Computers & Ballistics Software | Contributing Editor Robert Rinker - Tip

COMPUTERS & BALLISTICS SOFTWARE
Computers have proven themselves to be very useful tools for the reloader and shooter. Not only are they marvelous for calculating such complex formulae as free recoil or the necessary twist rate for a specific barrel length/bullet weight, but they also allow shootists to quickly install and easily operate software dedicated solely to ballistics.

There are some other great uses for computers in the gun room that are not as commonly known or practiced. Some of those would include:

Weapons Inventory

Equipment Inventory

Reloading Notes

Test Comparisons

Trip Planning

Trip Diaries

Names, Addresses and supplemental data on landowners

Scheduled Custom Work

Number of rounds through a firearm

Projection of needed supplies for reloading a known number of rounds

If you are like most people, (not wishing to learn new software) you can accomplish the above through your word processing or mathematics programs. If you would like the most versatile program for dealing with the above items, and a lot more, then an interrelational database management program (IRDMS) is your best option. dBase by Borland is an excellent program of this type. It does take some time to learn, as is the case with all IRDMS software, but then the potential for useful applications is truly limitless.

Ballistics software [for a complete review of all available software of this type, its features, capabilities and system requirements see issue #16] has opened new vistas to the sport shooter. These programs and a computer, along with a chronograph, make just about any shooter into a ballistician. While these programs are truly phenomenal, a couple of points should be remembered when using them:

When calculating bullet path, be sure to include ALL range data. This will include some information which is not easily acquired: wind speed, angle to target, altitude and humidity, for example.

These programs are not infallible.

Perhaps the best demonstration of the fallibility of these programs can be seen in the area of wind effects. There are three schools of thought on how a bullet is coaxed off course by air motion. Naturally, any program is only capable of calculating a bullet's reaction to external forces in one way. Hence, the readings you get for your bullet's lateral movement due to a specified wind will be relative to the accuracy of the ancillary data you key in (weather, altitude, BC, etc.) and the formula selected by that particular software company for computing effects.

What all this boils down to is simply this: the printouts you get from these programs are outstanding reference items, and indeed will get you close to actual field "drift." However, don't bet the ranch on the absolute precision of these numbers. Of course, the greater the distance the more error will be noticed in the numbers spewed forth from your silicone shooting buddy versus the actual field occurrence. You can readily see how this happens by walking 300 yards across a field holding an anemometer and a wind flag: wind speeds and directions will change as you move downrange.

By all means use your printouts afield -- they're worth it, but never forget to use the most powerful of all computers in calculating hold-off: your brain. It is your assessment of field conditions and your experience with the firearm and load that will predictably arc your missiles into their intended targets.

Didion's Formula

This is the formula which is seen most often. It is used to determine the amount of lateral displacement (sometimes called deflection or drift) a bullet will experience during its flight time. It's convenient to have a chronograph, this formula and a calculator when working up loads at the range.

D = Xy ( T - Vt)
D - Stands for the amount of displacement expressed in feet
XY - Stands for the velocity of the wind in feet per second, if not at 90 degrees
T - Stands for the time of flight of the projectile from the firing point to the target
VT - Stands for the time of flight of the projectile from the firing point to the target in a vacuum.

TEMPERATURE INFLUENCES
Early last summer I moved from the Midwest to Arizona. The hunting in the Southwest is not just good, it is "grrrrrreat," as Tony the Tiger would say, but hot weather can bring about some very special problems. The ballistic and health issues of high temperature can concern a shooter in any part of the country, so even if you live in Vermont and have no intention of hunting or shooting in the Sonoran Desert, the information gathered here should be beneficial.

From a ballistic point of view, we could simplify the subject by saying that little temperature changes have little effect and big temperature changes have a big effect. Of course some readers might disagree and say that big changes don't have a big effect, but this depends on the type of shooting involved. For just general hunting at close range, out to perhaps 150 yards, the ballistic changes from temperature will go almost unnoticed even at extreme temperature differences. But for long-range hunting and demanding benchrest shooting, the effect can definitely be described as "big."

Problems with high heat are not just associated with the desert. The heat, when we consider the tolerance of the residents, can be worse in the rest of the country. My daughter, who lives near Louisville, Kentucky, spends her summers in upper Wisconsin because of her husband's job. She reports that the local people are extremely miserable when the temperature reaches low 80s. Of course 85 in Louisville is just a nice comfortable summer day. On the other hand, I live in the Sonoran Desert of Arizona. A temperature of 110 degrees is so common the local people don't even complain. (At least not much.) In the desert, 85 degrees at night is cool and comfortable.

In 1999, Chicago, Illinois, had 104 degrees Fahrenheit on the 30th of July. The Illinois death toll was 95 by August 2. On the same day, the temperature in Louisville, Kentucky was also 104 and it reached 99 the next day. In North Carolina, the Raleigh-Durham International Airport reported a record-breaking 101, which was the third consecutive day with a temperature over 100. Alabama had a heat index, based on both the temperature and the humidity, of over 110. In the past, Chicago's death toll from heat related deaths were more than 700 in 1995. Excessive heat or cold can be a killer that none of us, young or old, should ignore, regardless of where we live or hunt.

Heat can and does increase the chamber pressure and give a higher velocity. The problem is caused when the pressure increase is more than safe. Hunters in the colder climates of the far north sometimes make up both winter loads and summer loads. There can be a problem with excess pressure in using winter loads in the summer. A very hot chamber from rapid firing can cause the same danger if a cartridge is allowed to stay in the chamber for a period of time before firing.

The heat that is transferred from the ignited powder to the barrel and action is not used for propelling the projectile and is lost and wasted. Only about 30 percent of the heat created is gainfully used to expand the gas and push the bullet. Another 30 percent is wasted on heating the barrel and the 40 percent balance goes to recoil, noise, and escaping at the muzzle. With only about 30 percent being useful in creating kinetic energy, a firearm is not an efficient machine.

The heat produced can exceed the melting point of the steel in the barrel. The short time the heat is influencing the metal is the only thing saving it from greater damage. The temperature might reach about 5,550 degrees Fahrenheit and most barrel steel will melt at about 2,500 degrees Fahrenheit. The heat is dispersed by radiation into the air and by conduction into the stock and anything else touching it.

A heavy barrel absorbs more heat than a thin barrel and the larger surface area gives an increased heat transfer to the surrounding air. The heavy barrel has advantages in rigidity. They are usually inconvenient to carry hunting, but excellent for benchrest and some other uses.

Air temperature enters into the ballistic picture in three distinct ways. One is with density, another is elasticity of the air, and the third is the effect on the cartridge, primer, and powder. Elasticity acting on the velocity of sound influences the effect of temperature on air resistance. The elasticity of the air affecting the velocity of longitudinal wave motion in the air is a function of the temperature. Within reasonable bounds, it is not influenced by density changes.

For constant pressure, a given quantity of air changes density with temperature approximately according to Boyle's law:

(pV = k where V is the total volume, p is the total pressure, and k is a constant.)

The temperature changes may be daily, seasonal, geographical, or irregular variations from weather factors.

Temperature has an effect on air density with cold air being denser and hot air less dense. As any aircraft pilot knows, the combination of high altitude and hot temperature can be deadly for aircraft performance but it also affects firearm ballistics. The effects on long-range shooting are well known. The change is caused not only by air density, but also by changes in cartridge pressure. A lower temperature will lower the pressure and the velocity. Shooters who compete at ranges out to 1,000 yards will notice this effect. At this extreme distance, a 10 degrees Fahrenheit change in temperature will need a 1 minute of angle change in elevation. Large temperature differences will cause large elevation changes, all else being equal. (Remember that we are discussing extra long-range shooting. There would be a change at a short range, but it can be disregarded.)

A temperature change of 17 degrees Fahrenheit will influence the air density the same value as an elevation change of 1,000 feet.

Cold temperatures retard powder combustion. A major powder manufacturer, DuPont, says that with smokeless powder, a velocity drop will occur below 70 degrees Fahrenheit of 1� fps per degree. Example: Temperature at 25 degrees Fahrenheit, 70 � 25 = 45 times 1.5 = 76.5. A 77 fps drop is not enough to make much difference in all but the rarest cases. If someone in the far North were hunting at 41 degrees Fahrenheit below zero, the drop would be about 167 fps. A full hundred lower than a 25-degrees but still not much for most firearms used in that area. Still, if hunting with a marginal caliber firearm, it should be considered.

Temperature, relative humidity, and barometric pressure should be considered in every serious shooting effort. The air resistance and cumulative drag are influenced by these atmospheric conditions which create an apparent change in the ballistic coefficient. Apparent because the projectile behaves differently only because of outside changes. The actual ballistic coefficient remains the same if based on standard atmospheric conditions. (Altitude at sea level, temperature 59 degrees Fahrenheit, relative humidity 78 percent, and barometric pressure of 29.58" Hg.) The ballistic coefficient will appear lower if the above conditions are reversed. Most factory figures are converted and reported as standard conditions.

Drag on a projectile is a function of air density. To correct for non-standard air density, we have to find the figure for the shooting range and the standard for the altitude. The correction factor is the ratio received by dividing the standard pressure by the shooting range pressure. The number obtained is used as a multiplier.

To correct for non-standard temperature, we must add 459.69 to each temperature to obtain a reading on the Rankine Scale. After the additions, the multiplier is obtained by dividing the standard temperature into the shooting range temperature.

The humidity correction is difficult to perform and it makes such a tiny change in the ballistic coefficient that it can be skipped.

For an example, we will use the standard conditions listed a short while ago and use the shooting range conditions as 90 degrees Fahrenheit, 29.20" pressure, and 88 percent relative humidity. For our example, the original ballistic coefficient can be .342.

The barometric pressure correction factor is 29.58 / 29.20 = 1.011.

The temperature correction factor is 90 + 459.4 / 59 + 459.4 = 1.059.

The temperature factor times the barometric pressure factor times the original ballistic coefficient = the corrected ballistic coefficient:

(1.059 times 1.011 times .342 = .366)

While we would like to think that this would give us an accurate figure, it will not. It will be closer than if we did nothing, but it will still be just an approximation. Tables are available that list multipliers made from the ratio between the actual density at the time and place involved and the standard. Corrections that complex are not normally necessary. In our modern times, when that amount of accuracy is required, it is usually figured on a computer with a ballistics program.

A little playing around with the On Target computer program was interesting. Their software permits changing the weather conditions for trajectory figures so I experimented with a cartridge with which I am familiar. I also chose two temperature extremes that I have experienced. Research was on the .22-250 Remington with a Winchester 50-grain Silvertip bullet sighted in at 400 yards and fired to 500 yards. In a 24" barrel, this will give a 3,810 feet per second muzzle velocity at the standard barometric pressure, humidity, and temperature. When the temperature is lowered to 20 degrees Fahrenheit, the velocity at 300 yards drops from 2,527 fps (at the standard 59 degree Fahrenheit.) to 2,436 fps. The 500-yard velocity drops from 1,836 fps (at the standard 59 degrees Fahrenheit.) to 1,710 fps.

When the temperature is raised to 110 degrees Fahrenheit, which is common here in the Southwest, the 300-yard velocity goes up to 2,630 fps and the 500-yard figure goes up to 1,979 fps. There is a corresponding drop or increase in energy. The trajectory, as we would expect, is flatter with the higher temperature. The 500-yard standard temperature figure was - 14.1" but that lowered to -16.8" at 20 degrees and flattened out to -11.5" at 100 degrees Fahrenheit. (Don't you get tired of all of these numbers? Nevertheless, they are important.)

Of course these are not huge differences, but they are still significant and should not be ignored. Cooking-off is the term used to describe a cartridge that is fired or exploded because of the application of heat. A blow on the primer is not applied. Testing by the National Rifle Association and Army Ordnance and other independent groups has shown that the average rifle cartridge will cook-off at high 200 to low 300 degrees Fahrenheit. Average is an important word in that statement. Some may be lower or higher.

The high temperatures that steel can obtain in direct sunlight, especially in the south and the tropics, can also cause a cook-off in firearms. If a few rounds are fired, the temperature can raise the few extra degrees needed. A cartridge that is confined in the chamber will expel the bullet and may be as deadly as any accidental shot. Military crash crews are taught how to disarm aircraft weapons at a fire scene and not to be in front of the gun until it is safe.

Excessive barrel heat from firing will change the point of impact. This is little concern to many hunters, as long as the rifle is sighted in for a proper hit with the first shot from a cold barrel. A target shooter, or a varmint hunter taking numerous shots, should learn what to look for. For accuracy, slower shooting requires the shots be spaced to hold a uniform barrel temperature, perhaps a little warm but not too hot. As stated many times, each firearm and cartridge combination will respond differently. If accuracy is important, experiment with the gun and ammo that you plan to use at the range where you plan to use it. Going hunting? Remember that the special, once in a lifetime shot, will almost certainly be from a cold barrel. If you have played around on the range at sighting-in and the barrel has become hot, let it cool before the final sight adjustment. If you are shooting in the desert or some other place where metal is heated to a blistering temperature, expect it to affect your accuracy.

Your firearm and trajectory are not the only things changed or affected by temperature. The human body normally can operate safely in a narrow temperature range, and excessive heat can be a killer. It is common here in the desert to find skeletons of hunters and hikers who have been missing for years, but in most cases, their deaths are pointless and uncalled for. When out in the heat, drink and eat regularly. This may sound like simple advice that doesn't need to be said, but many people die each year from not following it.

Dehydration from lack of water is always a serious consideration. In hot weather, hunters or hikers should carry a quart of water or sports drink for every hour they expect to be active. Up to two quarts an hour if the temperature is very hot. This is a lot more than most people believe is necessary or bother to carry. A small amount of water is not going to avoid problems. Soda is ineffective because most are high in sodium, which pulls water from body cells. This, in effect, does just the opposite from what is needed in hydrating the body. Also avoid alcohol or caffeine because they are diuretics and they will deprive the body of much needed fluids. (Sorry, but that cold refreshing beer is not good for you on a very hot day.)

Canteens are excellent for carrying water, but so are small plastic bottles, either with or without an insulating cover. Many people here in the desert fill the plastic bottles about three-quarters full with water, leave the cap off, and place it in the refrigerator's freezer compartment. When ready for an outing, the bottle can then be filled to the top with cold water and capped. It will remain cold and refreshing for a long time.

Electrolyte-replacement powders, such as Gatorade, can be carried along and added to water as required. Water is not the only concern. Hats, food, sunscreen, and good judgment also are necessary.

If a hot weather hunter drinks plenty of liquid but doesn't eat, he or she is at risk of hyponatremia, which is a serious heat illness. Salty foods are good but avoid foods that are high in fat or protein because they are difficult to digest and can cause an upset stomach from exertion. Carry plenty of snacks, raisins, crackers, peanuts, pretzels, and fruit. Eat or snack frequently. A hunting trip is not the time to try to lose weight. It is important to eat trice as much food as when staying at home. Yes, that is what the experts say, twice as much.

Dress in layers so clothes can be removed or added to suit any change in temperature. Take a breathable outer jacket that is waterproof, even if it is a hot day. If a problem arrives and the nights are cold, being stranded should not only be survivable but also comfortable. For that reason, carrying a small flashlight is not a bad idea.

If you are hiking to a hunting spot that is far away and the temperature is hot, stop and elevate your feet above your heart for five to ten minutes every hour. This helps to prevent cramping and exhaustion by removing metabolic waste from the lower extremities. While some people may consider this as slowing their progress, their movement will be stronger when they continue.

Never go alone, but if you do, let someone know exactly where you will be. Of course, if you have a cell phone, don't forget to take it along. Also, to quote the National Park Service, "Leave no trace." Carry out what you carry in and restore any damage.

- Bob Rinker