Jim Carmichael in his 1975 book entitled The modern rifle questions whether modern cartridges followed rifle design or today’s rifles came about to meet the demands of modern ballistics. He concluded that during the previous 75 years, few original basic rifle mechanisms and cartridges had been developed for each other. Generally, their developments had been independent.
However common to both cartridge and rifle was a modernization of their components. Clearly, because modern materials such as cleaner and slower burning powders, a .30-06 cartridge today is better than at the time of its introduction. The same can be said of the rifle chambered for the .30-06 cartridge. Steel is better, and depending upon one’s view, a fiberglass or laminated wood is considered to be better than a traditional wood stock.
Design is a bit more difficult to measure. To a large degree, design is a matter of taste and is difficult to find an agreed standard for its measurement. Even so, are rifle and cartridge design generally better today than at the turn of the century? Few would argue that automatic rifles are not clearly more advanced today than at the turn of the century, but can the same be said about other rifle designs such as the bolt action? Only a handful of bolt actions, primarily the turn-bolts, have been successful. Of these, the Mauser-type is the most widely used. The action has been modified over the years, but the basic design continues to be with us. Some would argue that a number of those modifications are not that good; i.e., control feed versus push feed etc. Many of us would agree with these arguments.
A close examination of the evolution of cartridge design, since the turn of the century reveals what appears to be a number of minor but significant changes. Generally, cartridge design has been more varied than that of rifles even though the overall appearance of today’s cartridge appears similar to its ancestor of the turn of the century.
In this essay, I shall examine the evolution of cartridge design during this past century. Of the cartridge components; i.e., bullet, powder, primer and case, the focus will be on the evolution of the cartridge case and will restrict the analysis to prominent generalizations rather than to details such as specific angles of cartridge shoulders. The .404 Jeffery and the .416 Rigby are identified as cartridge cases that have undergone recent parenting. Finally, I shall propose that the .505 Gibbs will be the next cartridge case to parent offsprings.
GENERAL: For a central alignment to the bore of the barrel, the case represents the focal component of the cartridge. The case must change little during maximum pressure and temperatures for it serves to seal the chamber and to prevent uncontrolled discharge of expanding gases. In addition, it must protect the primer and powder from accidental ignitions such as absorbing heat and preventing premature ignition; i.e., a hot chamber.
A variety of cases from different manufacturers are offered to the American shooter. Generally two categories exist: mass producers and specialty producers. Offerings from the mass producers have been consistent for a number of years; but within the past decade, premium handgun cartridges have become popular and within the last several years premium rifle cartridges have become available.
With regard to the specialty producers, we find Lapua Ltd. of Finland, Mast Technology of Las Vegas and Bertram Bullet of Australia, to name a few. Lapua’s cases have been known for quality for a number of years and recently have developed a case to withstand higher than normal pressures. Mast Technology provides high quality cases for Dakota Arms and Lazzeroni Arms. Finally, Bertram cases are best known as European and dangerous game cases. Generalizations can be made for these manufacturers. The quality of their brass is very good to high; special attention is paid to case head uniformity, case neck concentricity and uniformity and flash-hole uniformity; special features are offered such as strengthened webs; and/or the cases are for special or unusual calibers.
First Generalization: The American shooter will become more interested in high quality and specialty cases which will permit higher chamber pressures; i.e., low to mid-60,0000s psi., higher powder volumes, and ease of chambering. The specialty case producers will become more prominent in the future. Those mass-case producers that now offer specialty cases will expand this niche while those manufacturers that do not offer such cases will join the niche market.
ANATOMY – RIMS: Cartridges are identified as either being rimmed, rimless, rebated rim or semi-rimmed.
Rimmed cartridges represent the oldest case head which was ideal for early cartridge designs. Rimmed-case shoulders are long and sloping making it difficult to headspace on the shoulder of the cartridge. Major disadvantage of rim cartridges is the difficulty in reliable feeding from box magazines.
Semi-rimmed cases represented a major step in solving the feeding problem. Semi-rimmed cases usually have pronounced shoulders that allows headspacing from the case’s shoulder.
Rebated are the least common head designs. Logic of the design is to provide increased case capacity without enlarging an existing bolt face. A problem sometimes encountered with this type rim is that, upon chambering, the bolt face may pick up the body of the case rather than the rim.
Finally, rimless represent the most commonly used head-design today. One advantage of the rimless case is that it pushes the cartridge forward in the chamber until it stops at the center of the shoulder which represents the headspace datum point. Theoretically, as the powder begins to burn, the cartridge is held centered within the chamber allowing the bullet to be seated closer to the barrel’s axis.
No matter the type of rim employed, cases with uniform case heads are essential for consistent, accurate shooting. Case head uniformity impacts upon the action-barrel twist during firing which in turn, impacts upon accuracy. If the case head is not perpendicular with the axis of the cartridge, the gap between the case head and the bolt face will change with the chambering of subsequent cartridges.
Second Generalization: Of the several different rim types found at the beginning of the century, the one that became most successful is rimless. This should continue to be the case for the foreseeable future. Rimless cases will either be belted or non-belted with pressure from the shooting community to eliminate the former.
ANATOMY-HEAD COMPONENT (BELTS, WEBS WITH FLASH HOLES & ADJACENT SIDEWALL): At the time of their introduction, belts served as the positive headspacing of rimless cases along with providing a smoother, more reliable cartridge feeding from a box magazine than rimmed cases without belts. Belts were essential to the long, sloping shoulders of cartridges being used in both double and bolt-action rifles at the turn of the century.
The American shooter’s introduction to belted, semi-rimmed cartridge cases was the .300 and .375 Holland and Holland Magnums. The terms “Magnum” and “Belted” are often closely associated, but the association is not warranted. Modern magnums have case shoulders that are more than adequate for headspacing which means the belt serves no useful purpose other than perhaps, as a symbol of early magnum.
Most, if not all, of the current crop of gun writers continue to “plea” for the elimination of belts based upon the arguments that non-belted, semi-rimmed cartridge cases headspace on the center of the shoulder and physically center the cases in the chamber. In addition, semi-rimmed cartridges with belts are more difficult to feed from the magazine to the chamber.
From my prospective, the arguments are good; however, the arguments are based upon the assumption that the cartridge case’s sidewall and web are strong. A bulge in a case is seen where the elastic limit was exceeded and a permanent deformation remained after the pressure was lowered. The step beyond bulge is rupture. Unless the sidewalls and web of the case have adequate thickness, they may split or the base may separate from the body of the case.
A major disadvantage for rimless belt as well as rimless beltless cartridges is the thickness of the sidewall just forward to the web. During ignition, the first part of the cartridge case to expand is the neck and shoulder which closes the chamber by sealing off the expanding gases. During this time, the cartridge’s base is forward to the bolt face and is not supported. As pressures increase, the case is driven back against the bolt face which results in the stretching of the case, particularly the sidewall immediately forward of the web. When the case’s sidewall resist the outward expansion against the chamber, the pressure is more than adequate to stretch the case thereby increasing its length. The sidewall becomes thinner at that stretch point.
Weak webs and sidewalls immediately forward to the web are of particular concern with high-capacity cases that generated pressures greater that 60,000 psi. In modern solid-head cases, the hardness of the brass is the major factor that determines a case’s pressure limit before undergoing plastic deformation. One such example was encountered by Lapua Cartridge Ltd. of Finland with the .338 Lapua Magnum. Other than shortening and necking-down, the major modification included a hardness distribution ranging from the head and web (hard) to the mouth (soft) as well as a strengthened (thicker) case web and sidewall immediately forward of the web. As a result of the hardness distribution, strengthened web and sidewall, Lapua Ltd. is advertising that the .338 case can safely achieve a maximum average pressure of 400 MPa (~ 59,000 psi) and recommend not exceeding 460 MPa (~ 67,000 psi). Note: Using a Fabrique Scientific strain gauge, my tests of pressures up to 63,000 psi have reveal no visible morphological changes in the case. At this pressure primers begin to flatten.
It is generally accepted that flash-hole uniformity, or lack of uniformity, is critical to the proper ignition of powder. Burrs are a serious problems. Without uniformity variation in muzzle velocity becomes evident. Another concern deals with the overall configuration of the flash hole. The thickness of the web influences the configuration as exemplified in the thickened web of the .338 Lapua Magnum case when compared to its parent, the .416 Rigby (Figure 1).
Third Generalization: New-case design is moving toward one that is beltless, with a strengthened web, with improved flash holes and sidewall immediately forward to the web. These features will not eliminate case fatigue, but do prolong case life.
ANATOMY-OVERALL SHAPE: To set the stage for an examination of overall shape of cases, let’s examine exaggerated examples. At the turn of the century, long tapered cases were not unusual. Most were rimmed and originated from black powder cases of the previous century. Wildcatters during the 1950’s and ’60’s focused the logic that increased velocities are good, and they can be achieved with increased powder charges. A goal was set to increase the capacity of the case and to modify powder burning properties. Velocities greater than 3,000 fps were generally set as a goal. However, there can be too much of a good thing. Too much, and the good thing turns bad. Bad is defined as increased chamber pressures.
Ackley & Improved Cases. The key to the logic of Parker O. Ackley and others of the period is the concept of “bore capacity.” Ackley, in his 1962 Handbook for Shooters and Reloaders , defines the term as the measure of efficient powder burning in a restricted space. Variables controlling powder burning include the type of powder, the bore-groove diameter, the pitch of rifling, the sectional density of the bullet, the allowable pressure and finally, the case shape.
Ackley’s definition “feels” good to me but I wonder whether the variables controlling powder burning can be put into a formula and measured? Looking more recently than Ackley, Robert A. Rinker’s definition of “over-bore capacity” which is found in his excellent 1995 book entitled, Understanding Ballistics is “… A quality of a cartridge that has too much volume in relation to the bore. This does not permit the complete and efficient burning of the charge. To a lesser degree, barrel length, bullet weight and caliber are also involved.” A bit better, but not definitive. To me the concept of “bore-capacity” and “over-bore capacity” are easy to comprehend, but difficult to pin down because of the uncertainty of the variables that somehow influence the outcome.
At about the same time of Rinker’s book, Dr. Ken Howell in his 1995 book entitled Designing and Forming Custom Cartridges for Rifles and Handguns expressed frustrations similar to mine. “Gun literature teems with lines that say this or that cartridge is ‘over bore capacity,’ but I’ve never seen, read, or heard of an attempt to define it. So I’m going to stomp out onto thin ice next to a hole over deep water and drum up a definition.” Ken distills Ackley et al. to ” … bore capacity — the largest powder charge that can produce acceptable ballistic efficiency in a gun barrel of a given bore size.”
Ken continues by focusing on the theoretical energy content of powder versus muzzle energies. As an example, he examines the 25 caliber and calculates that 25% of the powder’s energy represents the threshold for this caliber. Similar values (i.e., 20 – 35%) for other calibers have been reported by others, particularly in reloading manuals published by gunpowder manufacturers. So progress is being made, at least with one variable; but I still don’t have a firm grasp of the subject. Until someone can identify and measure all variables, I’m afraid that we shall have to treat bore capacity as being analogous to pornography. Can’t define it, but know it when I see it!
Reading Ackley, I always sensed that he did a great deal of testing in order to challenge his views and logic, but most of his views may have been primarily governed by intuition and logic, each based upon many years of experience. Let’s review some of his intuition and logic. Ackley argues that case shape does have an effect on pressures. Short case versus a long case: in a short case, all of the powder grains are closer to one another versus those in a long tubular case. The latter would extend the ignition period which increases chamber pressure.
Very-tapered cases, with long-tapered shoulders reach high pressures long before their potential bore-capacity is reached. Ackley’s logic is that the powder grains are broken up, compressed and “funneled” trying to get out of the case ahead of the gas spreading ahead of the primer. Thirty-three years later, Rinker’s views are similar and he adds the view that a long-tapered shoulder will cut into bore life.
In contrast to a very tapered case, Ackley points out that an improved case (i.e. Ackley Improved) with fairly straight-sided walls, with a shoulder slope of 30 to 35 degrees, also moves the powder forward, but it compresses only from the rear — not from the sides of the chamber. The powder is supposedly blown into the throat of the barrel as a plug of bore diameter. The powder in the outer parts of the case has time to burn in the case, while the compressed center portion is ignited behind the bullet.
In addition, Ackley argued that erosion is partly due to the design of certain cases. The offenders have shoulder lines which converge to a point with the bore rather than in the neck of the case. With the former, the gas-velocities blow unburned grains of powder against the sides of the bore with such force that they serve to cut into the bore.
Rinker adds that big powder charges are better controlled in a case with a convex shoulder, while he states that it is debatable whether or not that sharp shoulder performs better in retaining the igniting powder within the cartridge case.
So far, the logic sounds good to me. I tend to agree with Rinker that cartridge cases of the future should have steeper and sharper shoulders rather than long-sloping shoulders. Shoulders with 25-35 degree should be ideal in retaining the powder in the case longer thus permitting better and perhaps more complete combustion (not to omit the reduction in brass flow), as well as providing a slight increase in powder load.
Reasonable logic supported by men we respect is probably the “logical” way to go. But there’s always a zinger. . . . a zinger waiting for us to become complacent. . . . a zinger that has been around for awhile that we tend to forget. Here’s the zinger. Rinker, in his book, points out that a series of tests was described in The American Rifleman during the summer of 1946 and mentioned again in 1981. Clearly Ackley had to be aware of the article. Three cartridge cases, all of the same capacity and all necked to fit identical .22 bullets, served as the variables in the test. Differences were one case had a conventional body taper with a long 14-degree shoulder, the second had a very long body taper with a 25 – 35-degree shoulder, and the last had little body taper with a concave-radius shoulder. Velocities and pressure were measured in all three cases. The latter by both electronic-transducer and copper-crusher methods. You guessed it, ballistic performance was the same within experimental error.
My sense is that some body taper is desirable while extreme taper or no taper are not desirable features. Extreme taper in a cartridge case may lead to a case longer than necessary, requiring a longer action, and may contribute to difficulty feeding from a magazine while, on the other hand, no taper or a straight-wall cartridge case may make it difficult to center when chambering. There is a modest increase in case capacity with a sharp shoulder.
Fourth Generalization: Case shape of the future is typified by the cartridge cases from Dakota and Lazzeroni. The body will be slightly tapered and the shoulder will be between 25 to 30 degrees with 35 degrees representing an extreme angle.
ANATOMY-VOLUME: It’s generally known that case volume of the cartridge varies from one manufacturer to another. In fact, case volume will vary between lots of the same manufacturer. The reason that this is important is that case volume plays a major role in the pressure and velocity properties of a cartridge. Cases from specialty manufacturers tend to maintain the same case volume from lot to lot. However, they would be the first to point out that the reloader should expect some minor difference.
One of the major goals of wildcatters was and is to increase the volume of cases while keeping the caliber of the bullet constant. A direct approach would be to construct a bigger case. To do so would result in the body of the case either being longer or with a larger diameter — or both. With a longer case, the rim diameter (and thus the inner-bolt size) could remain constant, but the action might require lengthening. As mentioned earlier, the consensus favors a short case over a long case when the volume remains the same. By increasing the case’s diameter, while maintaining its length, results in an increased-rim diameter (and thus the inner-bolt size). This would require a bolt face, but not necessarily an action, with a greater diameter. One could improvise, as Winchester did with its .284 or as Jim Davenport did with the .585 Nyati, by attaching a rebated rim, which is smaller than the diameter of the case body. As mentioned earlier, rebated rims can lead to feeding problems.
Another direct approach would be to find a larger, existing case and modify; i.e., shortening etc. Finding a larger case and modifying is the easier of the two approaches as the cost of designing a new case would be prohibitive. As a result, a number of cases became famous because they have become parental cases to a large number of wildcats. I’m attempting to make a complete list of parental cases and their offsprings. For example, the latest count of necked down (not necked up) offsprings from the .30-06 Springfield (a long action cartridge) is 69 and from the .300 H&H (a magnum action cartridge) is 37. The list has not been completed.
A less direct approach in the past was to “improve” the cartridge by blowing out the case in an “improved” chamber. This usually sharpens the shoulders of the case and eliminates the body taper so that its powder capacity increases — usually not by much. The literature during the heyday of “improving” is full of stories of how hard or how easy it is to improve certain cases. In some instances the improvement did not outweigh the steps needed to make the improvement. My incomplete list of improved offsprings from parental cases totals 72, many created by Ackley.
Today, attempts to find ways to increased volume of powder for a given bullet caliber is alive and well. Most attempts are seen with the large capacity cases — large capacity cases from the turn of the century; i.e., dangerous game cartridges. Again, two approaches are being used: designing a new case or modifying an existing case. With the former, John (Lazz) Lazzeroni of Lazzeroni Arms offers a range of cartridges based upon a novel beltless case which is manufactured by Mast Technology. The beltless case parents the 6.53 mm Scramjet, 7.21 mm Fire-Hawk, 7.82 Warbird and the 8.59 mm Titan. Apparently novel in design, the beltless case is reminiscent of the .416 Rigby.
Recent examples of using modified cases include the 7 mm STW parented by the belted .375 H&H (via 8 mm Remington Magnum.) as well as the Dakota series which include the 7 mm, .300, .330, .375 and .416 Dakotas, all parented by the beltless .404 Jeffery. The beltless .416 Rigby has parented the .30 Lapua Magnum., .30 Cody Express, .300 Phoenix, .338-.416 Barret, .338-.416 Rigby and the .338 Lapua Magnum — the first and last cartridge are offered by Lapua Ltd. while the others are wildcats.
Fifth Generalization: Case volumes per a given caliber will continue to be increased until that caliber reaches its “overbore capacity.” The days of increasing the volume by enlarging the case, from the manufacturer’s original bullet/case combination, in an improved chamber are probably over. The new approach is to utilize a larger case, particularly one that will be able to serve as a parental case for several calibers — all smaller than what the case was originally designed; i.e., Dakota, Lazzeroni etc. With increased volume comes increased pressures. Webs and sidewalls immediately adjacent to the webs will have to accommodate these increased pressures. This generalization does not address some of the newer powders that generate lower pressures.
ANATOMY-NECKS: Howell points out that the purpose of case necks is to hold “… the bullet tight for ballistic efficiency, stability, and consistency — and in line with the axis of the bore….” Thus, case neck concentricity affects bullet-bore alignment. If the case neck is not centered, the bullet will enter the rifling with a yaw and accuracy will be altered. It is generally known by the handloader that the wall thickness of the case neck often varies from one case to the next and from one side of the neck to the other side.
With regard to the proper neck-to-body ratio, Ken points out that a short neck sacrifices the neck’s grip on the bullet while a longer neck sacrifices some powder capacity; in some instances this can be true with a short neck. Finally, Howell feels the minimum neck length should equal the diameter of the bullet. A rule of thumb is to seat a bullet a minimum of one diameter. This may be difficult with some of the new, very low drag (VLD) bullets; i.e., very long bullets that may have to be seeded deep into the case in order to accommodate magazine-size limitation.
My experience is that there is something sensual about a cartridge’s neck to body ratio. Many, including myself, can look at a cartridge and sense if it’s a winner or not. Even though we are capable of sensing, it doesn’t mean we are always correct. This sense seems to come from whether the neck to body ratio “looks right” or not.
Sixth Generalization: The proportioned neck to body ratio seen in the past will continue in the future.
CONSIDERATIONS: Why was cartridge case shape from the 60’s and early ’70s so important to the overall evolution of case design? My sense is that in the ’60’s and early ’70’s Ackley and others were trying to assess their progress to date so as to continue to create new cartridge case designs that would yield higher bullet velocities. But was that possible? Were the velocities going up? Clearly, Ackley and others knew that the limiting factor to bullet velocity was chamber pressure. Perhaps Ackley was not intimidated, but clearly others were apprehensive of high-chamber pressures.
In 1975, Carmichael summed up the heyday of wildcatting when he observed that the reason wildcatting died out was that during the early 50s, chronographs were rare. As a result velocities were estimated – usually to the optimistic side. When chronographs became available, the velocities of many wildcats were found to be ordinary. Extra powder caused excess pressure and excess barrel wear.
Thus after the advent of smokeless powder, the goal of cartridge makers, wildcatters like Ackley and to some extent, manufacturers, had been to increase bullet velocity. To a degree they were successful, but increased powder loads that led to over-bore capacity resulted in high-chamber pressures, increased barrel erosion with little gain in velocity. This was the end to changes of overall shape of cartridges.
Today, do chamber pressures limit bullet velocity? If so, can continuation of case design reduce chamber pressure while increasing bullet velocity? Apparently not yet; however, the introduction of slower burning powders has been able to increase bullet velocity while maintaining or lowering chamber pressures using existing cases.
The introduction of most current reloading manuals will briefly review internal ballistics. Even today, chamber pressure versus bullet velocity is paramount to these reviews. For example, in VihtaVuori Oy’s Reloading Manual, 2nd ed. (as well as other manuals), we are told that chamber pressure and bullet velocity are affected by several factors. The word “several” turns out to be eight including chemical composition of the powder, burning rate of the powder, characteristics of the primer, properties of the case etc.
One can find a common theme that applies to these abbreviated descriptions of internal ballistics: burning characteristics of powder. Three of the more important characteristics are chemical composition, geometry of the powder grain and surface treatment of the powder grain. Today, we are seeing traditional single base powders being improved by adding energy enhancers (i.e., nitroglycerol etc.), by increasing the burning surface area of the powder grain and by adding retardant agents to the surface of the powder grain in order to slow the combustion process. The goal of these improvements is to achieve the required muzzle velocities with lower maximum pressures. Within the last two years, major cartridge manufactures have been selling rifle cartridges made with powders possessing these characteristics.
ONGOING EMERGENCE OF PARENTAL TO OFFSPRING CARTRIDGE CASES: We are now witnessing the emergence of new cartridge cases from pre-existing (parental) cases which can be divided into two categories. The first category is represented by cartridges that yield maximum muzzle velocity and energy and will fit into the shortest possible action; i.e., either medium, long and magnum, as appropriate. The goal of the cartridge is medium (150 to 250 yards) to medium-long range (300 to 500 yards) hunting, with maximum knock-down power using the lightest possible rifle.
Temporally, the second category is following the first and we are beginning to see it now. It is represented by cartridges that will be used for long to long-long range (500 to 1,500 yards and on) game hunting, paper shooting and tactical applications. Here, the primary goals are cartridges that yield maximum distance with adequate energy per given application. Of the three, game hunting may be a bit controversial unless it is long-range varmint shooting. With paper shooting, the goal will be to send a bullet with the highest ballistic coefficient possible the greatest distance and hit the a paper target. Tactical applications would be similar to paper shooting except terminal ballistics becomes an important consideration.
Cartridges that fit both categories are already here. With the first category, they can be found in calibers from 7 mm to .416, using the .404 Jeffery and .416 Rigby as parental cases, as exemplified with the Dakota and Lazzeroni series of cartridges. These are rimless, beltless cases with slightly tapered sides for easy feeding from the magazine to the chamber, a medium to slightly exaggerated shoulder slope up to 30 degrees, and lastly, “a well proportioned neck to body ratio.”
To some extent the .50 Browning Machine Gun (BMG) which has been used as a parental case down to a phonograph needle (just kidding) . . . has been necked down to .375 for long-range large game hunting by wildcatters and some manufacturers; i.e., State Arms. Receivers are too long and the rifle is too heavy to be carried by anything other than a mule.
The second category, lagging somewhat behind the first, are found primarily with the beltless. However, with long-long range hunting, the belted .30-378 Weatherby is a candidate for varmint while the belted .300 Winchester Magnum is marginal. With long-long range paper shooting, one can find variations of large capacity 30 and 338 calibers found in proprietary and wildcat cartridges. Thousand-yard benchrest shooters have their favorites. With the 1000-yard record becoming smaller and smaller, the next challenge will be matches with distances greater than 1000 yards. Finally, long-long range tactical is represented by the .338 Lapua Magnum and to a lesser extent, the .300 Winchester Magnum.
Long-long range paper and long-long range tactical shootings have a number of factors in common with the exception of terminal ballistics. The .338 Lapua Magnum, as a tactical cartridge, has come into vogue. Origin of the .338 Lapua Magnum cartridge is the Research Armament Industries’ (RAI) 8.58x71mm cartridge, based on a necked-down and shorten by 0.18″ .416 Rigby case. It originated in the early 1980s for a new .338/7.62x51mm NATO convertible Model 300 sniper rifle RAI built for the U.S. Navy. The Model 300 was the step below the .50 RAI Model 500, better known as the Haskins rifle.
Accuracy International promotes its .338 Lapua Magnum system as a lighter alternative to the .50 caliber bullet. Lapua Ltd. claims that it’s 250-gr. Lock-Base bullet with a ballistic coefficient of 0.662, and a velocity of 3,020 fps will yield 1/3 MOA at 500 meters and 1/2 MOA at 1,000-meter shot with a Keberst rifle. This would offer substantial improvements in downrange energy and effective range over the .300 Winchester Magnum.
The Sierra 300-gr. HPBT MatchKing bullet (#9245) has a sectional density of 0.375 while the Lapua 250-gr. Lock-Base bullet (B408) has a sectional density of 0.313. When sectional density is related to velocity, momentum is created. Even though momentum is not the same as bullet energy, momentum is a useful measurement and contributes to the ballistic coefficient. The ballistic coefficient of the Lapua 250-gr. Lock-Base bullet is 0.662 while that of the Sierra 300-gr. HPBT MatchKing is 0.768. The higher the ballistic coefficient, the higher the downrange velocity. Based upon a greater sectional density (bullet going farther) and a greater ballistic coefficient (bullet with greater velocity), the Sierra 300-gr. HPBT match with a precision rifle should be outstanding for long-long range paper and tactical shooting.
It appears that the modified .416 Rigby (aka .338 Lapua Magnum) might be finished for parenting new offsprings. A current ongoing project of mine explores a spectrum of .338 Lapua Magnum offsprings. I am using the .338 Lapua Magnum case as a parent to the 7 mm, .30 and .375. At this stage, my view is that the 7 mm-.338 Lapua Magnum will be over-bore, but its external ballistics should be better than the 7mm Dakota and the 7mm STW. The .30-.338 Lapua should be better than the .30-.378 Weatherby and the .375-.338 Lapua (seated to fit a TRG-S magazine) will be probably under-bore and its ballistics not very exciting (Figure 2). Data will be presented in a subsequent article.
What would be the next parental case? Reviewing the table entitled, “Comparisons of Large Bore Rimless Cases” the .505 Gibbs, .585 Nyati and the .585 Van Horn Express appear to be possibilities. Of the three, the .505 Gibbs has the smallest volume with the .585 Nyati and the .585 Van Horn Express being larger but approximately equal in volume. The .585 Nyati is composed of a .570 Nitro case and .505 rebated rim. The .585 Van Horn has a rim diameter that is greater than the .505 Gibbs and the .585 Nyati, which have the same rim diameter. This cartridge would require a greater bolt face diameter than the other two.
From my perspective, the disadvantage of the .585 Nyati is the rebated rim which may lead to chambering problems while the disadvantage of the .585 Van Horn is a large rim which would require a large bolt face. Even though smaller in volume, I’m betting the .505 Gibbs will be the next to be selected to serve as a parental case with .375, perhaps, representing the limits of the over-bore spectrum.
