ENVIRONMENTAL FACTORS THAT AFFECT ACCURACY


 


One of the fun things about owning and using a PC or Macintosh-based computer ballistics program is the opportunity you have to see how your particular bullet / powder combination is going to perform — at least theoretically.

By changing the various input factors that the program uses to calculate the trajectory and speed of the bullet (i.e. bullet weight, velocity, temperature, altitude, wind conditions, etc…), you can, with the stroke of a few keys, see the effect that these factors will have on your load.

In this discussion I’ll be focusing on five (5) “environmental” factors that affect accuracy — Temperature, Humidity, Elevation / Barometric Pressure, Wind, and Light. Four of the five elements can be used in a ballistics program. “Light” is the only element that cannot be taken into account by the computer.

Keep in mind that when we shoot a Long Range match we’re going to be required to make sight adjustments throughout the match to deal with changing conditions, sometimes on a shot-by-shot basis. Knowing the effect that weather factors have on accuracy and skillfully using that information when we make those sight changes can mean the difference between missing the target completely due to a wind shift, or staying on the target for an “8”, “9”, or “10”.

Temperature

Temperature affects our shooting in several different ways:

a) It affects the trajectory of the bullet.

b) It affects the temperature of the barrel.

c) It affects the ammunition we use.

d) It affects the performance of the shooter.


Trajectory Considerations

A simple fact of physics tells us that warm air is thinner or less dense than cool air. As a result, a bullet encounters less resistance and expends less energy as it travels from the firing line to the target on a warm day than it does on a cooler day.

Take a look at the numbers below which were generated by my RSI Shooting Lab program:

Load Data
  — Caliber = .45-90
  — Velocity = 1,300 feet per second
  — Distance = 1,000 yards
  — Ballistic Coefficient = .400
 

Air Temperature In Degrees

  40 60 80
Bullet Drop (Inches) 1,702.18 1,650.09 1,601.51
Impact Energy (Foot Pounds) 715 753 793

Clearly, this load performs better as the temperature rises. The bullet impacts about 50” higher (i.e. flatter trajectory) for every 20 degree change in temperature. And it retains about 40 pounds more energy upon impact as well.

Barrel Temperature

A second effect that results from warm or cool outside air temperature relates to how hot your barrel becomes during your relay.

On a relatively cool day (50 degrees), the outside air is able to absorb more heat from the barrel at a faster rate than it does when the air temperature is 80 or 90 degrees. This helps to keep the barrel cooler with fewer harmful side effects like “baking” the fouling in the barrel which in turn can result in a significant loss of accuracy.

Even on a moderate day (60 degrees) the barrel can become too hot to hold during the course of firing the 20 to 25 rounds that your relay may require, including sighter shots.

Ammunition Temperature

Heat — especially excess heat — can play havoc with your ammunition. Handloaded BPC ammunition that has not been properly stored or protected from outside sources of heat or direct sunlight can suffer significant performance deterioration. Not only will the explosion of the black powder in the case produce different results, but the lube on the bullet may melt from the excess heat and flow under the bullet or into the powder column itself.

Even if you have taken the proper steps to keep your ammunition cool prior to coming to the firing line for your relay, setting you ammo in the direct sunlight or chambering it in a hot rifle for any length of time can result in an errant flyer.

I always keep my ammunition in an insulated soft-sided cooler until I’m ready to transfer it into the ammunition block in the Shooter’s Tray that I carry to the firing line. In addition, the ammunition block that I use has a curved hood that covers the ammunition and keeps it out of the direct sun while I’m shooting on the line. Placing a white washcloth or towel over your ammo will serve the same purpose.

Shooter Temperature

Keeping the shooter cool and comfortable is an important element in the overall accuracy equation as well. If the shooter becomes overheated or dehydrated, not only does he or she become uncomfortable, but their physical senses are affected — vision deteriorates as moisture is drawn from the eyes and brain by outside heat, sweat running down the forehead can cause problems seeing the target, high blood pressure results from low water content in the body along with fatigue, weakness, lower back pains, inflammation, headaches, etc… — the list is a long one.

Staying fully hydrated during a match is very important for the shooter. Keep lots of bottled water (no carbonated beverages) within easy reach. Drink often during the day — don’t wait until you’re thirsty. By that time you are already dehydrated. It is not uncommon for some shooters to drink 4 to 8 bottles of water during the course of the day.

Humidity

Many shooters believe that humidity (i.e. the amount of moisture in the air) can have a serious impact on accuracy. The theory is that air containing a high degree of moisture must be more dense than dry air. As a result, it must offer more resistance to a bullet in flight than does dry air.

The fact of the matter is that the amount of humidity in the air has virtually no effect on bullet accuracy. That’s because a water molecule weighs less than a dry air molecule. Yes — that’s what I said. Moist air is actually less dense than dry air.

In fact, when the surrounding air is said to have 100% humidity (i.e. it is holding as much water vapor as it can at its current temperature), it still only contains 4% moisture. Look at the numbers for yourself:

Load Data
  — Caliber = .45-90
  — Velocity = 1,300 feet per second
  — Distance = 1,000 yards
  — Ballistic Coefficient = .400
 

Humidity Level

  0% 50% 100%
Bullet Drop (Inches) 1,629.66 1,626.88 1,624.12
Bullet Speed (FPS) 800 801 802
Retained Energy (Foot Pounds) 769 772 774

According to the numbers above, the trajectory of the bullet becomes flatter (with less drop), travels faster (higher speed), and retains more energy upon impact (retained energy) as the air becomes more saturated with moisture.

It is true that a high degree of moisture in the air does help to keep the fouling in the barrel softer than on a hot, dry day. But about the worst thing you have to worry about when the humidity is high is keeping your gear dry and preventing it from rusting!

Elevation and Barometric Pressure

Elevation and barometric pressure can definitely have a significant effect on the accuracy of your load. Why? Because they affect the density of the air, and therefore, the amount of resistance it exerts on the bullet as it travels to the target.

Elevation is generally expressed in “feet above sea level”. The performance of a load at sea level is much different than it is when fired at an elevation or altitude of 5,000 feet above sea level. Let’s plug some numbers into the ballistics program and see the results.

Elevation (Altitude)
  Sea Level 2,500’ 5,000’
Bullet Drop (Inches) 1,670.93 1,638.94 1,608.62
Bullet Speed (FPS) 782 797 810
Retained Energy (Foot Pounds) 736 764 790

As you can see, when compared with the bullet fired at sea level, the bullet fired at a 5,000 foot altitude has a trajectory which is 62.31” inches flatter, 28 feet per second faster, with 54 additional pounds of energy upon impact. That’s why it’s so important for hunters to re-zero their rifles once they arrive in camp when they travel to higher elevations to hunt such big game animals as elk, caribou, bear, and moose.

Wind

Many shooters immediately grimace at the thought of having to deal with wind when shooting at the distances that comprise Long Range shooting. But, as I pointed out in the article on “Dealing with Smoke” in this section of the web site, some wind is a good and necessary thing, especially when shooting Black Powder Cartridge rifles with their resultant clouds of white smoke.

After all, how else is all that smoke going to be cleared away or dissipated so you can see the target?

Wind is also a good thing when it is steady and predictable. As long as it acts as a constant and not as a variable, we can calculate its effect on the flight of our bullet and make the necessary adjustments to compensate for it accordingly. It’s when the wind is unpredictable, gusting, and erratic that it plays havoc with us. Over the course of 1,000 yards, the wind can actually be blowing in several different directions at the same time!

But just how much does wind affect the flight of a bullet in Long Range BPCR shooting? Well, because the ballistic coefficient of a BPC bullet is very low when compared to modern ammunition (.380 to about .420), wind drift is a major concern. Let’s take a look at just how much wind can affect the flight of the bullet when it is blowing at 5, 10, 15, and 20 miles per hour.

Load Data
  — Caliber = .45-90
  — Velocity = 1,300 feet per second
  — Distance = 1,000 yards
  — Ballistic Coefficient = .400
 
Wind Drift or Deflection

Wind from 9 O’Clock Direction

 

(In Inches)

 
5 MPH 10 MPH 15 MPH 20 MPH
74.44” 148.89” 223.38” 297.93”

Take a close look at the numbers displayed above. They tell us that at a distance of 1,000 yards, for every 1 MPH increase in the velocity of the wind, our bullet will be deflected from its flight path to the target by approximately 15”.

So, you can imagine the problems that can arise when you are shooting on a day when the wind is gusting or rising and dropping in speed. The secret is to hold-off or wait until conditions are as close to being the same for each shot that we take.

Reading the wind is a skill that is developed over time through experience and practice. To learn more about wind and how to read it I would suggest purchasing a small booklet entitled “Reading The Wind and Coaching Techniques” by M/SGT James R. Owens USMC (retired). Here are a couple of places where you can order it:

Creedmoor Sports: Part # C1225                               $ 9.95 + SH
http://www.creedmoorsports.com/Books_C1255.htm

Georgia Precision:                                                         $ 9.95 + SH
http://www.creedmoorsports.com/Books_C1255.htm

Jarheadtop.com:                                                           $ 9.95 + SH
http://www.jarheadtop.com/books.html


Light

The fifth and final environmental factor that we need to consider when shooting at Long Range is Light — or said another way, the quality and amount of light that is illuminating our target. Although a certain amount of light is falling on our target throughout the course of a day’s match, the intensity of the light and its effect on our ability to “see” the target clearly is constantly changing.

Take a look at the two targets shown below:


Target in Shadow                 Target in Sunlight

Which target can you see more clearly? Now get up from your computer and move back as far as you can go, and look at the targets below:

 

 

 

 

 

 

The farther away from the target you go the less definition the bullseye has within the target frame. Now, think about looking at a target 800, 900, or 1,000 yards away that is in shadows!

Changing light conditions will definitely have an impact on accuracy. Both you and your spotter need to be aware when the light has changed and whether or not you should make a change in your sight settings.

There is an old saying that goes:

“Light’s Up - Sights Up” and “Light’s Down - Sights Down”.

Basically what that means is this:

If you were shooting at the target on the left in the illustration above and suddenly the sun came out and was illuminating your target so that it looked like the target on the right, the sight settings that you were using when the target was in shadows would now cause you to shoot low on the illuminated target.

The reverse is also true. If you were shooting at the target on the right and suddenly a cloud passed overhead and threw your target into shadows, the sight settings that you were using on the illuminated target would cause you to shoot high on the shadowed target.

How much high or low I can’t tell you exactly. I have never seen any estimates or scientific way of calculating the change. But at 1,000 yards, even a small change at the firing line can amount to a big change by the time the bullet travels 1,000 yards to the target!
 


By Darryl Hedges
 

Copyright© 2010 All Rights Reserved.
 

 

HOME          DISCLAIMER          CONTACT US          SITE MAP
 

Web Site design and development by WebQueenWebSites.com