shot on.
PROPER ZERO OF YOUR RIFLE
When I go long range shooting I try and keep a few things in mind. I always try to
set up my range sessions like controlled scientific experiments. I try to maintain
as many controlled variables as I can. Rifle components, ammo, Scope, Rings,
shooting platforms ect. These are all factors that I have a sufficient amount of
control over and can manipulate at my will. This allows me to determine after a
sufficient amount of time behind a particular rifle how it will react to other
external variables out of my control. In effect, I'm getting to know my weapon and
building my self confidence in my abilities. The more I know, the better chances I
will have a making a first round hit. Why are these factors important? Because they
all affect our precision shooting and can mean the difference of a hit or a miss at
800 yards. To better understand how to adjust for them you must first realize what,
how, and why they affect your shot.
DRAG
Drag is known as the force that causes our projectiles to slow down while in flight.
Drag is calculated by 2 factors. AIR DENSITY which can also be thought of as the
stopping power of air, and BULLET EFFICIENCY. Which can be determined by a formula
known as a ballistic co-efficiency model. Each is broken down into its own individual
factors.
BULLET EFFICIENCY BC
Bullet efficiency is calculated using a mathematical formula known as the "Ballistic
Coefficient model" This is a complex mathematical model taking into account a
projectiles weight, cross sectional area, density, and shape. The result is a decimal
number of less than 1. The higher the BC (or closer to 1) a projectile is, the
flatter the trajectory, higher retained velocity, and better wind beating capabilities
of the projectile. In the end, greater accuracy. So what? The long and short of it is
that you want to shoot bullets with higher BC's. There will be more information and a
more detailed explanation in a different article.
AIR DENSITY
Air density can be divided into 4 main factors.
- Altitude
- Air pressure
- Temperature
- Humidity
All four factors go hand in hand in creating drag on your projectiles.
TEMPERATURE
Temperature affects air density in a simple way. The air that we breathe is made up of
several gases and water vapor. The laws of physics tell us:
- As the temperature increases the molecules in the air become greater in distance apart from each other.
- As the temperature decreases the molecules in the air to move closer together.
Think of it this way, Oil is more viscose ( runny) and easier to stir at hotter
temperature and denser and harder to stir at colder temperatures. The closer together
the molecules are in the air the more resistance a projectile will encounter between
the shooter and the target. Thus the more energy a bullet must use to stay it's
desired course. Heat also has a way of making a sniper feel uncomfortable. Whether in
a guillie suit or on top of a black tar roof a sniper must get comfortable in heat.
Drink plenty of fluids and try to keep your head cool with a cloth or a portable ice
pack. Heat exhaustion has claimed more than one shooter in our time. One rule of thumb
that has been taught (and its only a rule of thumb) is that for every 20 degree Fahrenheit
change from zero, your point of impact will be shifted 1 MOA. This is a rough rule of
thumb, but it gets you close at most ranges. So, if you zeroed at 60 degrees and were
shooting in 80 degree weather (all other factors being the same) the bullet will strike
1 MOA high. (hotter, less thick, travels faster). It works the opposite (lower by 1 MOA)
if you are shooting in COLDER weather than when you zeroed. There is a chart below that
provides another more accurate rule of thumb.
ALTITUDE
For this reference, altitude is a measurement of how far you the shooter are from sea
level. A rifle zeroed in Vancouver, British Columbia Canada {sea level} and then
transported Denver, Colorado USA {5000+ ft} will have a completely different zero point at 100 yards.
The reason for this is due to the relationship between gravity, and the earth.
As altitude increases the barometric pressure decreases, making the air thinner.
The closer you get to sea level the higher the barometric pressure increases.
The more barometric pressure is present the more drag on your projectile.
In short:
- The higher up in altitude you go = Less pressure
- The lower down or closer to sea level you go= Greater pressure
The speed of sound varies slightly at different elevations. This should only be a
consideration when taking long shots as your bullets dips in and out of the sound
barrier and transonic layer.
Air pressure is a contributing factor to the density of air as altitude changes.
More air pressure means more density exists surrounding the shooter. Thus the more
energy your bullet needs to expend to stay on course. At higher altitudes bullets
will strike higher due to their ability to maintain a higher velocity for a longer
period of time. At lower altitudes your bullet will strike lower. Here is an exact table
showing the impact of Air Pressure based on altitude. The table assumes the rifle has
been zeroed at sea level, and all MOA adjustments are needed to LOWER the impact (bullet
strikes higher the higher up you go)
Impact Air Pressure Has on a Projectile, GMM .308 175gr at 2600fps
| Range Yards | 1500ft MOA |
5000ft MOA | 10000ft MOA |
| 100 | .05 | .08 | .13 |
| 200 | .10 | .20 | .34 |
| 300 | .20 | .40 | .60 |
| 400 | .40 | .50 | .90 |
| 500 | .50 | .90 | 1.4 |
| 600 | .60 | 1.0 | 1.8 |
| 700 | 1.0 | 1.6 | 2.4 |
| 800 | 1.3 | 1.9 | 3.3 |
| 900 | 1.6 | 2.8 | 4.8 |
| 1,000 | 1.8 | 3.7 | 6.0 |
HUMIDITY
Humidity or relative humidity as it affects our projectile is the amount of moisture
or water vapor in the air. It is read as a percentile of 0-100%.
0% humidity means that the surrounding air is dry and unsaturated. 100% humidity
means that the air around us is heavily saturated with water vapor. How dose
humidity affect your shot? Simply the more moisture in the air, The more drag
on your bullets. The less moisture in the air the less drag on your bullets.
Do not confuse the previous statement as saying that more moisture in the air
means heavier air and thus more drag on our bullets. This is simply not true.
All in all these four factors only come into play at some point in long range
marksmanship. When taking shots within 200 yards with a high powered rifle these
factors barely take affect and are almost unnoticeable. At shots past 800 yards
these effects are more noticeable and might require some adjustment. So pack your
rifle and hit the range, the only way to see this for yourself is to do it.
Deviations From Zero for Temperature - Federal Gold Medal Match 175gr
| CHANGE FROM ZERO |
300M | 600M | 1000m |
| +30 Degrees or more | -2MOA | -3MOA | -4MOA |
+20 Degrees | -1MOA | -1.5 MOA | -2MOA |
-20 Degree2 | +1MOA | +1.5 MOA | +2MOA |
-30 Degrees or more | +2 MOA | +3 MOA | +4MOA |
The above table will give you a guided reference to show you how much of an
affect temp and humidity can have on your shots. This table has been in my data
book for years and has proven itself time and time again.
OTHER FACTORS
THE TRANS SONIC FACTOR
As a projectile in flight travel's from super sonic to sub sonic speed it encounters
some wobble before it settles back onto its path, it is hypothesized that the bullet
encounters some turbulence as it breaks from the sound barrier and settles into sub
sonic flight. One theory for this is that a super sonic projectiles sound waves catch
up with it as it slows to sub sonic speed some where around the 1150fps and thus
causing some turbulence until the bullet reaches around 1075 fps. At this point the
bullet settles back into it's subsonic path. Some have called the area in which the
transition takes place the "Trans-Sonic Region". The trans sonic region is not a
defined constant. It changes with variations in altitude and air pressure, so be
aware of this if you travel to different locations. Most experts say the trans sonic
region starts as high as 1600fps (depending on conditions) and goes down to the
subsonic velocities. To combat this shoot bullets with high ballistic coefficients
and try to keep your bullets above this transonic layer and super sonic for as long
as possible. There has also
been studies by Sierra and other companies that indicate the boat tail design can
help battle this wobble during transition. The classic example is the sierra 30 cal
168gr match king. This bullet was designed for 300 meter UIT competition and has a
steep 11 degree boat tail taper and is notorious for suffering greatly from the
transonic layer. Sierra discovered that by reducing the degree of the boat tail,
it helps keep the airflow attached during this transition, and has since incorporated
a 9 degree taper in their 175gr match king which is notoriously very accurate at
all ranges. To more fully understand this factor you need to shoot at these ranges
and see how your bullets are affected. Knowing and tracking your muzzle velocity at
different points in trajectory will help you in decrypting the mystery. The best way
overcome this obstacle is to actually shoot at long ranges.
MUZZLE VELOCITY
Muzzle velocity is the speed in which a bullet leaves the barrel of a rifle. Usually
measured with a chronograph, knowing your MV can be one of the most important factors.
MV is significant because it directly affects Time of flight, ballistic coefficients,
accuracy, remaining velocity, and terminal ballistic result. When determining what
the caliber your next rifle is going to be we all ask ourselves { or at least most of
us } what am I going to be using this new rifle for. Is it hunting sheep in the
mountains, or is it winning the next open shoot at camp Perry. When determining
cartridge selection muzzle velocity is at the top of my list. I ask myself what
will I be using my new rifle for. After I determine my needs I think of what type
of bullet will fulfill my needs. The laws of physics don't change for anything.
{no matter what your spouse says}. You cannot expect a 55gr 223 to hit the bulls
eye at 1000 meters. It's just not going to happen. Muzzle velocity and caliber
selection can be a science in itself. More on this later in another topic.
A NOTE ABOUT DATA TABLES
Data tables are wonderful things. We use them all the time and usually never question
there accuracy. Why should we when so many companies have devoted so much time and
research to create them for us. I find the most common mistake among shooters in their
interpretation of data tables to be the barrel length of the test rifle. Most think
that if it's written down it's as good as gospel. Nothing could be further from the
truth. Don't get me wrong when a friend sends or emails me a data table he or she has
been using I find all kinds of lost information. The most common are and in no
particular order. Barrel length of test rifle, twist rate, temp, humidity, proper
measurements i.e. was the test conducted in metric measurements or imperial, and
some times no measurements at all. It happens all the time. However the biggest
piece of information by far left out is the length of the test barrel.
Look at the table below:
The following is for Fed. 168 gr. GGM 60 Deg F ASL
| Barrel Length | Velocity FPS |
| 26" | 2694 |
| 25" | 2654 |
| 24" | 2616 |
| 23" | 2592 |
| 22" | 2572 |
| 21" | 2566 |
| 20" | 2524 |
| 19" | 2509 |
| 18" | 2477 |
| 17" | 2426 |
| 16" | 2414 |
Moving from a 25" barrel to a 24" barrel is no significant loss of energy There is a
significant loss of energy from the transition of a 26" barrel to a 20". It still
makes me laugh when I see a person on the range with his 308 tactical rifle
and 20" barrel aimed at the 1000 yard gong. He's getting madder by the minute
because he cannot connect with the thing. I usually go over and strike up a
friendly conversation and at some point ask what's wrong. "My rifle is broken", or
something to that affect is the response. It happens more than you think. After
a 20 min explanation he's back shooting groups and not wasting precious ammo.
PROPER CORRECTION FOR THESE FACTORS
The main thing to keep in mind is depending on the caliber these factors only really
start to barley affect your shots past the 300 yard mark and start to become
really noticeable at the 800 yard mark on drastically hot and humid days.
To properly understand these factors you need to start shooting on hot humid
days and lay witness to the affects for yourself. See how your rifle and ammunition
react. A properly managed data book can be an indispensable tool for study.
JUST DO IT BUT KEEP IT SIMPLE
To this point the 3 most critical factors a sniper must master are wind, distance
and proper adjustment, and target movement .You can sit there with a calculator
and punch numbers until your blue in the face. Nothing teaches you more than good
structured practice, and I don't mean getting 500 rounds and jack hammering them
down range until the target is nothing but a few strips in the wind. Because of my
busy schedule some times I only get to shoot 25 or fifty rounds per month if I'm
lucky. I combat the lack of range time by making each of those 25 or 50 rounds count
the most. After each shot, I look and take a note. When I get the chance to do
a 200 round session.( if my body feels up to it.) I make sure to go out on a windy
day to practice wind calling or maybe I'll find a place to work on my angle precision
fire. If I have some down time and it's raining out I grab my gear and pray for rain
at the range. No I don't have any screws loose (at least not legally)I will have the
range to myself because no one wants to be there on a miserable day. Even more so I
will have priceless practice time. Anything to bump up the challenge and make me work
mentally harder and smarter.
GADGETS, GIZMOS AND OTHER PRODUCTS TO SPEND YOUR MONEY ON
As I sit down to write the rest of this article I am disturbed by a knock at the door.
It's none other than my friendly neighborhood mailman delivering the bad news of
bills. No bills today, but a slew of new catalogues from an array of vendors. I
stop writing and start to flip the pages like a kid in a candy store. When I was
younger I would save my hard-earned money from my part time job to buy the latest
shooting gear. Before I made any purchase I would consult my father on if he thought
it was a wise decision. One night my father sat me down and we had a little chat.
The firearms industry is a 6 billion-dollar a year market. He picked up a gun magazine
off the coffee table and started to flip thought it. He pointed out that not one page
in the whole magazine was without a substantial portion devoted to advertising. He
picked up the brownels catalogue and started to turn the pages. We could only find
6 things in the whole catalogue that we really truly needed to become better shooters.
There is no magical technology that will instantly transform you into a super
marksmen. Fads come and go. instead of buying that new gizmo that so and so is
endorsing spend your money on an extra box of good quality ammunition. Nothing and
I mean nothing takes the place of good structured practice.
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