At the Moment of Firing and Fit of pellet to the rifling

INTRODUCTION.- One of the most misunderstood aspects of airgunning is the series of small, but important events that take place at the moment we pull the trigger.
Firearms enthusiasts know well the series of events that transpire inside the brass case the moment the trigger is pulled, but let us review them for the sake of those that are not so familiar with powder burners: the firing pin impacts the primer, the pre-stressed mixture ignites, sending a jet of very hot gases through a small hole into the main/powder charge cavity of the case where the granules of smokeless or the flakes of black/synthetic powder ignite in a somewhat random fashion. The further high temperature and pressure of the gases defeat the coating of the granules and ignite them in somewhat of a chain reaction. This makes the pressure inside the case soar to levels that are high to really grasp in our normal everyday mind as we never encounter in our everyday lives things that work at between 16,000 and 55,000 pounds per square inch (PSI).

Just to put that into perspective: imagine in your mind’s eye a 27 ton Sperm Whale (like Moby Dick) standing on its nose supported by a steel billet that is 1” on each side, square.
If you think Uncle Ted stepping on your toes was bad this last Christmas, I can assure you this would be many times worse.

So, there is a lot of pressure, and that pressure impinges on the BASE of the bullet. That base may be solid, or hollow; flat or boattail, but in general we are talking of something substantial and even the Minnie style hollow base bullets are solid at most 1/6 of the way forward. Yes lead is “soft”; anywhere from Brinell 5 to Brinell 22, and if it is a jacketed bullet, then you are talking of Brinell 35-125 (if the jacket is pure copper - gilding metal). Main point to remember here is that NOTHING in the firearms world approaches the softness of a skirted pellet that is hollow almost halfway through its length and made of alloys that, in the hardest of cases, contain still less than 3% antimonium (non-lead pellets are different, and while those require special attention, they still are much softer than the softest jacketed or plated bullet).

But, let’s go back to the point where gases are starting to impinge on the base of the bullet, and this pressure, exerted over an area means a force. The force created thus pushes the bullet out of the case, pushes the case walls out into the chamber walls and seals the neck of the brass case to the steel neck of the chamber, thereby providing a really good seal. This is what keeps us safe from all the mayhem inside the brass case when we shoot.
The bullet is forced forward VERY rapidly. So rapidly that the front section of the bullet does not want to move and this forces the rear section of the bullet to expand into the rifling, sealing the gases behind. If the bullet is solid, there is little “upset” as this phenomenon is called, but if the bullet is hollow based or soft, this upset has to be taken into account when designing the bullet. Some is good, too much might not. For a short while, under these pressures and forces, the metal of the bullet is in almost a malleable/fluid state; and in a short time after this, the front of the bullet finally gets accelerated and also upsets, filling the rifling completely.

In the cast bullet world, it is generally understood that bullets have to fit within 0.001” of the LAND / CALIBER / GROOVE diameter, and that the hardness of the alloy has to be such that no material gets stripped off the projectile and gets embedded into the rifling. At present, we do not use as many rifling designs as we used to: Pope, Forsyth, Alex Henry, Segmental, Obermeyer, Ballard, Green, Ratchet, MicroGroove, Enfield, Polygonal, Elliptical, Whitworth and a host of other styles, but, for the present, the list is quite shorter: MOST airgun manufacturers will use the land and groove style with equal angular spreads for lands and grooves called Enfield or Ballard or the Polygonal style now in use by Lothar Walther barrels, VERY FEW manufacturers will go to the trouble of designing a barrel for a specific projectile, as it has been proven that the general style does work over a wider range of speeds, shapes, materials and duties.
The exception, of course, are those barrels designed for military projectiles (where the potential of a government contract makes sense to go through the development process) and, to our knowledge, the 0.20" cal CCA barrel designed specifically for the JSB 13.7 grains Exacts (and made by Lothar Walther).
The MAIN duty of the rifling is to swage the projectile to its final shape and make it turn.
Why so many styles for such a simple duty? you may ask. 
The reality is that the design of the rifling is also responsible for a number of things:
How much energy is needed to get that projectile swaged to its final shape?
How much interference there is between the original shape and the final shape and therefore, how much cleaning the barrel will need.
How UNIFORM can the barrel be built or made.
For a time, I experimented a bit with the Forsyth rifling (in airguns also known as the "Career" rifling. But that experiment, alas, was not as successful as I wanted. And yet it produced an interesting result (more on that later).

IN THE AIRGUN WORLD.- What happens is, perhaps less dramatic from the numbers and units standpoint, but given the SHAPE of the classic Diabolo Pellet, the proportion of forces, pressures and temperatures (especially in the spring-piston airguns), AND the extremely low level of available energy it is probably much more important than in the case of firearms where energy is plentiful. So, let’s analyze step by step the events inside the barrel when we pull the trigger. We will need to divide the discussion between Springers and PCP’s because the pellets suffer a slightly different process and even within the springer class the process is slightly different in the case of those guns with long transfer ports and those with short transfer ports. But let’s take it case by case:

In a PCP, when the trigger is pulled this releases, in MOST cases, a hammer that impinges on a valve that then pops open, allows some quantity of high pressure gas to flow through it and then closes. Depending on whether it is a regulated or a non-regulated gun, the pressure admitted into the expansion chamber may be anything between 3,000 PSI’s and 1,150 PSI’s. Some guns work at very high chamber pressures (like the Walther 300, the HW-100, or the Talon), some guns work at very low chamber pressures like the USFT. But we are still dealing with a force that in the BEST of cases (1,150 PSI’s applied over 0.177” diameter means around 28 lbs), is still substantial in relation to the pellet’s material and shape.

Go back to the mental image of the whale and now imagine a single pellet supporting the weight of a young child. Yes we are not dealing with the fantastic numbers of the 7X66 Vom Hoffe but, for a humble pellet, this is indeed a great stress.
The saving grace is that, like some things in life nothing lasts forever, and this force is applied over a VERY short time to the pellet. So short that if you could apply the same proportion of force in a step in the same amount of time, you could walk on water. As a matter of fact, that is how some lizards actually walk on water.
So, the high pressure gas impinges on the pellet’s hollow skirt and drives forward the pellet. Just as in the case of the bullet, the pellet’s front section (the head) wants to stay put through inertia, and while the skirt is pushing forward, the head is resisting and this places all the stress in the waist or in the column that links the two parts (head and skirt) of the pellet.
As is the case in firearms, the high pressure gases also exert sideways forces and these tend to blow out the skirts of our pellets:

On the Left a fired Marksman pellet, on the right an unfired one. Look at the skirt and how it deformed (blew out). This pellet was fired from a Talon SS with a Lothar Walther barrel.

Careful experiments have shown that pellets DO DEFORM upon firing. And one of my preferred methods of choosing the best possible pellet for a barrel involves soft-capturing fired pellets and measuring how much they expand at the waist or the column. The more they expand, the less are we in control of the final shape of the pellet once we have pulled the trigger.
Once the pellet starts to move, depending on how the chamber was machined (or not), the head engages the rifling before the skirt and this also exerts a TORSIONAL stress on the waist or the column. At times, this stress may be enough to deform the pellet substantially, in most cases it is not, but it is still mentioned here to point out to problematic barrels that, in reality, need only to have their chambers fitted to the pellet the shooter wants to use.

The JSB Heavy MkI (original model) unfired on the left, fired from a Talon in the middle and fired from a Steyr LG-100 on the right.

By now, our pellet has been pushed, it has already upset in the rifling, and is travelling down the bore. And then, it reaches the choke. The choke is a constriction at the muzzle that is used to improve on the accuracy of the pellets. It is a historical aspect that came about when ranges were short (think less than 30 yards/meters) and the lack of standardization in the manufacturing of pellets was so blatantly lacking that some countries even went to a numbered bore system instead of a caliber. Back in those days the waisted pellet was an anomaly, it was produced in roller dies and the most common pellet was a slug, sometimes with a felt base (for lubrication). Chokes then made some guns shoot well a variety of pellets that could deviate in caliber almost a thousandth of an inch either way from what was SUPPOSED to be the NOMINAL caliber.

We now keep the choke not only as a historical habit and custom, but in part to STILL handle the differences in manufacturers’ specifications. The choke’s constriction, then becomes the last obstacle in the race of the pellet from chamber to target. In this case, it is the HEAD the one that suddenly encounters a resistance and it is the inertia of the skirt what tries to deform further the waist.

So, by now, our pellet has been pushed, shoved, blown out and squeezed. And we still expect it to have the same original shape and ballistic coefficient? I think that is a little naïve on our part. And if we think that is extreme treatment of our favourite projectiles, let’s look now at what happens in a springer.
​
In the case of the springer, when we release the trigger, the large mass of the piston gets accelerated forward, the seal seals when the pressure between the pellet’s resistance and the reducing chamber of the compression chamber reaches the point where the parachute opens (unless it is an ORing’ed piston). The piston continues to move not only due to the constant force applied by the mainspring, but also due to the inertia it has now acquired. By the time the piston reaches the end of the compression stroke, the pressure inside the compression chamber can be as high as 3,000 PSI’s and the TEMPERATURE can reach up to 2-3,000 F. Ideally, this set of conditions turn the air into a plasma and this plasma has very little internal friction (viscosity), so that it can flow through the transfer port and into the chamber.

IF the transfer port is too long, then the plasma has time to expand, cool down and return to a state of highly compressed gas. This MAY be beneficial in some guns shooting particularly thin skirted pellets, but in general, it reduces the efficiency of an airgun substantially. In those cases where the transfer port is short (most instances of sliding compression chamber guns), the plasma hits the pellet’s base not only with a blast of air, but with a blast of very hot air. The higher the temperatures and pressures getting to the chamber, the higher the efficiency of the gun will be. Once the gases hit the pellet and the pellet starts to move, the rapid expansion of the available volume cools down the air, so the pellet does not suffer from the high temperatures BUT, what DOES suffer from the high temperatures is the breech seal. Even the smallest of defects in the seal will begin allowing the hot plasma to exit between the seal and the barrel’s breech and this will create real “flame cutting” grooves in seals that do not fit well, are ill designed, suffer from poor quality materials, or are overstressed in relation to their initial operating parameters.

What the pellet DOES suffer is the incredibly more abrupt acceleration that a springer applies in relation to what PCP’s do.
Those shooters that have been shooting airguns for more than 15 years will remember the “Flying Trash Can” pellets, and how they sometimes blew up so badly as to become complete cylinders.
Luckily, the pellet making industry has advanced by leaps and bounds and we no longer have to worry with such extreme cases as long as we stick to quality ammunition and reasonable operating regimes. But it is still a concern where extreme precision is looked for with a spring-piston airgun, as paying attention to the internal ballistic aspects of the shot cycle is a must for those endeavours.
After the first, initial, blast starts expanding, the course of the pellet’s life inside the bore is pretty similar in either powerplant. The pellet will still travel through the bore and will still encounter the choke.

A completely blown up pellet

What we can do with this information.-
If you are a user/shooter:
Do note that the pellet designers have two basic ways of thinking. Let’s say, two “philosophies”:
In the case of JSB, H&N, RWS, and others, pellets are designed so that the head, USUALLY, rides the bore, but the SEAL is performed by the skirt at the groove. This allows for a little more efficiency from the system, as less energy is expended on re-shaping the hardest part of the pellet to the rifling.
It also implies that the skirt size needs to be closely matched to the GROOVE, depending on the specific design of the rifling. For example, if one rifle seems to be accurate with very large pellets (4.53’s or 5.55’s), then it MIGHT be worthwhile to test that rifle with pellets in the 4.50 or 5.51 sizes, as the difference is to make the head engage the rifling, or just ride on it.

On the left are Crosman Premiers. Do note how deep the head engages the rifling, as opposed to the Marksman pellets. Both pellets were equally accurate out to 50 meters from this Talon, even though the Premiers look less deformed.

BUT, In the case of the Crosman Premier, the Defiants, and others, the obturation (seal) is done AT THE HEAD, and the skirt is just along for the ride. The pellet is not as “hollow” as other designs and it has to be made of harder alloys. It is impossible to make these pellets “ride the lands”, as there would not be an effective seal and much energy would be lost.
So, each shooter has to decide what he wants to try and be prepared for behaviours that do not necessarily match pre-conceived notions of what 'should' work and what should not.
SOME guns may benefit, from the accuracy standpoint, of using one design style over another, BUT what is critical is that the consistency of the manufacturing demands is much higher when BOTH functions are assigned to one side of the pellet, as opposed to splitting the duties. On top of that, there will be a small efficiency drop, but a few fps is not important when you consider that the objectives of the airgunner are achieved through precision, not power.

Another important factor is that pellets that are designed to seal and guide at the head also impose a higher degree of stress to the material. And that is why they are made of harder alloys, either by adding antimonium to the lead, or using tin. In those cases, the harder alloys benefit MOST from using a good bore lubricant; applied directly to the bearing surfaces of the pellet, it will not only reduce the energy available for the generation of harmonics, but will also protect the bore from leading.

If you ever get interested in designing a rifling.-
You need to define what "philosophy" you want to use: will your rifling engage the head fully all the way to the grooves? Will the rifling be designed to make the head "ride the lands" and only the skirt needs to seal? What rifling pitch will you use? What metal spec will you use to manufacture the barrels? Ordinary mild steel? What happens in the case of breakbarrels where the barrel is the cocking lever? Are you sure of the REAL, not the NOMINAL dimensions of the pellet you selected?
If, by some reason, you want it to shoot well ALL the pellets, by now it must be clear that such a magic barrel would be almost impossible.
Just to give you an idea, here is a figure from a rifling patent, showing how many dimensions need to be specified for a rifling:

And in addition to the seven dimensions hereabove noted, you need to specify the material, the rifling pitch, and the manufacturing tolerances.

​Now, all the theory in the world (even when proven by a few experiments) is useless in the field if in YOUR particular gun pellet ‘A’ shoots better than pellet ‘B’. So, ALWAYS test a number of pellets. ALWAYS keep an eye out for new introductions, ALWAYS keep an open mind as to what the future may bring, and ALWAYS remember that YOUR gun is really unique. It is up to YOU to discover what is BEST for you and your system.

 
Keep well and shoot straight!