Internal Ballistics

All About Accuracy

Studying internal ballistics is all about accuracy—what accuracy is, where it comes from, and how the shooter can improve it. It's possible for the shooter to do something about accuracy problems once he or she understands the origins of their problems and takes the right steps to correct them.

Factory Ammo vs. Handloads

Factory Ammunition

Factory ammo is, and must be, manufactured to specifications that will ensure its functioning satisfactorily—despite a wide range of chamber tolerances in a variety of bolt, slide, lever, autoloading, and single-shot rifles, and revolvers and pistols both domestic and foreign. Factory ammunition must necessarily forego some accuracy potential for the individual rifle to perform satisfactorily in all the different rifles in which it may be used.

Handloads

Handloads, however, can be made up for the individual firearm and its chamber without compromise and so can bring out its best capabilities. Handloading your own ammunition—ammunition of excellent quality, custom tailored to your own rifle or handgun and your own shooting needs—could hardly be easier or safer.

It requires only these four different components:

And this basic equipment:

Our purpose here is to help clarify certain aspects of shooting and reloading that are frequently unknown or misunderstood. The more secure your knowledge of these complex factors, the better will be your chances of obtaining the ultimate in accuracy and performance from your firearm and ammo.

To get detailed, step-by-step procedures for reloading rifle and pistol cartridges, purchase your copy of the 10th Edition Hornady® Reloading Manual.

Inside the Chamber

To explain what actually happens in the chamber of a rifle or handgun when it is fired, as well as how the case and bullet are affected, we will use illustrations with exaggerated clearances, which would otherwise be difficult to see.

Rimless Cartridge

The rimless cartridge [Fig. 1] is a factory round that fits the chamber rather loosely; its bullet is not in perfect alignment with the bore, and the case doesn't contact the front of the chamber [Fig. 2]. As the firing pin strikes, it moves the case forward to contact the front of the chamber [Fig. 3], giving a little headspace—but not a dangerous amount [Fig. 4]. Notice that the violence of its flash backs the primer part way out of the pocket [Fig. 5] — and as the powder is ignited and pressure builds up, the brass case expands to fill the chamber completely [Fig. 6], preventing any escape of gas to the rear.

Rimless Cartridge Figures 1 - 6

As the pressure continues to build, the case is forced so tightly against the chamber wall that it cannot move, but since we had a gap between the base of the cartridge case and the face of bolt or breech block—what we termed a little headspace—the case itself must stretch in the head region circled [Fig. 7] to force the case head back. As the case is moved rearward, the primer is re-seated in its pocket. When the bullet exits up the barrel, the pressure drops, the case cools, and the brass contracts enough to permit extraction of the fired cartridge case from the chamber [Fig. 8].

The concept of headspace is one every shooter should understand fully, both in the interest of more accurate shooting and personal safety. In the Illustrated Glossary of the 10th Edition of the Hornady Handbook, we summarize four different headspacing systems and define the term very simply as "the fit of a cartridge in a chamber measured as the distance from the breech face to that part of the chamber which stops the case's forward movement." As we have just seen with this rimless case, an imperfect fit of the cartridge in the chamber will result in the case stretching in the critical head region.

Fired in a chamber having considerable headspace, this .308 Winchester case (Fig. 9) has had its wall thickness substantially reduced at the critical head region—and would almost certainly have separated on the next firing—as it did this case [Fig. 10]. Such a rupture might permit gas to escape rearward through the action, endangering the safety of the shooter.

Rimless Cartridge Figures 7 - 10

Rimmed Case

Next, we'll illustrate another type of case with a different head-spacing system: the rimmed case. When the primer, in this case, is struck by the firing pin [Fig. 11], the case itself cannot move forward appreciably because it is the rim and not the shoulder that stops its forward movement and thus positions it in the chamber. As the powder is ignited and pressure builds inside the case, its thin walls readily expand to contact the chamber, but because the rimmed head is virtually in contact with the bolt face or breech block, the case [Fig. 12] will not stretch as much in the critical head region as did the rimless case we first illustrated—unless the action used is an exceptionally weak one. 

Belted Case

The belted case—often used in so-called "Magnum" cartridges—is the third type of design frequently encountered. The "belt" is, in effect, a rim moved to the front of the head rather than being at the rear. When this type of cartridge is struck by the firing pin, it is moved forward only slightly, regardless of the space at the shoulder. When it has been fired [Fig. 13], there is very little stretch in the critical head region. However, if headspace is not close, the case will stretch just above the belt.

Rimmed & Belted Cases Figures 11 - 13

The Role of a Cartridge Case

A cartridge case plays a demanding role in the process of firing. It must contain hot gasses under immense pressures and seal the chamber to prevent their rearward escape. Despite safety features incorporated in modern rifle actions, case failure resulting from faulty headspacing poses possible dangers to the shooter. The careful reloader who understands the concept of headspace can take steps to minimize the risks of case separation.

To review our discussion up to this point, we've established that:

  • Factory ammunition, manufactured to function in a wide variety of actions and chambers of varying dimensions, will be an imperfect fit in the individual rifle and handgun.
  • The bullet will not necessarily be in perfect alignment with the bore.
  • The case will expand on firing to conform to the dimensions of the chamber when the cartridge is fired.
  • If headspace is not close, there will be some stretching or thinning of the case wall in the process.

Deciding How to Resize

While our case conforms perfectly to the chamber in which it has been fired, one important dimension has been changed so it cannot be reused as is: The case neck has expanded to release the bullet and is now too large to hold a new bullet securely. This brings us face to face with a decision that plagues and confuses many reloaders: whether they should full-length resize their cases, returning them to standard dimensions; or whether they should resize the necks only.

Neck Resizing

Let's examine the process of neck sizing to see what advantages it affords. Most cases can be neck sized in a normal full length die just by unscrewing it slightly, though a proper neck sizing die is preferred. Since the body of the case will not be worked in the die, it is not necessary to lubricate the full case. It is necessary only to dip the neck lightly in a dry lubricant, such as powdered graphite, to prepare for the resizing operation. A new primer, some new powder, and a new bullet get the case ready for reuse.

Though some rifles deliver their best groups when full-length resized, neck sizing alone usually promotes better accuracy, because when our reloaded cartridge is returned to the chamber, it is almost a perfect fit. Headspace is just right with all cases, whether rimmed, belted, or rimless; and most helpfully of all, the new bullet is almost perfectly aligned with the bore.

Observe what happens when this reloaded cartridge is fired: The striker does not drive the case forward because the shoulder is already in virtual contact with the chamber, and headspace is minimum [Fig 14].  The case is not subject to stretching in the critical head region as it was when fired originally [Fig 15].

Full-Length Resizing

Some reloaders may wonder why it would not be possible to adjust a full-length size die to eliminate excess headspace and obtain the advantages we've just noted for the process on neck sizing only. To get the answer to this question, we'll return to our cutaway illustrations. The full-length sizing die, though adjusted for minimum headspace, "works" the brass, eventually making it flow into the neck area, thereby lengthening the case [Fig 16].

When the reloaded cartridge is returned to the chamber, it doesn't have excess headspace, but it's almost as loose a fit as it was when brand new [Fig. 17]. One benefit full-length resizing provides (if done with minimum headspace) is in aligning the bullet with the bore [Fig. 18], though some misalignment may still be possible due to the case or chamber eccentricities.

Repeated full-length resizing will keep lengthening the case until it must eventually be trimmed, possibly after only three or four rounds. The reasons for this are simple. The case body expands on firing, and its diameter is reduced in full-length resizing. The brass displaced must go somewhere—and it does; it is pushed upward to lengthen the case.

Neck & Full-Length Resizing Figures 14 - 18

Overlength Due to Full-Length Resizing

When an overlength case is chambered, the mouth or edge of the neck will come up against the throat [Fig. 19] before the bolt has fully closed or the case shoulder has contacted the chamber [Fig. 20]. The camming action of the bolt is so powerful that it will actually crimp the case mouth fully into the bullet [Fig. 21] and wedge the case so solidly between the bullet and the throat that the neck cannot expand to release the bullet. Chamber pressures in this situation can and most certainly will go dangerously high [Fig. 22].

There will be ample evidence to confirm the existence of dangerous pressure levels directly attributable to the overlength cases:

  • The shooter may get belted sorely in the cheek.
  • The bolt may have to be hammered open.
  • The case head may present clear indications of too much pressure [Fig. 23].
  • The primer will be excessively flattened.
  • There will be a crater around the firing pin indentation.
  • Brass may have been extruded into the ejector slot. 

If the case head is miked it may show expansion [Fig. 24] - and even half of one thousandth of an inch (.0005") increase in the diameter of the case head is an indication of high pressure.

Case lengthening produced by repeated full-length resizing will shorten case life. Continuous working of the brass between the chamber and die stretches it, eventually producing cracks in the case and finally complete head separation. Hot loads, even if necks are trimmed as called for, speed up this process.

Overlength due to Full-Length Resizing Figures 19 - 24

Priorities & practicality

If optimum accuracy and prolonged case life are important to the reloader, our advice is to neck resize alone whenever practical. There are, of course, situations in which it is advisable to full-length resize. Shooters reloading for pumps, lever-actions, and autoloaders must do so to facilitate reliable chambering. A lever-action, for example, doesn't have the powerful camming action of a bolt action and may not easily chamber cases larger than factory standards. Shooters who reload cases from one bolt action for another will also find it necessary to full-length resize. Even shooters reloading for one bolt action will occasionally need to full-length resize for the sake of easy clearance; over repeated firings, the case may conform more and more tightly to chamber dimensions, making chambering and extraction increasingly difficult.

Bullets & Accuracy

Bullet Seating

While the issue of the cartridge case to chamber fit is of vital importance to the subject of accuracy, bullet seating has a considerable effect on accuracy in many rifles—and on pressure and velocity, as well.

The section ahead of a rifle's chamber just before the rifling starts [Fig. 25 & 26] is called the "leade," throat, or "freebore" and is really the bore of the barrel with the rifling reamed away. As our bottom illustrations show [Fig. 27 & 28], it varies considerably in length and in the angle at which the rifling is cut, depending on the views of different factories, cartridge designers, and barrel and gun makers. Normally, it is quite short and the rifling starts perhaps only 1/32nd of an inch from the bullet, but in some rifles, the freebore might allow over half an inch of bullet travel before it contacts the rifling firmly.

Bullet Seating Figures 25 - 28

bullet travel

To illustrate the effects of variations in bullet travel before the bullet enters the rifling, we'll compare a standard load with adjustments made only in the bullet's seating depth.

In a "normal" load with the bullet seated to allow about 1/32nd of an inch gap [Fig. 29] between the bullet and the initial contact with the rifling, pressure builds very smoothly and steadily even as the bullet takes the rifling. Pressure remains safe throughout the powder burning period [Fig. 30], and the velocity obtained—3,500 feet per second (fps)—is "normal" for this load in this rifle.

Seating the bullet deeper to allow more travel before it takes the rifling, as in these next two illustrations, permits the bullet to get a good running start [Fig. 31]. Powder gasses quickly have more room in which to expand without resistance, and their pressure thus never reaches the "normal" level. Nor does the velocity; with the same powder charge, it only comes to 3,400 fps [Fig. 32].

When the bullet is seated to touch the rifling, as in the accompanying illustrations, it does not move when the pressure is low [Fig. 33], and not having a good run at the rifling as did the other bullets, it takes greatly increased pressure to force it into the rifling. As the rapidly expanding gasses now find less room than they should have at this time in their burning, the pressure rise under these conditions is both rapid and excessive [Fig. 34]. Velocity is high at 3,650 fps—but at the expense of rather dangerous pressure. Many rifles deliver their best groups when bullets are seated just touching the rifling. Seating bullets thus can be done quite safely if the reloader will reduce his charge by a few grains. The lighter load will still produce the "normal" velocity without excessive pressure. 

Bullet Travel Figures 29 - 34

Many rifles deliver their best groups when bullets are seated just touching the rifling. Seating bullets, thus, can be done quite safely if the reloader will reduce the charge by a few grains. The lighter load will still produce the "normal" velocity without excessive pressure.

Standard Loads & Accuracy Fall-Off

This brings up another pointer on accuracy for shooters who may have a few thousand rounds through their rifle barrel and have noted a fall-off in the accuracy they can obtain with their standard loads.

Hot gasses from the shots previously fired through the barrel erode the throat and thus increase the distance a bullet must travel before contacting the rifling.

By loading longer bullets and seating them further out so they'll touch the rifling—making powder charge adjustments as necessary, of course—accuracy can often be improved.

Other Effects on Accuracy

The number of factors affecting the performance of a cartridge is remarkable. We've looked at:

  • The cartridge-to-chamber fit
  • Alignment of the bullet with the bore
  • The depth to which a bullet is seated
  • The amount of travel the bullet undertakes before contacting the rifling

Primer

The primer, a component many reloaders take for granted, may influence performance if it's not up to the task of igniting the densely packed smokeless powder. Some powders burn more uniformly than others and contribute to better accuracy.

Firearm

The firearm itself may be the cause of inaccuracy if:

  • The muzzle is burred
  • The throat is eroded in the barrel
  • The trigger is so jerky the shooter cannot maintain his hold from shot to shot

The stock and action must be properly bedded to maintain a uniform fit, or inaccuracy may result. Even this list does not exhaust the possible causes of poor accuracy.

Hornady® Accuracy Standards

A continuous test program is employed to check on our production quality. Our laboratory is equipped with the finest test barrels available and with machine rests that eliminate human variables in shooting so we can isolate shot-to-shot dispersion associated only with the bullets being tested.

The two targets shown [Fig. 35] were made firing the same bullets but tested on successive days. The small group met our accuracy standards and illustrates the kind of performance we demand of the product. The larger group was fired from bullets produced after the press making them developed only a few thousandths of an inch play in its cup-feeding mechanism. This evidence of maladjustment brought the production to a halt so the press’s problem could be analyzed and corrected.

As we said earlier, accuracy doesn’t just happen. You have to make it happen, by paying constant attention to these vital thousandths and ten-thousandths of an inch. No matter how perfect the basic design of bullets may be, they aren’t going to be consistently accurate unless we make them all to closer tolerances than, say, a Rolls-Royce engine.

Perfect balance is perhaps the most critical factor in bullet accuracy. The attainment of this goal is the major responsibility of design engineers, tool makers, production personnel and plant management.

They have the task of designing production machinery that will maintain near-perfect concentricity in the copper cups from which our jackets are formed in various punch presses. Not only are there multiple steps through which our gilding metal must pass on its way to becoming a finished jacket, the concentricity problem is compounded by our need to internally shape the jacket to control expansion in our hunting bullets. If the finished jacket is not of uniform thickness around its entire circumference, if it varies by even so little as five ten-thousandths of an inch, the resulting bullet may be unbalanced sufficiently to veer from its intended line of flight.

a Closer look

In the accompanying drawings, we will let the green dot represent the center of form of this bullet, a point at the actual dimensional center of the bullet. The red dot indicates the center of gravity of the bullet; both of these points should coincide exactly [Fig. 36]. But, because the jacket of this bullet was made with a thinner wall on one side, there is more lead there [Fig. 37] and the center of balance is moved ever so slightly in the direction of the heavier side, perhaps less than a thousandth of an inch [Fig. 38].

As long as the bullet is in the barrel, it rotates around its center of form [Fig. 39]. But, when it leaves the barrel, it spins around its center of gravity [Fig. 40], and this causes it to veer slightly off its intended course at a tangent to the spiral described by its center of gravity as it went up the bore.

Hornady® Accuracy Standards Figures 35 - 40

Less than half a thousandth of an inch in jacket concentricity can and does have a detrimental effect upon a bullet’s course. And, because we cannot chamber each bullet with its center of gravity similarly aligned in the barrel, subsequent shots will diverge at arbitrary angles, slight though they may be. The final result is a group with more dispersion than we would like.

It is only by minding all those ten-thousandths of an inch and tenths of grains in all stages of production that we are able to make millions of bullets capable of exceptional accuracy and in a variety of calibers having expansion characteristics suitable for target, varmint, and big game hunting.

Reloading Makes an Impact

We believe shooters need and want the kind of accuracy we’ve discussed in this short essay. That’s why the people at Hornady Manufacturing take accuracy so seriously. The effort to produce accurate bullets, to make accuracy happen, is a joint effort involving many individuals, their skills, and their dedication to the final goal.

Doing your shooting with extremely accurate and effective cartridges that you, yourself, have loaded provides more shooting for your money as well as better shooting. The accuracy factors that we discussed have indicated why it is possible to make better ammunition than you can buy.

And reloading can also give the shooter an invaluable sense of pride in his own craftsmanship. It’s the same pride that we at Hornady Manufacturing share in producing quality products that can be used confidently and effectively.