SMc ™: Introducing a New Era
M.L. (Mic) McPherson and Byrom Smalley, January 2002
Synopsis: After more than a year of work, the authors — McPherson is a ballistics and handloading expert; Smalley is a retired solid-fuel rocket motor design engineer — have concluded that an optimum chamber design does exist and that during the history of cartridge design, steady progress — based upon evaluation of empirical results — has approached but has not achieved that ideal design. We are sufficiently convinced we have solved this problem so that, after considerable soul searching, we embarked upon the expensive and time consuming effort toward patenting various principles we identified as being ideal. These characteristics promise superior accuracy and ballistics along with improved barrel life and efficiency and other benefits.
As long-time PS readers may know, that I have followed a quest toward understanding if any particular chamber design is inherently superior and, if so, what it is that makes a good chamber design and, thereby, what constitutes the ultimate chamber design. To this end, I have formerly explored unusually short and fat cartridges and those using spherical shoulder designs. Other than the unusual difficulties emanating from requisite case forming operations in such highly shortened designs (refer to an earlier Precision Shooting magazine article, Breeding Carbide Toothed Beavers), I have seen no evidence that such designs have any inherent flaws.
In response to that article, Byrom Smalley, retired Principal Engineer on the Thiokol solid-fuel rocket motor design team, called to suggest that I was on to something. A question posed in that piece, “…does an ideal design exist…?” could, Smalley believed, be answered in the affirmative. Moreover, Smalley believed that by working together, he and I could identify and develop that design.
Nearly a year later, we had resolved the fundamental issues to achieve an understanding of what really happens inside a cartridge, after primer ignition and before the bullet leaves the muzzle. With that information in hand, it was a simple matter to prove a specific design was ideal from the perspective of maximizing barrel life, accuracy and ballistic potential, for any given case capacity and in any given bore size.
We determined that the ideal design should have an internal body diameter at least twice bullet diameter and that the case shoulder interior should be elliptical and lack any significant reverse radius at the shoulder-to-neck junction (in relatively smaller capacity versions a spherical shoulder works almost as well and has some advantages). Each of these characteristics — ratio of interior case diameter to bullet diameter, body-to-shoulder transition design, shoulder configuration, lack of a significant shoulder-to-neck radius and several other characteristics are covered in our patent application.
We formed a corporation and filed the SMc trademark, for cartridge headstamps and associated marketing purposes. (SMc stands for Smalley/McPherson and is pronounced, SMACK or SMICK — take your pick.)
We must note that, for various reasons, our decision to patent this invention came only after considerable debate and consideration — this was not an easy decision for either of us. On the one side, this path is in direct opposition to what had come before.
Historically, such design work has been wide open to the individual experimenter, the resulting wildcatting has been most beneficial to the shooting sports and we did not want to dampen that aspect of this game. Moreover, those who had formerly attempted to benefit from the patenting of a specific cartridge (or chamber) design were effectively foiled by the simple fact that any minor design change — e. g., a difference of a few thousandths of an inch in any pertinent dimension — constituted the creation of a new chambering, one that would not be covered by any dimensionally-based patent.
However, the situation with our invention is quite different. Because our effort is based upon design features, rather than specific dimensions, and because it incorporates unique parametric design characteristics (such as shoulder design), dimensionally-specific limitations do not apply — cartridges based upon minor design alterations of an SMc design will still incorporate the key (patented) design characteristics and will, therefore, still be covered by this precept. Moreover, this entire concept falls under the province of intellectual property — we have gone where none has gone before in development of idealized cartridge (or chamber) designs. Chiefly for these reasons, we decided to undertake the expense of patenting these concepts.
For various reasons, having to do with the complexity of the issues involved, concerns over patent issues and simple lack of available space, the following discussion is not fully developed and it is limited in scope and completeness. We are satisfied that we can demonstrate each of the characteristics involved and that we can do so, based upon fundamental principles or derivatives thereof.
As an overview, the critical issue is what happens to a propellant charge after the primer ignites. In a typical rifle cartridge it is proven that the primer never directly ignites every granule in the charge — some portion of the charge, which is a continuous mass located most distal from the primer (behind the bullet), is merely compressed by that blast. Henceforth we will refer to this heterogeneous mass (granules and entrained gases forming an essentially impermeable mass) as the propellant mass, therewith the analogy with solid rocket motors. This mass initially ignites only along the rear face. This gives two distinct burning regimes — those granules ignited along the entire surface and those burning only along the exposed (rearward) face of the propellant mass.
The former will burn in accordance with well-understood characteristics related to adsorbed deterrent surface treatments and confining pressure (temperature). The latter will burn as a function of confining pressure and the bulk burning rate of the average granule composition (with a significant increase in effective burning rate resulting from increased exposed surface area, due to continuous exposure of included voids and differential burning rate within the exposed granule layer — this burning face will retain a texture, which will increase surface area).
Failure of the primer to ignite granules within the compressed propellant mass stems from three distinct factors:
1.) No significant pathway exists whereby primer-generated gases, or those from nascent propellant ignition, can freely pass through the charge mass — case, barrel and bullet provide a nearly perfect seal in front of and to the sides of this mass (this eliminates significant heat transfer into the propellant mass);
2.) Squeezing of propellant mass (granule deformation and compression of entrained gases) soon seals off any existing permeability so that energetic gases cannot continue to infiltrate this mass, in response to increasing gas pressure behind this mass;
3.) As primer gases pass through the charge mass, those gases rapidly lose energy to the cool granules; this attenuation eventually renders primer-generated gases impotent toward further granule ignition.
It is worth noting that quenching of granules that happen to be adjacent to the case walls or to the bullet base can also render impinging primer gases incompetent toward ignition. This is one reason we find unignited granules expelled from combinations such as a 38 Special, firing a load using 2.7 grains of Bullseye, which ignites quite easily, despite ignition by a primer that easily touches off a load using more than 20 grains of H110, which is very difficult to ignite. While we cover a mitigation of this problem in another aspect of our patent, said aspect is of little interest to most modern target- or hunting-rifle shooters so we will ignore it herein. However, those aspects of our patent can significantly benefit shooters using typical blackpowder and handgun cartridges; we anticipate covering this information in a future article.
As nascent combustion of ignited granules creates gases that increase pressure within the combustion chamber, granules and entrained gases within the propellant mass continue to deform. Through this process, propellant mass volume decreases by perhaps 15%. For complicated reasons (including compressibility factors), little additional compression occurs beyond that point — as applied pressure exceeds about 3000 psi, the relatively incompressible granules deform significantly, resulting compression of entrained gases heats and pressurizes those until gas density becomes sufficient that ideal gas laws no longer apply.
Compression of gases within trapped inter-granule pockets does result in significant adiabatic heating of those gases but those pockets are generally far too small to contain sufficient heat energy to provide granule ignition through this means. Conversely, in rocket motors, larger gas pockets can and do result in such point ignitions, thus so many early explosive failures in solid-fuel rocketry! Similarly, this effect might contribute to detonations when careless handloaders use relatively reduced charges of slow burning propellants in large cartridges.
Depending upon many variables, as chamber pressure reaches perhaps 3000 psi, the bullet begins to move into the barrel. At this point, one of several things can happen. Generally, in a typical bottlenecked case, a shear zone forms through the propellant mass and thereby an essentially impermeable, bore-diameter plug of unignited granules pushes the bullet into the bore while a cylindrical mass remains trapped behind the case shoulder. (One could say the plug merely follows the bullet into the bore but that is not accurate; because this plug is largely impermeable, it must transfer some portion of the combustion force to the bullet.)
Vagaries of shoulder design significantly affect what happens to the cylindrical portion of the propellant mass. Any of the following things can happen to some or all of this mass: squeeze down and extrude into bore; tear asunder — as granules exposed at the interior surface are ripped free by turbulence generated in rapidly passing gases; remain trapped behind the case shoulder.
In any instance, this shear zone (created as a bullet-diameter plug of propellant pushes the bullet into the bore) creates a substantial increase in ignited surface area. The burning front at the exposed surface of the propellant mass thereby extends forward along the surfaces of this shear zone. Thereafter, propellant within four distinct burning regimes exist:
1.) Individual granules that had been ignited by the primer and continue to burn;
2.) Granules that have been separated from the cylindrical mass (propellant trapped behind case shoulder) and beginning to burn along the entire surface — similar to (1) but not as far along toward complete combustion;
3.) Cylindrical mass trapped behind case shoulder, burning along base and some portion of shear surface (interior wall);
4.) Plug, burning along base and along rearward portion of exposed shear surface (as the front end of the plug moves into the case neck, quenching between the relatively cool case neck and propellant granules retards or prevents ignition along the perimeter of the foremost portion of the plug) — if the case neck is long enough!, which is a critical factor in barrel life and explains why short-necked case designs are notorious for giving short barrel life and, sometimes, ballistic fliers (ballisticians refer to such designs as poorly behaved, the 243 Winchester is the single most notorious example).
One can easily prove that the ideal goal for ballistic efficiency and uniformity is to have every propellant granule ignited before bullet movement begins. Consider that any granule that does not ignite at all or that does not ignite until the bullet has already exited the bore cannot contribute anything to bullet acceleration and that such a granule must absorb some of the energy that would otherwise contribute to bullet acceleration — either through granule heating or through granule acceleration or through both effects. Then consider that the sooner any given granule ignites, the longer the energetic gases generated by the resulting combustion will have to work on the bullet — more time, more work.
Because, in any conventional cartridge, it is fundamentally impossible to achieve ignition of all granules before the bullet begins to move, we opted for the next best thing — ignition of as many granules as soon as possible and uniform burnout of the remainder. For this reason, we optimized our design for several important characteristics, which work toward maximizing the percentage of propellant granules ignited at any point during bullet acceleration toward the muzzle.
The following characteristics apply:
1.) An unusually short and fat case design increases percentage of granules directly ignited by the primer.
2.) A two-to-one ratio (approximate value, ideal is likely closer to 2.1:1) between the diameter of the case interior and the bullet contributes to simultaneous burnout of plug and cylinder.
3.) Single-radius shoulder (body-to-shoulder radius only) helps trap cylindrical propellant mass, thereby exposing this material to the highest possible pressure (temperature).
Owing to gas acceleration through the bore, pressure drops as a function of distance from interior face of case web. Combustion rate depends upon pressure, and granules entrained into the bore experience progressively less pressure as those accelerate away from the chamber. Because propellant is a progressive-burning substance, this pressure differential results in a significant difference in burning rate.
4.) Cylindrical mass trapping also limits energy losses associated with acceleration of unignited solids and the bore erosion such burning particles induce.
5.) Elliptical shoulder design minimizes primer-generated shock energy transfer to bullet base. This minimizes potential for the primer blast to prematurely move bullet. This design also focuses waste energy from the primer blast into propellant located directly behind the bullet base (this portion of the charge is the last to ignite and is therefore least contributory to bullet acceleration); the resulting compressive heating of this portion of the charge speeds subsequent ignition and burning of this portion of the propellant charge.
6.) For any given case volume, compared to more conventional cartridge designs, the unusually short and fat configuration of these cartridges reduces case interior surface area. This reduces heat loss and thereby further improves efficiency.
7.) This design reduces heat transfer to the case because the trapped propellant cylinder provides an insulating boundary until that material burns through to the case wall.
We could also argue for a reduction of shock-induced barrel vibrations resulting from the primer blast because the SMc design spreads out (in time) this shock wave impact on the case shoulder. Arguably, this should contribute to improved accuracy potential — chiefly, because it will reduce the magnitude of barrel vibrations; thereby, it should reduce the sensitivity of the gun to barrel time, with regard to accuracy of any given load.
At this time, it became necessary to develop a computer program to define the shoulder contour, as a function of both case (interior) and bullet diameter and case interior length (within the body portion). For any given bore size, each specific case capacity requires a unique shoulder design so primer shock energy reflecting from th case shoulder will properly focus behind the bullet. Manual calculations (Smalley) required eight pages of math for each design — this invited error, whence the necessity for a computer driven algorithm.
As an aside, we might also mention that these concepts, particularly case shoulder reflection of primer waste energy and how simultaneously that shock wave hits the case shoulder are obvious explanations as to why certain conventional case designs have proven so excellent. A too-shallow shoulder angle allows this waste energy to reflect onto the bullet base, thereby encouraging heat loss and premature bullet movement. A too-steep shoulder angle design increases sharpness of the impact of the primer blast on the chamber shoulder and thereby increases barrel vibrations; it also reflects waste energy back toward the primer, where that energy does little good.
In this regard, a quick analysis (Smalley) proved two things. First, the trial and error path that led to the relatively short, 30-degree shouldered case (PPC and similar), as are now used almost universally in benchrest competition, was predictable — in that class of cartridges, 30-degrees is a very good compromise for the angle of a traditional (conical) case shoulder. Equally, this analysis verifies the value of Ackley’s 40-degree shoulder as an excellent compromise for the normally longer hunting cartridges to which it is usually applied.
(Currently, only Pacific Precision Tool & Gauge is licensed to produce SMc reamers.)
A note of caution, our patent is quite broad and specifically includes any design utilizing any similar shoulder configuration, whether elliptical, parabolic, spherical or some combination thereof, as well as those with an inside case diameter that is two or more times bullet diameter.
In general, owing to the unusually large case diameter, SMc cartridges of any given capacity are relatively short. Therefore, outside of 17- and 20-caliber designs, it is very difficult to form SMc cases with reasonable capacity. As case manufacturers create appropriate factory cases, this problem will disappear (we are now working with Norma). For now, a very efficient 17-caliber can be built on a 221 Fireball case by reducing body taper, a la Ackley, necking it down and incorporating the patented SMc shoulder. Similarly, we have created two 20-caliber versions, based upon a shortened full-length 6mm Norma BR case, and we have acquired ready-to-install Pac-Nor barrels for a Savage rifle.
Soon after Newlon die blanks become available, to make dies, testing will begin on those designs. Design of the smaller version is intended to afford 3500-fps muzzle velocity with Hornady’s 33-grain V-Max (now obsolete!); similarly, the larger version is intended to produce 4500 fps with this bullet.
Moving to larger calibers, the 284 Winchester or 376 Steyr case are the correct diameter for use with 22-caliber SMc designs. Trouble is, when we sufficiently shorten either of those to achieve a reasonable capacity, the case wall becomes too thick at the shoulder (thereby preventing the desired two-to-one propellant-column to bore-diameter ratio). Case shoulder and neck also become increasingly difficult to form and excessive neck turning is then required.
One correct solution is redrawing of cases, to shorten the tapered portion of the case body wall and to thereby reduce minimum body wall thickness (because SMc cases are necked down so far, case neck thickening is unusually great, beginning with a thinner case wall mitigates this problem). We could solve this with the help of Buffalo Arms (208-263-6953) which specialize in redrawing cases. It can modify cases to suit our needs, see below.
Similarly, the new WSM and Short Ultra cases are ideal for 6mm SMc hunting cartridges. It looks likely that we will be able to simply neck and shorten one of those basic cases sufficiently to provide a case with capacity similar to the 6mm Remington. Simple conversion to a case with capacity similar to the 6mm-06 is almost a certainty.
These cases will also work (marginally) for a 25-caliber version but we have not given that bore size much consideration, despite our personal biases toward the quarter-bore, bullets now available in 6mm and 6.5mm sizes suggest very little real need for a 25-caliber cartridge, either in hunting or target work. (A preferred deer chambering of Smalley’s is the 257 Ackley Improved; his 120 load in his rifle outperforms many typical 270 Winchester factory loads.)
Cartridge designation in the SMc system is as follows: bore diameter (caliber or mm), followed by a slash, then case capacity in grains of water — as measured to the interior junction of shoulder and neck. Because, in SMc designs, this junction is where the bullet base should be located in the loaded round, this number essentially equates to usable case capacity (excepting the issue of a slight variation depending upon bullet base configuration — e.g., boattail, and seating depth).
Because bullet base location is relatively critical in the SMc design (too-shallow seating is not particularly harmful but too-deep seating is quite detrimental), chamber throating must match the length of the intended bullet (or bullets of similar length), else some bullets that might be used in the chambering must have a significant jump before engaging the rifling. This is essentially opposite the practice in traditional chamberings, where cartridge overall length was generally held as the important issue. For this reason, SMc reamer design is complicated.
Our 6.5mm testing forced us to use the largest diameter case that is readily available, the 416 Rigby. Our initial foray into building such a rifle (McPherson) is the 6.5mm/60 SMc, a long-range target design. As the name implies, this is a 6.5mm cartridge utilizing a case holding 60 grains of water to the base of the neck.
To make these very short cases, I had Buffalo Arms redraw a batch of 416 Norma cases, to shorten the tapered section so it ended at a point just over 1-inch from the base of the case and so the case wall from that point forward was 0.010-inch thick. Because we did not know exactly which 6.5mm cases we would form from these stretched cases (we have also designed the 6.5mm/70 SMc, which is intended as a hunting cartridge), we had Buffalo Arms leave these blanks quite long.
To form the 6.5/60 case, all that was required was to shorten those cylindrical blanks, then carefully square and deburr the case mouths, anneal the mouths, run each through the press (using appropriate neck-bushing dies) eleven times, re-anneal the necks, slightly expand the necks to fit K&M Services outside neck turning mandrel, turn the necks, trim all cases to final length, deburr each case, load and fireform all cases. (I omitted various critical steps from this short list!) As a friend noted, one might conclude that breeding carbide toothed beavers and training those to whittle productively on hardened brass might have its advantages!
By minimizing body taper and using a slightly loose chamber base, we could also use the modified 416 case for 7mm versions of the SMc cartridge, but the ratio of case interior to bullet diameter is marginal for best results — the 505 Gibbs case would work far better. For a 30-caliber SMc, we would need a ready supply of cases with a base diameter similar to the 505 Gibbs, which is, again, marginal for best results. For anything larger than the 416 Rigby case, currently available mainstream bolt-action receivers are not appropriate. We recognize that this is a legitimate problem, but it can be solved.
Meanwhile, until we can get Norma to produce special, short, thin-walled cases (in the style of the 6mm BR Norma) factory-necked to the desired caliber, few shooters will want to get into this game. For now, it is far too much like copious quantities of hard work! Moreover, with the difficulties and extreme measures required in case forming, significant accuracy penalties (resulting from limitations in finished case quality) seem inevitable, or, at least, hard to avoid.
One of us (Smalley) has considerable experience with two 22-caliber SMc designs. We throated the 22/28.5 SMc for use with 69-grain bullets; we designed the similar 22/29.5 SMc for use with 52-grain bullets.
We intended the former to compete on an equal, usable capacity, basis with the 223 Remington. In this number, we have found several loads that easily launch a 69-grain bullet (sans moly) at well over 3200 fps from a 24-inch tube. Considering the unconventional use of the 6mm BR case, where the case head is fully 0.030-inch smaller than chamber diameter, and the fact that these Norma cases are not necessarily the hardest ever produced, this performance level seems sufficient to prove the ballistic merit of this design.
The latter, a similar version intended for benchrest, easily generates more than 3600 fps with 52-grain bullets, using propellant charges near 28.5 grains, depending upon propellant type used. To date, accuracy is disappointing owing, we believe, to problems with converted case quality.
Preliminary results with the 6.5/60 are interesting. This number easily launches the 130-grain Norma Match bullet (moly-plated) at 3400 fps from a 28-inch Pac-Nor tube. These loads resulted in precisely zero additional case-head expansion (the fireforming load had induced between 0.0010- and 0.0015-inch case head expansion).
Christer Larsson (Chief Ballistician at Norma) believes that lack of such expansion in these particular Norma-based cases suggests that peak chamber pressure must have been less than 70,000 psi. While 70,000 psi is certainly significant pressure, it is not very far above the pressures used in many modern cartridges. QuickLOAD suggests that loads at 3300 fps will generate perfectly normal pressures (less than 65,000 psi).
Unfortunately, as a first test toward drawing these case walls so thin, Buffalo annealed the case sidewalls twice. Our hardness testing shows that in doing so they created an unusually soft case body. For this reason, the case body does not spring back from the chamber walls sufficiently to provide for easy extraction, even with loads generating perfectly normal pressures. To maintain usable extraction, loads at about 3200-fps are maximum with the 130-grain bullet.
Interestingly, with a 28-inch Pac-Nor barrel, launching this bullet at 3200 fps from the 6.5-284 Norma requires top loads — pressures therein are almost certainly close to 65,000 psi. Therefore, it looks as though the 6.5/60 easily produces 100 fps more velocity at any given pressure. Because these cartridges have almost identical usable capacity, we have a tantalizing suggestion that the SMc design does produce superior ballistic efficiency. Of course, we do not yet know and that lack of data is frustrating. Fortunately, Norma will soon begin testing for this effect using a 6mm SMc cartridge designed to duplicate 6mm Remington capacity.
Meanwhile, we have ordered another batch of modified 416 Rigby cases. Buffalo Arms will redraw those, using the least amount of annealing possible (preferably none). We will see how that goes. These will also give us a chance to spend two (more) days making cases!
We hope the result will be worthwhile. These cases should have harder sidewalls and that should allow use of higher pressures with normal extraction. We have returned the reamer and barrel to Pacific Precision and Pac-Nor, respectively. We had a mistake in the throating (far too long) and, inexplicably, one of us (McPherson) cut the case necks 0.050-inch too short on all 100 of the original batch of 6.5/60 cases — you cannot imagine his consternation at that discovery. The modified reamer (to correct both errors) has gone to Pac-Nor, where they have rechambered the barrel — which is on its way to McPherson’s shop, as this is written.
We have now fired some of those first-attempt cases six times. Interestingly, none shows any measurable case neck lengthening, despite testing involving a marginal amount of case body resizing, along with a slight shoulder bump before each reloading.
Through careful lathe work and using the finish chamber reamer, we converted a Newlon die body to form a combination neck bushing and shoulder bump die. We now have additional Newlon die blanks. We will use the finish chambering reamer to form a resizing die from the 12L14 steel blank and then have Fireball case-harden it — this will induce sufficient shrinkage to adequately resize the case body. Meanwhile we have been using an unhardened die, which tends to gall cases.
Similarly, we will use the modified chambering reamer to convert the 416 steel blank into a proper seating die, which Fireball will harden through heat-treating. The system we have been using to date has not provided good bullet-to-case alignment. Just one more problem to be overcome.
We continue to explore the potential of these designs. Should Norma find merit, it is likely they will eventually offer BR style basic cases (short tapered sections and thin walls) in various base diameters and perhaps even a new line of factory offerings, based upon the SMc concept. Meanwhile, experimental handloaders who are sufficiently serious to consider the effort involved worthwhile can order reamers from Pacific Tool and Gauge. We can provide correct reamer designs to accommodate any bore diameter, usable case capacity and bullet selection.