A Look at Shotshell Ballistics

By Randy Wakeman

This is not the type of article written for entertainment. If you happen to just want a shotgun that goes ‘bang’ and have no interest in patterning your own shotgun, I’m sorry, this won’t be of any interest to you. We must appreciate what representative sampling is, the difference between anecdotal "evidence" and empirical data as opposed to theoretical or emotional data.

Can you possibly tell me what brand of ammo will shoot best out of my Marlin 39A rimfire? Let's try a more specific question; how many #7-1/2 6% antimony pellets from a 1 oz., 1200 fps load in my A-5 20 Mag will land in a 30 inch circle at 40 yards with a factory Browning Invector choke marked "Modified"?

Naturally, there can be no answer, it is all test and trial. Does my A-5 shoot to the point of aim, or not? Does it shoot to the point of aim at 20 yards? 50 yards? With what loads?

There is no possible way I can look at a shotgun and a shotshell and tell you what POI will be at range, and I sure can't tell you what the next pattern will look like. I can't draw an accurate picture of it; no one can. Oberfell & Thompson picked the right title for their book, to be sure. Whether we choose to think of it this way or not, as O. & T. said, "the distribution of pellets over the pattern area is governed by the theory of probability, sometimes called the doctrine or rule of chance." All we are attempting to do with patterning is attempting to predict future results.

Without individual patterning, we are shooting blind. There is nothing particularly "real-world" about breaking an artificial clay pigeon tossed in the air by a mechanical device. No professional ballistician would consider such an event as being meaningful at all when attempting to predict pattern performance. One pattern itself is meaningless; once-incident reporting is bad data. Five is better, ten consecutive shots far better yet.

It is hard to separate emotion from the actual and factual. Brand loyalty and enthusiasm for a particular gun, gauge, or shell does not accurately predict patterns. Good patterns are where you find them, and they really have to be found.

I've been very successful at being unsuccessful. I have been able to produce horrible, inefficient, ineffective patterns from every brand of shell, every gauge, every choke constriction, and every brand of shotgun I've ever tested. I've had "cylinder" chokes that have produced modified performance, and "full" marked chokes that gave barely "improved cylinder" performance.

Folks ask all the time what "the best choke is" for "xxx." They do it all the time; they do it right here in letters to the editor. And, they get "answers." Many, many answers. All these answers without knowing the gauge, the gun, the shot size, the shot's hardness, the payload size, the muzzle velocity, the range the dove is being taken at (for example) and so forth. A "scientist" should find this quite amazing, for even knowing all this no precise pattern can be predicted from a shotgun, only generalities. Not knowing all of these things makes rumination about patterns absurd. To find out what your gun may or may not do there is no substitute for pulling the trigger, nothing even close.

Ignorance can be bliss, and if I didn't pattern my own shotguns I might be a bit more blissfully ignorant than I am today. I know people don't think they need to pattern, and most people can't be bothered to do so. We all happier thinking our shotguns shoot to point of aim, and our patterns are "good." Fine by me, it is naturally none of my business what folks care to hurl through the air.

Selection of guns, choke, and shotshell by personal patterns makes anyone a far more effective shooter, whether breaking clays or picking up more birds. Most folks I know are FAR better shots than they think they are; it is just that they haven't taken the time to find the most effective pattern for their needs.

Scattergun ballistics are rarely looked at, but having a working knowledge of the salient points can help make all of us more successful hunters and shooters. It is just as important to understand the basics of exterior and wounding ballistics with shotguns as it is with any other firearm.

Round balls are horribly ballistic inefficient projectiles by comparison to conical bullets, that much is common knowledge. They are also not particular stable in flight, unless we spin them. One of the greatest advances in small arms accuracy, in history, was the introduction of the rifled barrel. Where smoothbores were horribly inaccurate, rifled muskets were a quantum leap forward in accuracy. Though patch for the ball and rifling twist cannot possibly change the aerodynamic deficiencies of round balls, it can and does make them more stable in flight and give us accuracy. We don’t have that luxury with shot payloads, but we have a lot of pellets to help us out. We still shoot patterns, not groups, but patterns can be placed on game with great effectiveness.

Perfectly round pellets beat out of round or flattened pellets quite handily; a flat surface pushes too much air. Technically, a perfectly smooth surface can increase drag compared to a surface that breaks up air pressure differential of a sphere like the dimples on a spinning golf ball, 336 for American, 330 for British golf balls. The dimples create turbulence; the rotation of the ball carries air round with it. Big differences, though, a golf ball can spin as fast as 8000 rpm. We induce no such spin to shotshell pellets. Other shapes, like conicals, are far more aerodynamic than golf balls or round balls in general, but I suppose they are not as easy to putt with.

It is the “Magnus Effect” (named for German Physicist Hienrich Gustav Magnus) that accounts for many of the esoteric movements of balls in several sports such as curveballs in baseball, slices in golf, and perhaps most noticeably in ping-pong, or table tennis. Though this stuff can make, and has made some interesting fodder for ad-copy, it is not relevant to shotshell ballistics. Tiger Woods can launch a drive at 180 mph; a 1200 fps shotshell equates to 880.68 miles per hour. More relevant is density. A golf ball is very close in density to that of water (water being 1.00 g / cc). Elemental lead is 11.35 g / cc, over eleven times as dense, so it becomes obvious based on velocity, induced spin, and density that comparing golf balls to shotshell pellets is a ridiculous notion.

Mass and density are the keys to quantifying shotshell range. Antimony hardens lead, yet antimony is lighter than lead and more expensive. To reduce pellet deformation on initial setback, we add antimony, even though it makes the shot less dense. Every high performance target load made today has high-antimony lead shot for that reason. We know from testing that round wins downrange in shotshells. In a technical report from the Canadian Police Research Centre (TR-03-98, Sept., 1995) involved extensive testing. In this series of tests, “Various lead shot sizes were fired into bare gelatin, gelatin covered by heavy clothing and gelatin embedded with swine rib at three different distances. Shot deformation, and penetration were observed and recorded.” Wounding ballistics expert Duncan MacPherson noted “Sphericity must be obtained to insure desirable and repeatable performance.” Perfectly round shot not only wins from an exterior ballistics perspective, it also wins in wounding ballistics. As Dr. Martin Fackler, an authority on wounding ballistics, has noted time and time again, penetration is paramount in effective terminal performance. Round shot penetrates deeper than shot that is not.

Though rounder is better, we still have a problem aerodynamically; the round ball gives us a miserably poor return for increased velocity and recoil. Consider the horrid flight characteristics of #8 lead shot as published by Lyman: nearly half of our muzzle velocity has vanished by the time we hit 40 yards. A 1220 fps load of #8 shot has only 665 fps left at 40 yards. There is only one way to better time of light, decrease drop, and decrease windage, and increase impact velocity (and thus penetration) with a 1220 fps load of the same density; that being the use of larger shot. We increase velocity as well, but that may quickly give us diminishing returns in terms of pattern quality, recoil, and is generally not particularly effective. No matter what we do with #8 shot, it won’t equal #4 shot. Number four shot, launched at 1220 fps, retains nearly the same velocity at 50% more range (655 fps @ 60 yards) than our #8 shot has at 40 yards.

Just as sectional density is a useful barometer of terminal performance and penetration in rifle bullets; so it is with shot. Sectional Density is calculated by dividing a bullets weight by its diameter. With all other variables being equal, a bullet with a higher SD will lose less velocity, shoot flatter (less drop and windage), and offer better penetration. More weight (density) is our only path with spheres of the same diameter.

It boils down to generalizations, not absolutes. Even those skilled in the art have their proclivities upon relying upon the unreliable, the intangible, the emotional conclusions brought on by subjective observations rather than repeatable clinical studies.

We can show that a 99% chance of a 4-pellet hit on the vitals of game birds is sufficient pattern quality for quick, clean kills. We can show that hard lead is a superior material for shotshell use, compared to most readily available substances. We can show that more pellets give us the potential for denser patterns.

We can show that as ranges increase, we need tighter chokes, and larger pellets to achieve the same pattern quality. We also need larger pellets at longer distances to give us similar penetration to that achieved at close range.

We know that excessive recoil from shotguns hinders follow up shots, and that shotgun recoil can exceed levels considered dangerous in shoulder-fired rifles. Payload and velocity increases are not without practical limit, and velocity increases with spheres give us very little return on investment.

We know that neither gauge, brand of shell, brand of shotgun, nor the markings on a screw choke equates to any specific pattern percentage, at any range. We also know that none of these elements equates to effective pattern spread or evenness.

We know that significantly lighter shot materials than lead decrease effective range. Lead showing up as 11.35 g / cc on the periodic table. We know that “steel” loads (7.86 grams / cc), even when loaded with shot sizes three sizes larger than lead are horrible cripplers when attempted to be used at the same ranges as the better lead loads. Where extremely competent duck hunters have demonstrated the ability to cleanly, ethically take their birds at 60 yards, these same seasoned hunters now have struggled with an approximate 35-40% decrease in effective range. Even the short-lived revival of the 10 gauge and the introduction of the 3-1/2 in. steel shotshell has in no way allowed approaching, much less the duplication of the now traditional “old” hard lead loads in effective range.

Patterning alone, in our gun, quantifies a reasonable prediction of what our next shot may produce at a given range; the range we want to shoot at. As the ranges grow, our pellet size must increase in concert to offer the same penetration potential. This is with the assumption that that the pellets are of the same gravimetric density and sphericity, lead being the benchmark. The only practical approach of bettering wounding ballistics at longer ranges or on tougher birds using pellets of the same size is to use denser shot materials, rounder shot materials as they exit from the muzzle, or more velocity, with velocity giving the worst return on investment downrange.

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Copyright 2007 by Randy Wakeman. All rights reserved.