Primer Plume Penetration Study
M.L. (Mic) McPherson, February 2003

SYNOPSIS: A series of tests, made possible through procurement of an inert smokeless propellant simulant with physical characteristics that are very similar to Rl-15 (an extruded smokeless propellant), has provided answers to several questions about propellant ignition (both smokeless and, to a lesser degree of certainty, blackpowder) upon which I have long pondered.  Those questions are:

CAUTION: Some tests described in this article might well or certainly do entail an entire genre of potential hazards any one of which could result in significant property damage, personal injury, or death.  Keep this in mind — Please, do not attempt this at home!

Perhaps ten years ago, I began advocating use of relatively mild primers for many applications: lever-action rifle loads, reduced velocity loads using very small charges of pistol or shotgun propellants in any rifle cartridge, and many blackpowder loads.  In this context (and generally), by mild, we mean a primer type that generates relatively less explosive shock (brisance) and relatively more heat (thermal energy) per unit of energy released and which may generally produce less total energy, compared to some other primer type.  The RWS 5341 and Remington 2½ are classic examples fitting the description mild while the Winchester WLRM and Federal 215 are at the far opposite end of the spectrum and, by any measure, are hot primers.

By the way, one dares not miss an opportunity to state the following: While the Federal 215 has long held the reputation of being the hottest primer available to handloaders, by every ballistic measure one might wish to test, Winchester’s WLRM is far hotter and always has been.  While the latter may generate less pressure and velocity in the occasional load combination, in the majority of instances, it will generate significantly more pressure and velocity.  Please refer to the included tables.  (Evidently, this longstanding misconception stems from an unfounded rumor, begun by ignorant gun writers, upon discovering that the 215 had been developed specifically for use in factory loads of the largest Weatherby Magnum cartridges and who then assumed that that fact demonstrated that the 215 just naturally had to be the hottest primer available.)  However, this does not mean that the WLRM will always give better ignition than the 215 does, nothing is that simple when it comes to primers!  I will bet that, despite the results of this testing, there are combinations in large cases where the 215 is, by any measure, superior.

Finally, Federal developed and uses the 216 for every load in every case larger than the standard belted Magnums.  These include the 470 Nitro Express, the 30-378 Weatherby Magnum, the 416 Rigby and all related cartridges.  Federal had a very good reason for doing so — it recognized that any existing Large Rifle primer sometimes generated squib loads when used in such cartridges and likely someone there knew that such a squib load is only one step removed from a detonation.  Wanting to avoid an inevitable lawsuit, it developed the 216.  If you understand this and you load for such large cartridges, you must ask yourself why Federal will not offer the 216 as a handloading component.  I have asked.  The answer, given sotto voce, and explicitly off the record is: “Because handloaders are too stupid to follow directions and that primer, if used in smaller cases is almost certain to result in dangerous loads that could well kill the shooter and bystanders.”  So, it seems Federal is in a bind in this regard.  If it does not offer this primer, sooner or later someone will detonate a rifle because they could not get the primer they needed; conversely if Federal does offer this primer, sooner or later someone will explode a rifle because they are too stubborn, stupid, and foolish to follow directions, no matter how explicit, direct, and forcefully those might be presented!  Federal has my sympathy.

I had done considerable testing with various full power 30-30 and 45-70 loads and cast bullet 5744 loads in lever-action and single-shot rifles, along with a modicum of testing of blackpowder and 5744 loads in various single-shot rifle chamberings such as the 44-90 Straight.  In the full-power loads in the lever rifles, I had used the CCI-300 and had discovered superior accuracy to that produced by any rifle primer! I had tested loads with these primers for two reasons: first, the shorter cup provided added safety against possible magazine chain-fire during recoil; second, the softer cup seemed likely to provide more uniform response to the typically mild striker energy typical of such rifles.  (This fact was considered in one of my earliest published articles, Handloader’s Digest, where I described MOA loads developed for my Marlin 45-70.) In blackpowder and 5744 testing, I had seen very good results with the Remington 2½, which I had guessed might offer superior performance because those who engineered it claim that it has the lowest brisance and highest heat content of any Large-Pistol primer.  With the results of all that testing, I had not been disappointed.

Based upon decades of experience with magnum revolvers, I well knew that the primer blast can move the bullet and that propellant granules do not ignite instantly (a physical impossibility).  One can examine myriad internal ballistics results to demonstrate the validity of these points but a few should suffice here.

If the primer blast did not, sometimes, in some specific load types, move the bullet and if the charge ignited practically instantly, it would be very difficult to resolve the following results (and many others).  First, it is quite common for an increase in neck tension in 44 Magnum loads to result in significantly higher chamber pressure; conversely, substitution of a primer that is otherwise demonstrated to be hotter will, in the same load, generate significantly less pressure.

For example, in testing how far a primer will move a bullet, without a charge and with almost zero neck tension, the CCI-350 proved far more capable than other Magnum pistol primers — see table.  Yet, when used in a certain (common) class of 44 Magnum revolver loads, having a significantly compressed charge of a very hard to ignite propellant (W296), substitution of this hotter primer, where a milder primer had formerly been used, routinely resulted in a significant decrease in both muzzle velocity and apparent peak chamber pressure.

One could conjure all manner of explanations for both of these results but only one follows the tenets of Occam’s Razor (providing a simple explanation, rather than relying upon ever more convoluted conjectures).  That simple explanation is: greater neck tension tends to reduce bullet movement in response to primer blast, which occurs before effective propellant ignition begins — it results in a smaller boiler room; conversely, the more violent blast from the hotter CCI-350 moves the bullet farther before effective propellant ignition begins — it creates a larger boiler room.  If propellant granules ignited essentially instantaneously, these results would be mutually exclusive.

Many readers may know that, often, in such loads, use of a smaller-diameter expander ball results in greater peak pressure and velocity (within reason, use of a smaller expander increases resulting bullet-to-neck tension and hence increases resistance to bullet movement).  Similarly, experienced handloaders have long recognized that in 38 Special midrange loadings, slight differences in crimp alter internal ballistics: less crimp, less pressure; more crimp, more pressure.

Excepting one fact, one could rightly argue that greater crimp or greater neck tension delays bullet movement longer, after the primer explosion, so that chamber pressure is higher before the bullet begins to move, equally as well, one could argue that greater resistance to bullet movement reduces primer-induced bullet movement, so that boiler room is smaller when the charge ignites.  That one restricting fact: Otherwise identical loads always show significant primer-induced bullet movement when tested without any charge! — the hotter the primer (i.e., CCI-350), the greater the bullet movement.  Evidently, in these particular loads, the main difference is how far the primer moves the bullet before propellant ignition and subsequent combustion generates significant pressure.  These examples are sufficient to persuade me that smokeless propellant ignition is significantly delayed, with respect to the primer blast.  These tests would seem sufficient to demonstrate that, in many loads, force from the primer blast does move the bullet before propellant ignition generates significant chamber pressure.  For the past three decades, I have pondered how to go about proving (or disproving!) this contention.

Testing Primer Impulse

A 1997 test with the 45 Colt, using a fired case that had sufficient elasticity to retain a bit of crimp roll at the mouth and Hornady swaged lead bullets demonstrated vast differences in propulsive force for various large-diameter primers.  The included table shows results of that study, where the primed case (modified to work with both Pistol and Rifle primers) held a finger-inserted Hornady 250-grain Swaged lead (Cowboy) bullet at identical normal cartridge length in each test.  We used the same chamber of a standard Ruger 45 Colt revolver as the test vehicle.

One major concern with the value of the study noted above was that lack of propellant might dramatically alter results from what would occur in real loads.  Typical boiler-room-filling charges occupy about 60% of available space in the case, air fills the remainder of the boiler room (within interstitial pore spaces).  At first consideration, it seems obvious that adding the propellant charge will increase compression within the boiler room, in response to primer gasses, hence, this should increase pre-ignition bullet movement.

However, reality is not so easy as this simplification would suggest.  First, granules offer a relatively large amount of cool surface, which interacts with the hot primer gasses to cool those.  Besides the immediate and direct pressure loss associated with cooling, cooling also results in condensation of primer gasses, with a resultant significant pressure reduction within the boiler room of the case.  Hence, if the granules provide a sufficient heat sink, adding the charge might not increase maximum primer-induced pressure within the boiler room and this could even reduce primer-blast-induced pressure!

With regard to the above noted study, I had the foresight to save the remaining primers in all those boxes, I even have more of the bullets; one wonders why I did not have the foresight to save that test case in a place where I could find it as easily! Because I did not, I cannot readily repeat that test, using added simulant with all other characteristics held constant.

I have since done related testing which proves that in typical rifle loads, I tested with the 308 Winchester, adding the charge absolutely reduces the chance of the primer blast moving the bullet.  This is true, regardless of propellant type, primer type, or degree of case filling, including charges where bullet seating significantly compresses the propellant.

Comparing Internal Ballistics of Loads using Various Primers

Several years earlier, I had conducted an extensive study of primer performance with three different propellants in otherwise normal 30-06 loads.  This data, presented in the next three tables, was obtained in January 1996 at the Accurate Arms ballistics lab with the help of then chief Accurate Arms ballistician, Bill Falin.  We made every possible effort to ensure that these results represented only primer-induced differences.  For a more complete discussion, refer to my book, A Short Primer on the Primer.

 

We present the preceding Accurate Arms Study here to give the reader a feel of the real-world effects of primer changes in typical rifle cartridge loads and for one other important reason.  One would hope that the results of this very careful study would do one thing — suggest to the prudent reader that anyone believing they can predict, a priori, the pressure, consistency or velocity consequences of any particular primer substitution in any particular load is, more often than not, bound to be very much surprised.

Unusual Loads Give Unusual Data

One possible conclusion one might make in response to evidence from unusual revolver loads, where increasing charge mass in an already highly-compressed loading, tends to decrease both peak chamber pressure and velocity, is that increasing the charge volume might always increase the boiler room pressurization in response to the primer blast.  If this were true, increasing the charge mass in any loading would always result in greater pre-ignition bullet movement (if any occurred) and hence increased pre-ignition boiler room volume so that, depending upon the degree of charge increase and the rate of charge combustion, peak pressure might increase, decrease, or remain constant.  However, this example is misleading: The charge in such loads is so highly compressed that significant elastic force is transmitted from deformed propellant granules to the bullet base; hence, increasing the charge, progressively lessens the primer impulse required to initiate any given amount of bullet movement.  All else being equal, a heavier charge generates more primer-induced bullet movement even if the effective primer impulse does not increase.  Moreover, in such highly compressed charges, the primer plume might not penetrate as deeply as normal; therefore, less surface area exists where primer plume cooling and condensation can occur, hence the plume induced pressure can remain higher longer.  This situation does not occur in most normal rifle loads.

Testing Primer Impulse:
Primer-Only, Versus Primer-Plus-Inert

Because time for testing and the volume of simulant material available were both limited, we only tested the CCI-350 primer in this particular study — a primer which, according to the above tables and other test results, generates propulsive force this is similar to that produced by most Large Rifle primers.  The results of this test are startling for a number of reasons.  Despite the very high bullet pull used (tight carbide sizer followed by 0.425-inch expander, and Brinell 14 hardness bullets, seated 0.385-inch into case, test shots moved all bullets >0.65-inch, which means that practically the entire bullet shank had engraved into the rifling.

If we divide 0.85-inch by 0.70-inch, we see that adding simulant reduced average bullet movement 21.4% (about 1/5).  These results are unequivocal: Adding simulant material reduces primer-induced bullet movement, even in a load with an unusually high degree of charge compression (by rifle-load standards), and, by logical extension, so would adding propellant, see table.  This result offers dramatic support for the contention that heat transfer from primer gasses onto propellant granules outweighs any effects stemming from decreased air space in any normal loading in any reasonably large case (wishing I had the Meacham 22 Hornet for some related testing!).  This result occurred despite the fact that the charged test cases showed zero evidence of primer leakage on outside of cases, while primed-only cases were almost completely blackened — report was perhaps ten times louder with primed-only cases.  In progressively larger cases, difference in potential bullet movement will bias toward progressively less bullet movement with the charge installed.

As noted, rifle loads seldom generate 10% propellant compression during bullet seating.  Therefore, it is unlikely that, excepting combinations such as the 458 Win Mag or the 45-70, where long heavy bullets are seated deeply into a nearly cylindrical case, propellant compression will ever be adequate in rifle loads as to compress charge sufficiently so that elastic granule force will contribute significantly to the potential for the primer blast to initiate bullet movement.

Hence, it seems that a simple test will prove whether the primer will move the bullet and, if so, the maximum likely amount of such movement.  If a primed-only test shows no bullet movement, it is unlikely that the loaded round will show bullet movement in response to the primer blast — because any such pre-charge-ignition movement is apt to result in varying degrees of boiler-room enlargement before propellant ignition occurs (see test results), it is likely that such movement is always detrimental to utmost accuracy.

Because this result is so important, I will repeat it: If the primer blast alone does not move the bullet in your rifle load without the charge installed, it is unlikely the primer blast will move the bullet in any normal loading.

Some benchrest shooters attempt to get around the, bullet-movement-equals-variable boiler-room-space, limitation by seating bullets directly against the rifling, so that bullet movement requires rifling engravement, which requires significant force.  However, this approach may not work, as one benchrest competitor discovered when he failed to charge one case during a match.

The 205M primer drove his 6mm, 70-grain bullet downrange about ten feet! In disbelief, after the match, he experimented and found relatively consistent results, in his unusually small, tight-fitting and chamber-sealing cases, sized to provide only modest neck tension, using moly-plated bullets in an unusually smooth bore, the force of the primer blast always drove the bullet through the barrel, with enough velocity to move it several feet before it dropped to the ground.

How Blackpowder Differs from Smokeless Powder
(in response to the primer blast)

In the ways that it responds to the blast from a primer, blackpowder has two significant differences from smokeless powder (an interesting term for a product that is neither smokeless nor a powder!).  First, blackpowder might ignite in response to a somewhat different amount of heating.  Second, while smokeless granules are significantly plastic and particularly tough, blackpowder granules have very little strength and essentially no plasticity.  Hence, while smokeless granules throughout the charge can withstand the intense shock of the primer blast relatively unscathed, blackpowder granules toward the base of the charge will be massively fractured by the shock associated with any conventional primer generating sufficient energy to produce rapid ignition.  This fracturing will reduce or possibly eliminate permeability — at some point, the fracturing front may extend forward of the effective ignition plume so the remaining charge cannot ignite directly, just as with smokeless but for a different reason!

How far this fracturing propagates into the blackpowder column is an interesting question and might fully explain why hotter primers often generate higher blackpowder load velocities — more of the charge pulverizes, and, hence, more of the charge burns faster.  However, this hypothesized pulverization depth does not equate to effective ignition penetration depth.  One can envision situations where a relatively mild primer with longer flame duration (e.g., the RWS-5341) could well generate more primary ignition by penetrating farther into the charge without pulverizing as much propellant.  As one can imagine, resolving the issue of an overall burning rate increase resulting from pulverization versus an overall burning rate increase resulting from increased primary ignition depth would be somewhat difficult!

Testing Primer Plume Penetration

An Epiphany!  After contemplating for many years how one might go about testing how far the primer plume reaches into propellant the charge with sufficient energy to directly ignite granules — my feeble brain finally kicked in and functioned! Very likely, this occurred because, for the first time, I had an effective inert smokeless propellant simulant in my hands.

The test was pure simplicity; Initially, I chose a conventional extruded smokeless propellant with granules of similar size and shape to those of the simulant material (VihtaVuori N160, which is a reasonably typical propellant, in terms of deterrent characteristics, etc.).  I then fully prepped a batch of once-fired W-W 45-70 cases.  I prepared each of those in the following manner: full-length resized, bellmouth expanded (0.455-inch expander), flash hole prepped (K&M), primer pocket prepped (K&M), and inside mouth deburred and polished.

I then calculated various combinations of simulant (placed over the primer) and N160 (placed on top of the simulant and hence under the bullet) for testing.  In each test, I seated the Remington 405-grain JFP to an overall length (OAL) of about 2.56-inch, using an RCBS seating die, adjusted to provide a slight roll of the case mouth.  Because significant theoretical work by Byrom Smalley and me, on the SMc patent, had suggested that effective primer-plume penetration depth was unlikely to exceed one-half inch, the first test used sufficient inert material to form a compressed simulant column about 0.9-inches long; when this failed to result in propellant ignition, I tested progressively shorter simulant columns until propellant ignition did occur.

I will note here that I did subsequent testing with a wide variety of large-diameter primers, including standard and magnum rifle and pistol, including the mildest, least brisant, hottest low-brisance, both of the hottest primers available, and several primers from the middle of the spectrum.  Equally, I tested propellants of all types and burn rates.  And, I tested with inert columns of 0.7-, 0.6-, and 0.5-inch compressed height.  All of these tests were done with a greater degree of charge compression.

What this proved was that, with a single exception, a hotter primer did not result in 0.6-inch effective penetration to achieve ignition of any propellant.  Equally, the mildest primer and every other tested primer always resulted in 0.5-inch ignition depth and never with 0.6-inch ignition depth.  Finally, with the noted single exception, all propellants had a 0.5-inch maximum ignition depth.  Of course, with finer regulation of inert column height, I might have proven that measurable differences do exist, but I am satisfied that any such differences are modest.

As you might imagine, these tests were particularly nerve-wracking and difficult to load, with the faster propellants, I had to use a double charge of smokeless, with a slow rifle propellant atop the fast propellant.  When all was said and done, I had fired several thousand primers during this testing.  Could I have done more?  Sure, all it takes is time and money.  For now, I am satisfied with the validity and usefulness of the results I got with this testing.

Results of this test were a startling confirmation of every aspect of our theory — with typical rifle propellants, effective primer plume penetration will be about one-half inch; among large-diameter primers, primer choice makes very little difference in penetration depth (however, a hotter primer can generate effective ignition significantly further toward perimeter of case — although penetration depth is not significantly greater, more granules within the ignition depth are initially ignited); within normal typical bounds, percentage of propellant compression makes very little difference; granule packing scheme can have a modest influence.  (Subsequent preliminary testing, suggests that the plume from certain small-diameter primers, particularly Standard Pistol types, can demonstrate significantly less effective penetration.)

Some unusually slow-burning rifle propellants demonstrate less effective penetration depth, while unusually fast-burning rifle propellants demonstrate greater effective penetration depth.  (Particularly fast-burning handgun and shotshell propellants show significantly greater effective penetration depth).  This is exactly as would be predicted by anyone who understands how granules extract heat from the penetrating plume and how granules ignite: Compared to granules with a medium amount of deterrent in surface layers, granules that are more highly deterred require a longer bath in hotter gasses to achieve ignition temperature, while less-deterred granules can ignite in response to a shorter bath in cooler gasses.  (Deterrent always increases ignition temperature proportionally, the higher the degree of deterrent, the higher the ignition temperature.

A: In order to move the bullet this far, as necessary to cause significant rifling engravement,
it is likely that a few propellant granules ignited.  (While this might seem odd,
it is quite possible for such interrupted ignition to occur.)

B: Swirl Charging involves pouring granules though funnel in a manner that promotes uniform propellant packing scheme and an unusually high bulk density — slowly pour granule stream from scale pan against side of funnel cone with snout of pan tangent to side of funnel.

1: Audible hangfire, perhaps 0.3-0.4 second delay; muzzle velocity may have been near 800 fps.

2: Audible hangfire, perhaps 0.3-0.4 second delay; muzzle velocity may have been near 700 fps.

3: Simulant material solidly welded and stuck in case — the primer plume alone was
proven in the All-Inert test incapable of creating such effective welding
of simulant material; hence, partial ignition must have occurred.

4: <500 fps; dim, dull red fireball.

*: Same Test; ** Same Test

All charges thrown from Hornady L-n-L powder measure excepting test marked (SC) see note B.

All estimated lengths taken by measuring dowel protrusion from barrel, before
and after attempting shot.  All other lengths taken with dial caliper.  (On all misfires,
after length ignores several-thousandths-inch protrusion of primer from head of case.)

Relating this testing to Real Cartridges

An interesting and important characteristic of the 44 Magnum tests and the Inert-Only test in the 45-70 is that primer residues extended the full length of the case interior and onto the bullet base.  This deposition diminished from base of case, toward base of bullet.  It was obviously concentrated on simulant located near the primer and on the case walls near the web.  This deposit on the case interior wall showed obvious indications that granules in contact with the case walls had moved forward after the initial primer residue deposition.  This, scraping of primer residues from the case walls was concentrated near a point about 1/3-inch forward of the case web and diminished progressively toward the bullet base.

Besides being interesting in its own right, evidence that primer blast significantly compresses the simulant (by driving the base of the charge toward the bullet) might help us solve another aspect of internal ballistics that this simple inert-under-propellant test cannot resolve, specifically: in a normal smokeless load, in addition to the percentage of the propellant charge directly ignited by the primer, how much additional propellant is ignited by gasses from nascent propellant ignition before significant bullet movement occurs?

Ample evidence from theoretical considerations and from various types of testing suggests that such nascent ignition gas derived from propellant cannot reach particularly far into the initially unignited propellant for two reasons.  First, momentum transfer from the primer blast, along with the pressure build up toward the rear of the propellant column from the primer gasses and gasses generated by nascent granule ignition, works to compress the forward portion of the charge, rapidly and significantly.  This compression squeezes interstitial pore space so that both porosity (air space) and permeability (pathways through which gas can freely move) are reduced, the latter soon effectively disappears — in the subsequent 0.001 second before the bullet reaches the muzzle, if gas cannot move through the mass, it is impossible for sufficient heat to transfer into the mass to ignite granules trapped therein.  With the granule sizes used in small arms propellants, compressive heating of trapped gasses cannot generate sufficient local heating to cause adiabatic granule ignition — however, in some instances of detonation this effect might play a part.

Second, because the front of the charge is effectively sealed (while some gas can escape past the bullet, this effect is extremely modest); due to boundary-layer considerations and the almost vanishingly short time available, even while permeability still exists, very little heat energy can pass through the unignited mass.  Therefore, granules that are well removed from the zone of primer-induced ignition, cannot achieve ignition temperature through indirect gas heating.

All subsequent granule ignition occurs after the bullet begins to move and results from various sources, depending upon case configuration and a few other variables.  These sources include combustion along the rear face of the initially unignited mass; combustion along any sheared face that occurs in the initially unignited charge (such as occurs in many bottlenecked cases, when the perimeter of unignited mass is trapped behind the case shoulder while the core is driven forward behind the accelerating bullet); turbulent ignition, where granules are torn free from the plug of trapped material by gas flow or friction with the case walls and bore interior; and possibly other causes.

In blackpowder loads, little, if any, secondary ignition is likely to occur because the inherent permeability of blackpowder is so low to begin with and because (almost certainly) the shock from the primer blast pulverizes and compresses propellant near base of charge into an essentially impermeable mass.

One anticipates that many readers will object to using an inert smokeless simulant in place of an inert blackpowder simulant for this test.  I certainly object most vigorously! However, we know of no inert blackpowder simulant with sufficiently correct physical properties to facilitate such a test — all the following and perhaps other particle characteristics must be very similar: density, size, strength, plasticity, shape, and thermal properties.  Meanwhile, for myriad reasons, having to do with permeability and other properties, we will contend that the simulant used will probably accommodate effective primer penetration to a greater distance from the flash hole than would blackpowder or any (hypothetical) inert blackpowder simulant that was effective for such testing.

If this contention proves true, then it is fair to conclude that the Remington 2½ primer is indeed significantly milder and therefore apt to pulverize granules a shorter distance into the charge; that the RWS 5341, despite being almost as mild as the 2½ by every other measure, probably generates as strong an ignition pulse as does any hotter primer, but with far less charge pulverization; that the Federal 215 and Winchester WLRM perform reasonably similarly.  Specifically, excepting with the 2½, and likely many mild small-diameter primers, penetration depth is similar to that seen in smokeless loadings — evidently, to begin burning, both blackpowder and smokeless powder require reasonably similar ignition pulses.

Conclusions

While it is by no means the last word on this subject, this study at least partially answers all questions posed in the synopsis:

 

This photograph seems to be lost and so are the components needed to retake it.

45-70 simulant, blackpowder, simulant test examples with test bullet (Bear Creek Supply,
360-grain).  At middle is fired case from 215 primed test round (above is inert chunk
recovered from barrel, near muzzle).  At right is misfired round with same ratios of
inert and FFFg using 2½ primer.  Note that loaded OAL placed this bullet solidly
into the rifling; we did not observe primer induced bullet movement.