Ballistics. Everyone's heard of them, but few people know much about them. For many folks, all they've ever heard about "ballistics" was when McGarrett would have Dano run a bullet down to "ballistics" for testing.
Sure, lots of people know that a bullet is engraved with the impressions of the lands and grooves from the bore's rifling. That's a given. People understand that, in many instances, a fired bullet can be used to identify the weapon it was fired from.
As I tend to read a little more about ballistics than "the next guy," I thought I'd offer a few words on the subject that might help some of you in your shooting endeavors. I don't claim to hold a degree in forensic science or physics, but I think I might be able to help some of you who are novices when it comes to the study of ballistics. If nothing else, perhaps I can get you to go out and do a little reading on your own about the subject.
There are actually three areas, or categories, of ballistics: internal, external, and terminal. Each one is important to the shooter, and, at a minimum, a shooter should understand a little about each one if he is to be proficient in his craft -- and most especially, if he is to avoid injury.
All the things you've read about in gun magazines, concerning things like chamber pressure, initial velocity, powder burn rates, and things of this nature, deal with internal ballistics. As simply as I can put it, internal ballistics is limited to those things "internal" to the firearm you're using in relation to the ammunition you are firing.
Especially as a handloader, you should be concerned about neck tension on the bullet. If it is too tight, your pressure will build to a higher level, perhaps with dangerous consequences. Also, a contributor to higher pressure is the distance from the ogive of the bullet to the location where contact is made with the lands and grooves. Often, having the bullet touching can increase pressure.
Powder charges, as well, have a direct effect on pressure, especially compressed loads.
Other things, like temperature (both ambient and chamber) will affect the level of chamber pressure in your weapon.
Commonly, you will hear experienced shooters tell you that "the" way to determine if your pressures are too high is to look at the fired primer... that is, if it's flatter than a pancake and has no roundness left to it, your pressures are too high. Indeed, if the primer has flattened excessively, and certainly if it has backed out of the primer pocket, you may have some valid issues to address about the chamber pressures that are resulting from your ammunition. However, visual observation is NOT "the" way, the ONLY way, the SAFEST way, the MOST RELIABLE way to determine if you are experiencing excessively high chamber pressures. Such measurements are BEST taken by equipment intended for the purpose, such as firearms manufacturers use when testing their rifles, handguns, and shotguns.
It is always wise to rely on your reloading manual, verbatim, until you KNOW you have gained the experience necessary to start "experimenting." And, frankly, YOU ARE A FOOL if you do not wear high-quality eye protection when you are priming your cases and when you are testing your ammunition.
Everything that goes on inside the round, from the time the primer explodes until the time the bullet exits the muzzle, deals with internal ballistics. I strongly recommend that you buy the best ammunition you can afford, or that you use the best handloading tools you can afford when "rolling your own." Buy not ONE, not TWO, but at least THREE reloading manuals, preferably from folks like Sierra, Hodgdon, Speer, Vihtavuori Oy, Nosler, Barnes, Hornady, Accurate Arms, and so forth. Ideally, I'd recommend at least one comes from a bullet maker, one from a powder maker, and one from a company that sells loaded ammunition. Once you buy them, READ them... don't just start looking up data and load recipes. Read them, learn the language, talk the talk, U-N-D-E-R-S-T-A-N-D them... in short, know what the heck the manuals are talking about. If you don't, then don't do a THING until you have gotten answers to the questions you have! Don't load a single round until you KNOW what you're doing!
After the bullet has exited the muzzle of the barrel, you're dealing with external ballistics. Now you're concerned about things like trajectory, wind drift, stabilization, "going to sleep" (the bullet, not you), deformation, firing angle, cant (not "can't" as in "I can't stand rap music" but "cant" as in whether the firearm is tilted -- at the time of firing -- to the left or to the right.
From the moment of the bullet leaving the barrel until the time the bullet impacts the target -- or anything else that stops its flight or alters it's course -- you are concerned with external ballistics.
Stabilizing the bullet is a function of velocity and the bore's rifling working together. The twist rate of the bore (1:9, for instance, meaning the bullet will complete one full turn in nine inches -- even if, as in the case of a handgun, the barrel is only seven and one-half inches long) is just a ratio, nothing more, and indicates how much distance is needed to turn the bullet one time. Combined with velocity, enough "spin" is imparted to the bullet to better enable it to "fly."
Think of "the perfect spiral" that an expertly-thrown football has once it's left the thrower's hand. Now, think of the throws you've seen where the football wobbles in flight... and ask yourself which one flies more efficiently. It's the same with bullets. Stabilization is the name of the game.
If a rifle is canted, that is, if the rifle is (from the firing position) tilted to the left or to the right (I'm not talking about moving the muzzle or the butt of the rifle up or down -- that's dealing with elevation), you will be "throwing your shots" to the left or right, respectively, as the bullet travels in flight. The farther you're shooting, the more of a concern this becomes. A relatively inexpensive device called a ScopLevel, which mounts to the top of your scope, goes a long way to completely eliminating this type of shooting error.
"Putting the bullet to sleep" refers to when the bullet actually stabilizes. If you've ever heard of someone getting "okay" groups at, say, 200 yards, and relatively "better" groups (not "worse" groups) at, say, 400 yards, the person isn't necessarily nuts... though you might think so. Some bullets, fired from some firearms, don't "go to sleep" (i.e., stabilize completely) until they've traveled a certain distance. Yes, you CAN get a larger minute of angle (MOA) group at 200 yards than you might get at 400 yards. If you fly airplanes, then you know something about pitch, yaw, and attitude (not "altitude," but "attitude" -- and I don't mean anything about the personality of your flight instructor). You can relate such things to the flight of a bullet as well... and you should.
Wind drift has to do with how much the bullet will move, or "drift," to the left or right, depending on which direction the wind is blowing. Bullets with a higher ballistic coefficient (BC) do better in the wind (i.e., the wind has less effect on the bullet as it travels, and does not drift as much as a bullet with a lower BC). There is a SUPERB article in the August, 1996, issue of Precision Shooting on bullets with a tungsten core. These bullets, about ten or twenty times more expensive to produce, enable a shooter to worry less about wind drift than he ever could before with bullets having lead cores. Essentially the same (visual) configuration as bullets from Sierra or other bullet manufacturers, these tungsten-core bullets WEIGH MORE, contributing to a higher BC, and allowing shooters to "defeat" the wind better than was previously possible.
Wind roll is another area that serious shooters have been giving more and more attention to in recent years. There have been some very fine articles on the subject written in the past year or so in Precision Shooting magazine. Depending on the direction of rotation of your bullet, and on the direction the crosswind is blowing, a bullet will actually climb or drop -- more -- than if there was no wind at all. Literally, it is as if the bullet is rolling "up" (or "down") on the wind. Yaw also contributes to the degree of wind roll. Makes for interesting reading.
Trajectory and firing angle are important considerations, too. From the moment the bullet leaves the barrel, gravity starts affecting it. Though the bullet may actually climb initially (a subject for further discussion at another time), and will cross the sight axis twice (over longer distances) -- once on the rise, and again when dropping -- all projectiles can be expected to produce some sort of arc in flight. This arc is affected by a number factors, besides the BC of the bullet, to include humidity, ambient temperature, and velocity. The firing angle, such as whether you are firing up or down hill, is often a "hot" subject with shooters... and I'll likely discuss it in more depth, too, at another time. Understand for now, however, that while the laws of gravity do not change with uphill or downhill shots, your point of aim (POA) must change to facilitate a change in point of impact (POI). As I said, this is a topic that is argued over, often and with conviction... though there is little reason for argument.
Deformation deals with how much the bullet was changed as it traveled through the bore. If the bore of your weapon is rough, you may be losing more jacket material (i.e., bullet weight) than you would like. How deeply did the lands and grooves imprint the bullet? How much did the bullet get squeezed as it entered the bore. These things will affect the bullet's flight. And, if the bullet hits anything "unexpected" in its trip to the target, "deformation" REALLY gets serious... but I'm not here to talk about ricochets.
When mounting your scope, try to use rings that will keep the sight axis of the scope as low to the bore axis as possible. This will make it easier for you to calculate changes in elevation as they are needed, especially over greater distances.
This last category of ballistics deals solely with the engineering and manufacturing characteristics of the bullet. Does the jacket separate from the core? Does the bullet "mushroom" (expand) at all, and, if so, by how much? Does the bullet penetrate for a short (or long) distance before it expands? Does it not expand at all but, rather, penetrate the target completely? Is the jacket of the bullet so soft that it leaves excessive amounts of material in the bore of the weapon? These are all questions that belong in the world of external ballistics. As varied as are the designs of bullets, so, too, do the results they yield vary. Some bullets disintegrate on impact, some mushroom, some break into a few pieces, and sometimes they don't expand at all but just leave entrance and exit holes in a given target. Especially when hunting, it is important to "know" the game you are hunting, and it's equally important to "know" the characteristics of the bullet you are using on that game.
There you have it. A short and sweet presentation of ballistics. As a subject, it is something you can never know enough about, one that I'm still studying almost daily. Everytime I go to the range, I'm always trying to "put it all together" and make things work for me.