How Far Does a Primer Plume Penetrate?
M.L. (Mic) McPherson, December 2002
Synopsis: The primer blast does not ignite all the granules in a typical rifle load. A fundamental characteristic, and ballistic limitation, of conventionally primed sporting cartridges is penetration depth of the primer plume (the pyrotechnic jet produced through the flash hole). Depth of effective penetration depends upon many factors and is certainly somewhat variable; however, in typical rifle loads, this variability may be surprisingly small. Conversely, several lines of evidence demonstrate that, regardless of cartridge configuration or propellant type, direct ignition of the majority of propellant granules in any zone cannot occur more than perhaps about 1/2-inch forward of the case web interior surface and that under most circumstances such ignition depth will exceed about 3/8-inch.
The following discussion may seem to ramble around the issue of what the primer blast does to the propellant column, rather than focusing directly upon how far the primer blast penetrates that column. This is because an accurate understanding of precisely what the primer blast does and how that affects the propellant column is critical to understanding why a primer blast does not directly ignite every granule in the charge in typical rifle cartridges and loads.
Disclaimers: Unlike some, I would never deliberately claim to have the last word on this matter, or any other. I derived the facts behind the following discussion from decades of experience, while testing hundreds of chamberings, from the 22 Hornet to the 50 Alaskan, with thousands of loadings, using nearly every propellant available in the last 50 years, and in combination with at least 30 distinct primers — since 1985 I have used up the ink in three cartridges for the Oehler 35P printer during such load testing. (Ken Oehler expressed surprise that any individual could do so much testing!) I have experience in several ballistic laboratories testing unusual loads and unique components of various kinds. In my handloading career, I have had serious internal ballistic discussions with the following persons and groups of people: Bill Davis, Elmer Keith, Parker Ackley, Homer Powley, Ellwood Epps and others that many readers would recognize by name and reputation; myriad esteemed handloaders; and practically every ballistician in the trade, This represents a combined group with experience across hundreds of chamberings, thousands of loadings, and millions of rounds. If one could total the extended related experience of these people, it would reach into thousands of years.
In general, besides just talking handloading, I have made an effort to ask about and discus the strange and unusual experiences each had known. Moreover, I have gleaned information from various tomes on the subject of internal ballistics (from Krupp onward) and tried to separate wheat from chaff, as it were, which is no easy task. As a scientist, I always strive to make logical deductions, based upon first principles. Nevertheless, despite what some might suggest as significant credentials in this matter, this nimrod makes no claim as to having God’s wisdom. Some might suspect that those who claim to have such insight might exaggerate that matter!
A personal note, if you will forgive me. I find hypocrites to be among the most offensive of persons. For example, consider a person who would, decades after Elmer Keith’s death, berate Keith for simply having used the wrong terminology more than one-half century earlier. As late as last year, said person was still holding a torch because Keith had used the wrong terminology when expressing a belief in the need for use of better steel in the production of reloading dies. One might be forgiven for expecting one who would deride Keith for such an innocent and harmless transgression to be very cautious in his own publication submissions. Alas, that particular writer has failed to do so, repeatedly.
(It occurs to me that Keith may very well have suffered from an editorial transgression in this matter; Keith may very well have specified a reasonable type of steel, only to have some ignorant editor fix the text with the belief, or hope, that his uneducated readers might understand it better; because I have had, on five separate occasions, different editors make such a fix to one single word — adsorbed, incorrectly changed to absorbed! — I am definitely sensitive to this likelihood.)
Finally, I am attempting to present information for the typical reader, rather than for the professional explosives expert. Hence, some definitions might lack scientific preciseness — my hope is that these and the associated arguments will carry the intended message, despite this transgression.
Alternative Priming Systems for military applications, with bigger cartridges, the standard system with which handloaders are familiar proves inadequate to provide uniform ballistics and maximum performance. For such cartridges, two alternative approaches are generally used.
The first system, used in intermediate chamberings, is the flash tube. This device simply delivers the primer flash to the forward end of the case, so that ignition initiates first near the bullet base and progresses backward. Advantages of this system are twofold; first, essentially no energy is wasted in accelerating unburned propellant; second, the burn tends to be more uniform from one shot to the next because the propellant mass is relatively stationary (it is trapped at the rear of the case).
The second system, used in the largest chamberings, is the spit tube. This system uses a hollow, perforated tube that extends from the primer to approach the bullet base. Designers often fill this tube with a booster priming charge but the system does not depend upon that characteristic. The advantage of this system is that the entire propellant column ignites at the core so that, as it combusts, total reacting surface area tends to increase — automatic progressivity. Combined with the correct priming charge and the right propellant burning rate, this system can provide significant velocity increases and shares the consistency advantage of the flash tube.
Manufacturers have adopted the standard ignition systems to sporting cartridges not because of superiority but simply because of affordability. This is a matter of scale, as the size of the cartridge increases, the additional cost of these priming systems diminishes, as a function of total cost, until the ballistic advantage becomes worthwhile.
Background and Definitions have been discussed in previous articles by both Byrom Smalley and me, particularly those works pertaining to the short-fat case concept, ideally, to maximize both cartridge performance and ballistic uniformity, several primer blast (or plume) related factors are critical. With this in mind, the ideal primer blast would:
One might speculate that we could identify other salient factors but this list seems reasonably complete and should certainly illuminate the importance of primer function to ballistic uniformity. The problem is, no conventional priming system (Boxer, Berdan, or rimfire) can accomplish every prerequisite requirement to ultimate ballistic uniformity in typical cartridges with typical loads.
Before proceeding, with the goal of assuring that the average reader can follow the content of this essay, a few definitions might be useful (italicized words within this list are also defined within this list):
A Critical Point about How Primers React
Many readers may have an impression of the primer as a detonating explosive — once the striker hits the cup, the pellet explodes and thereby generates hot gasses and incandescent particles. While accurate, in as far as it goes, this is far from a complete and accurate picture. What the primer pellet actually does is burn extremely rapidly (technically a deflagration). This deflagration does, in fact, generate a stream of hot gasses and fine particles but within this stream is a significant admixture of material that is actively reacting, long after the plume passes into the propellant chamber. This latter characteristic is critical to understanding how a primer actually works in a sporting cartridge. This continuing reaction results from two sources: first, pieces of unreacted primer pellet entrained within the plume; second, partially reacted molecules that continue to react toward a more stable (and more energetic) reaction end product, thereby generating lighter, hotter molecules.
Salient to this discussion is the fact that the volume of plume material produced through the flash hole is only a tiny percentage of the final volume of the plume material generated! This is a significant reason why the generated plume tends to compress the propellant column, rather than blast a hole through it.
The included photograph of a thoroughly typical unconfined primer blast (produced into the atmosphere through the flash hole of a case head, with the case body removed) irrefutably demonstrates this characteristic. Note that the final pyrotechnic flash generates a roughly cylindrical column perhaps 8-inches in diameter and about 18-inches long. Those who understand how a high-pressure jet propagates through and beyond an orifice (the flash hole) will instantly recognize this picture as representing an impossible situation, impossible, that is, if the produced jet is composed of final reaction products. Such a jet would generate a relatively longer and thinner, cone-shaped plume.
For purposes of the following discussion, one can envision the plume as being made up of a relatively small volume of gas and particles blasted into the propellant chamber and then rapidly expanding many times in volume through decompression, and generation of additional heat and gas molecules.
Primers in the Real World
The effectiveness of priming in conventional sporting cartridges is constrained by several factors. In typical rifle loads, and in most revolver and pistol loads using a reasonably full case of typical propellant, any primer blast containing sufficient energy to theoretically carry effective ignition to the bullet base would necessarily also dramatically compress the charge toward the front of the case and might even pulverize a significant percentage of granules that happen to be proximal to the flash hole. These two factors would necessarily work to defeat the implied purpose for using such a primer. Specifically, increasing energy of a conventional primer tends to increase both percentage of granules that the blast might pulverize and total compression of the forward portion of the charge.
The former result would produce smaller particles at the base of the charge, which would dramatically reduce permeability, limiting the ability of the primer gasses to penetrate the propellant column. The latter result reduces pore space, which reduces both permeability and porosity. The net result is that the igniting plume of a hotter primer cannot penetrate dramatically farther into the granule charge; hence, under certain circumstances, a hotter plume might not effectively penetrate any farther — one could envision a situation where it would not penetrate as far! Certainly, real world experience suggests that often (perhaps, almost every time) a hotter plume does penetrate slightly farther but the difference is nowhere nearly commensurate with the actual difference in primer energy.
The hottest Large Rifle primers have at least three times the energy of the mildest Large Rifle primers, yet in some loads these two types do not generate significantly different ballistics. See my Precision Shooting articles on the subject of primer energy, as measured by distinct methods. (PS 1996 Annual, page 288 and an article published in ’97 or ’98 come to mind.)
It is long since proven that the plume from primers generating mostly gasses, created from a relatively milder blast (less brisance) with more total heat energy (more fuel and oxidizer in the pellet mix), tend to effectively penetrate farther than the plume from primers generating relatively more brisance, or a greater percentage of hot particles, as a percentage of total energy.
Before leaving this section, it is critical to understand that the above facts indirectly support many points discussed subsequently in this article. Specifically, if primer plume particles do not effectively penetrate as far as primer plume gasses (which is well demonstrated), then it follows that propellant column permeability and porosity are key to the ability of a primer blast to effectively penetrate into said mass.
What Really Happens when a Primer Plume
Blasts into a Propellant Charge
Propellant granules do not ignite instantly. Compared to generation of the primer plume from the flash hole, and the subsequent continuing reaction of plume material within the powder chamber, granule ignition delay, while not an eternity, is rather significant.
The smallest propellant granule is many orders of magnitude more massive than is any constituent of the primer plume. Hence, owing to initial permeability and porosity of propellant column, a significant volume of plume material will penetrate into the base of said column before the plume jet transfers a significant amount of momentum to granules near the column base.
After entering the propellant mass, these reacting gasses will spread to fill a roughly spherical, or perhaps a roughly hemispherically ended cylindrical, volume of propellant. Subsequently, expansion of gasses within that volume (due to decompression and continued reaction, which increases plume volume, and due to nascent propellant granule combustion) will tend to expand that impregnated volume at the expense of the unignited mass volume.
It is critical to note that as the plume penetrates into the propellant mass, the granules preferentially screen out heavier (still reacting) molecules and reacting solids. This happens for two main reasons: first, generally, such components are necessarily moving at a lower velocity, so those will naturally tend to lag behind the lightest molecules; second, such components simply cannot ricochet around the intervening particles as efficiently as lighter molecules (more massive components give up a greater percentage of momentum to intervening granules at each interaction — conservation of energy and momentum). Hence, at the instant the plume reaches plume penetration depth, the plume tip will contain essentially no material that will continue to react (and thereby generate more heat) while the plume base will contain a large amount of such material. Continuing reaction within the plume (which grades from nothing at the tip to maximum at the base) locally dramatically expands plume volume. Of coarse, because this is a dynamic event, tip penetration depth and base reactions are concurrent.
In addition to reaction-related expansion, the plume will continue to undergo decompressive expansion but this cools the plume and hence reduces available ignition energy. However, a second source of additional energy is the nascent combustion of ignited granules, which occurs first within those granules that were proximal to the flash hole when the primer ignited.
At this instant, two dramatically significant things occur. First, through transfer of momentum from the pyrotechnic jet, the charge column accelerates away from the web of the case. Second, as noted, some portion of the rearmost end of the propellant charge is expanding, due to continuing plume reaction and nascent granule combustion within that volume. Because chamber volume is essentially fixed, the only possible result of this expansion within the rearward portion of the charge is acceleration of the charge base toward the front of the case, which results in additional compression of the forward portion of the charge. As we will discuss farther on in this piece, this compression is not theory but is a well-proven fact, recognized by those who have actually studied such matters.
Obviously, initially, these generating gasses will achieve some penetration into the unignited mass but for an entire suite of reasons, this penetration cannot reach the bullet. As these gasses move into the propellant mass, several things happen: first, cooling of plume gasses; second, compression of gas entrained within the propellant column, ahead of the plume, which builds interstitial pressure and hence thwarts plume penetration; third, continuing compression of the unignited propellant mass, so that, eventually, all permeability disappears, through compressive deformation of granules and subsequent closing of the initially interconnected openings between and within the granules (tubular small arms propellant granules have an endwise perforation).
Charge compression coincidentally deforms granules, while shrinking interstitial pore spaces between granules. Critical to this discussion is the empirically demonstrated fact that 3000 psi will reduce volume of a typical propellant column by about 15% and that such a level of compression will effectively eliminate porosity — data gathered by me, circa 1996, and first presented in edition seventeen of Handloader’s Digest.
This loss of porosity prevents gasses that might penetrate into the mass from doing so. Moreover, gas penetration that occurs before such loss of porosity occurs and any that occurs as a result of any residual porosity through the mass, cannot be significant because no place exists where such gas can go, after reaching the front end of the charge mass — by deliberate design, the front of the cartridge presents an almost perfectly sealed container. Hence, as nascent granule combustion generates gas pressure within the zone near the base of the charge, that zone expands and thus drives the remaining charge forward and; in response, the forward portion of the charge is simply compressed.
Within this compressed zone, to a certain depth, granule combustion gasses can penetrate with sufficient heat to result in granule ignition; beyond that depth, it is impossible for sufficient gas to pass into or through this portion of the charge to introduce sufficient heat to ignite the granules. This discussion represents a reciprocal argument — that portion of the propellant mass that the primer does not directly ignite is the compressed portion described above. The point here is to understand that, in most reasonably large cases with normal loads, such an initially unignited volume always exists. Ballisticians within the military munitions industry have proven this in testing with guns equipped with glass windows in the barrel. They then use high-speed cameras to take photographs at increments along the bullet path.
Such photographs, taken near the chamber, always show a plug of propellant pushing the bullet through the bore — of course this plug is burning along its rearward face and it can burn out long before the bullet reaches the muzzle.
Stress-versus-strain properties of propellant granules are an important consideration in this discussion. Testing proves that when subjected to extremely rapid loading (stress), as occurs with the shock of a primer blast coming through a flash hole, propellant granules react as an extremely rigid solid (resistant to deformation) and an extraordinarily tough material (resistant to shattering). At lower loading rates (as with the buildup of pressure during nascent granule combustion), propellant granules deform more easily but are still quite resistant to shattering.
Additional support for the contention that the primer blast does not penetrate significantly into the propellant column and certainly does not simply blast a hole through that column comes from a simple test. Filling a primed 44 Magnum case with W296, which is notoriously difficult to ignite, and firing the primer results in unignited W296 spraying from muzzle with very little, if any, propellant left in case, chamber or barrel. If the primer blast simply bored a hole through the propellant column, this test should leave a significant portion of propellant in case, cylinder and barrel.
How Granules Ignite and how Granules Contribute to Bullet Velocity
For simplicity sake, we will consider these basic types of granule ignition (for a typical rifle cartridge):
With regard to item 1, above: It is important to recognize that smokeless propellant granules are generally extremely tough plastic objects that will normally tolerate surprisingly violent abuse — if this were untrue, internal ballistics would be an entirely different science! Hence, despite the brisance of the primer plume, as produced from the flash hole, under normal circumstances, this jet pulverizes very few, if any, granules and it cannot simply bore a hole significantly far into propellant column, a process that would require either enormous energy (to move granules aside, almost instantaneously), or pulverization of all granules in a direct path toward the bullet (so pulverized material could either instantly combust or somehow rearrange and thus move out of the way, which does not happen — nothing burns instantly and pulverizing a material does not decrease the bulk volume, the pulverized material could simply move into the pore spaces but that would take time, and the momentum required would have to come from the much lighter gasses, so the plume would run out of gas before moving more than a few-tenths-grain of propellant aside, which would halt penetration within a few-tenths-inch of the flash hole!).
At extremely cold (arctic) temperatures, some types of flake propellant granules can become extremely fragile, with potentially tragic results. One 50-million round batch of NATO 9mm Pistol ammunition was loaded with such a propellant; under normal usage conditions, performance of this ammunition was perfectly normal; however, when these rounds were laboratory tested at extremely low temperatures, peak pressure was monumental, suggesting that the primer plume was driving the entire charge against the bullet base with sufficient force to pulverize many or all the granules; upon ignition, the resulting pulverized propellant essentially detonated, which set up the famous standing-wave syndrome demonstrated in the Krupp tests, circa 1880, which is seen whenever the entire charge is ignited when positioned at either end of the propellant chamber.
In a sufficiently short case, it is possible that initial penetration of the pyrotechnic plume can reach far enough into the propellant charge so that essentially the entire charge is contained within the primary ignition volume. It is also possible that in a relatively large case, charged with a sufficiently small volume of a sufficiently easily ignited propellant, the primer plume can ignite essentially the entire charge, but see the paragraph at the end of this section.
Readers should note that any smokeless propellant could become sufficiently brittle under arctic conditions as to make firing the gun patently unsafe. Several years ago, our esteemed Editor forwarded a letter to me from an Alaskan shooter who was experiencing unusually high pressures with his 300 Winchester Magnum load using a charge of 4350 that should have produced perfectly normal pressures — as I recall, the correspondent had to hammer the bolt open. He was testing in the dead of winter at temperatures near minus 40-degrees. I wrote him a long explanatory letter and begged him to cease and desist such testing — because I have not heard back, I have to wonder if my letter arrived in time! Shooting in temperatures below about minus 40-degrees is particularly dangerous, in this regard, and, also, because the steel of the gun becomes progressively more brittle as the temperature drops — a classic double-edged sword. Worse, some types of steel embrittle catastrophically below about minus 40-degrees.
With rifle loads using partial charges of unusually fast-burning propellants, the less deterred — faster burning — granules require much less heating to ignite and can also ignite and burn so fast, in response to gasses generated by primary ignition, that even a less-than-complete primary ignition can generate results that are practically indistinguishable from those that would occur in response to 100% primary ignition. This result occurs partially because such types of loads ordinarily generate far slower bullet acceleration, so that much more time exists for secondary ignition to occur before the bullet has accelerated significantly into the bore, compared to the situation with full-power loads. This is not the situation with typical, full-power loads with case-filling propellant charges. In such loads, using more heavily deterred propellants, primary ignition requires significantly greater granule heating and is associated with a relatively longer delay (with regard to how far the bullet moves during that delay); secondary ignition and subsequent progressive progression of burning rate is relatively so slow that once bullet begins to move, gasses generated by any newly ignited granules will contribute much less energy to bullet acceleration.
Ignition in Sporting Cartridges
Sans serious testing, which Smalley and I are heading toward, I am loath to suggest that any specific numbers suggested herein are necessarily anything more than an educated guess, but I will offer such general suggestions. Please, keep in mind that I offer these only as a means of indicating degree.
In typical sporting cartridge loads, several lines of evidence support the contention that plume-generated ignition cannot reach far beyond about 5/8-inch forward of the case web and that plume penetration depth is often less, perhaps closer to 1/2-inch. Perhaps equally important, minimum primer plume penetration is approximately 1/2 inch. As discussed in previous sections, primer characteristics have a surprisingly small influence upon characteristic plume penetration depth (at least as applied to primers that are both readily available to handloaders and that are adequate for the job at hand). Conversely, several factors do have a significant influence upon effective plume penetration depth; these include but are not necessarily limited to:
Perhaps a series of examples from experience will be helpful in understanding these differences. First, let us consider the 44 Magnum, a cartridge with a propellant chamber that is about ½-inch long. A typical midrange load in this case might use about 8 grains of Unique. Experience proves that, in such a load, the primer plume always reaches the bullet base — as evidenced by primer residues on the base of recovered bullets. Because this charge fills only about 60% of the propellant chamber, it is unsurprising that the plume can reach the bullet base. Conversely, a typical full-power load in the 44 Magnum using about 25 grains of W296, which fills essentially 100% of the propellant chamber, when ignited by the very hot CCI-350 primer, acts far differently (CCI-350 is hotter than most rifle primers). In such a load, the primer blast never reaches the bullet base — I have examined hundreds of recovered bullets fired from a 44 Magnum; only those fired with typical midrange loadings (similar to above example ever show any significant evidence of primer residues on bullet base and such bullets always show such residue. The same comparison holds for other cases of the same general length. Shorter cases do not follow this pattern for two reasons. First, obviously, the propellant chamber gets shorter, so it is more likely the plume will approach the bullet base. Second, such cases generally use faster (more easily ignited) propellants, so effective penetration depth naturally tends to increase.
Conversely, longer cases, despite the potential use of hotter primers, tend to act similarly to the above W296, 44 Magnum example. In such longer cases, with any normal load, effective ignition depth never reaches the bullet base. Use of smaller diameter cases, larger granules and some types of primers might increase effective ignition depth, to perhaps approach 3/4-inch, but it seems unlikely that it ever significantly exceeds 3/4-inch.
Evidence from military tests with the cylindrical version of the 20mm cannon prove that, in that cartridge, which uses a dramatically hot primer, a significant portion of the charge is unignited by the primer and instead simply follows (or pushes) the bullet, as a compressed heterogeneous mass (deformed individual granules with entrained gasses trapped in the remaining interstitial spaces), burning from the rear face forward. Conversely, in the bottlenecked 20mm cannon load, the case shoulder disrupts this unignited mass, as it moves past the shoulder, this disruption allows the almost instantaneous radiant ignition of all exposed granule surfaces within the initially unignited mass, so the unignited plug is much shorter — despite the fact that it is composed of harder-to-ignite granules that burn much slower.
Similarly, QuickLOAD, a software program that models internal ballistics, based upon how individual granules burn, predicts very accurate results for typical bottlenecked cartridges, such as the 308 Winchester; this fact suggests that it models internal ballistics reasonably accurately when the vast majority (perhaps >90%) of granules are ignited before the bullet has moved more than about two inches — which, as in the bottlenecked 20mm cannon, is the situation for the 308 Winchester. However, in bottleneck cartridges with a wider and steeper shoulder, the program tends to under-predict velocity, perhaps due to the delay associated with shearing a particularly long plug. Predictions in such cases tend to run about 100-fps slower than actual velocity.
The 458 Winchester Magnum has a propellant chamber essentially identical to the 7mm Rem Mag — same diameter, same taper, same length. The only significant differences are that the 458 case is not bottlenecked and typical 458 loads use a much faster (less deterred and hence more readily ignited propellant). In the 458, and in all similar cylindrical or near-cylindrical cases (where no shoulder exists that is sufficient to disrupt the moving plug of unignited propellant), QuickLOAD predictions are always dramatically and systematically faster than actual performance. Predictions run about 150-fps faster than actual velocity, and, of course, pressure is commensurately lower than the prediction.
If either the primer or secondary ignition ignited all granules before the bullet had moved significantly into the bore, QuickLOAD the prediction bias in these two chamberings should be equal. Independent QuickLOAD results show no bias within the propellant descriptions, or with any other factor, that might explain this dramatic difference in predictive accuracy.
The following table compares these two numbers with various nominal loads. I chose these cartridge designs because this pair shares an extremely similar propellant column size and shape. A comparison between the 308 Winchester and the 45-70 shows a similar result, see blow.
Looking at progressively shorter pairs of cylindrical cartridges leads one to conclude that primary ignition depth is usually less than about ¾-inch, maximum. Generally, this systematic error is directly related to propellant column length; the longer the propellant column, the greater the error; the shorter the propellant column, the lesser the error (with a short enough propellant column, the error disappears, specifically because the primer does ignite most of the granules in such loads). As those who accept the arguments given here might expect, in cylindrical cartridges, that the propellant column length where QuickLOAD predictions and actual performance coincide is near 3/4-inch and shorter. (Those who run these calculations and compare the results with actual performance must take into consideration that in such loads, the primer blast often moves the bullet significantly, before significant propellant ignition occurs.)
Support from SMc Experimenting
Significant testing of various SMc cartridges by Smalley and me suggest the validity of the above contentions. The SMc cartridge design incorporates case body and shoulder features designed using first principles, as necessary to maximize both ignition and combustion efficiency, when compared to a more conventional cartridge of similar capacity. In order for these features to come into beneficial effect, the propellant column must be long enough so that some significant portion of charge is not included in primary ignition zone. Testing with three lengths of 22-caliber SMc numbers shows a definite relative ballistic advantage (compared to conventional case designs with similar capacity) but only when the propellant column length exceeds about 1¼-inch. In such cases, the SMc advantage becomes worthwhile, and it increases with progressively with increasing propellant column length, through the capacity of a typical magnum case.
Shear Versus Compressive Strength
One more point, some have identified the unignited heterogeneous mass as a potential bore obstruction. Such a deduction evidently stems from ignorance about the characteristics of such materials. Just as with sand, granular materials can exhibit tremendous compressive strength, while having very little shear strength. Because the unignited material can shear at relatively low stress, it cannot represent a significant bore obstruction — in a significantly bottlenecked case, the center of the unignited mass simply shears free from the perimeter (which is trapped behind the case shoulder) and pushes the bullet down the bore. (Granules can either slide past one another or shear through at a relatively modest pressure.)
Reviewing the above discussions supports the deduction that the primer plume never penetrates full-length of the charge in a typical rifle-type cartridge. Conversely, in most (shorter) pistol and revolver cartridges, the primer plume usually does reach the bullet base unless unusually dense charges of relatively slow burning propellants are used, and even that influence has a limit near ½-inch of propellant column length.
Without significant evidence from direct testing, attempting to place absolute values on effective depth of primer ignition penetration would be foolish. Nevertheless, we can say from experience, anecdotal information, QuickLOAD, SMc ™ results and limited laboratory data that it probably commonly exceeds 1/2-inch and seldom significantly exceeds 3/4-inch.
The dynamic process that occurs after the primer begins to burn is very complicated. In typical rifle cartridges with typical loads, the primer blast results in direct (primary) ignition of a significant portion of the charge but certainly not 100% of the charge — a significant portion of the granules are not ignited by the primer. Those granules not ignited by primer are concentrated toward the front of the case.
Interactions between the plume and granules near the base of the charge work to transfer much plume momentum to the propellant column. This factor, along with nascent combustion of granules in that zone, works to accelerate the base of the column, thereby compressing the forward portion of the charge.
Compression of the granule mass within this closed system retards infiltration of hot gasses from nascent granule combustion, so the front of the charge cannot ignite, except at the rearward surface (individual granules, while still hidden within this mass, do not ignite).
The case shoulder can act to disrupt this heterogeneous mass, thereby exposing additional granules, and entire exposed surface area to ignition sources. In a cylindrical case, this disruption does not occur.
A note about the sketches elsewhere on this web site: we have presented sketches as simplifications of what happens in typical instances of conventional priming, when applied to loads using case-filling charges of relatively dense propellants.