Connecticut Custom Airguns
  • Welcome!
  • Hector's Airgun Blog
  • Products and Products Blog
    • One-Off's
    • The "Héctors Special'" scope by Sightron
    • K1050i FT
    • The Hex Louver or "Secret Sunshade"
    • Pellet Path Calculator >
      • Questions, Answers and Comments on P-P Calc
      • Privacy Policy for PP-Calc
    • The Nautilus SideWheel
    • The X-10 TiltMeter
  • Zimmer-Silhouetten
    • Results 2017-2018
    • Results 2016-2017
    • Results 2015-2016
    • Results 2014-2015
    • Results 2013-2014
  • References and Links
  • Contact us
  • Store

Hector's Airgun Blog

Where we discuss, CIVIILY,  anything airgun.

Return Home

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

1/19/2016

19 Comments

 
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).
Picture
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:

Picture
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.
Picture
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.
Picture
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.
Picture
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.


Picture
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:
Picture
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!
19 Comments
Alejandro O. Martinez
1/19/2016 20:57:51

I'm gonna have to read this article ... over-and-over ... very comprehensive. Well worth your time to read.

Reply
Hector Medina
1/20/2016 09:51:43

Well. if I am confined to bed with a cold, the least I can do is write something, no? LOL!

Thanks for your kind words.

Un abrazo!



HM

Reply
Tim
1/22/2016 14:03:30

Thank you very informative article. Makes you wonder if the FX Smooth Twist barrel would be less consistent than a fully rifled barrel?

Reply
HECTOR J MEDINA GOMEZ
1/23/2016 00:57:30

Tim,

The "Smooth Twist" is nothing but a modern variation of the old Paradox shotgun barrel. In the old days, the Paradox was used to impart a spin to lead slugs/balls. To do this properly, the paradox had a ratchet rifling design, as the soft lead, but heavy, slugs/balls would otherwise strip the rifling.

The current ST barrels are basically the same idea, but at the muzzle end of the barrel they are polygonally rifled by putting the smoothbore barrel into hydraulic dies. It is, therefore, MUCH cheaper to manufacture than a conventionally rifled barrel.

As all things in life, it really depends on the exact conditions of usage whether this will be a good solution for you or not.

All serious accounts I have read place the Smooth Twist barrel in a favourable light when used at very high energy levels.

For moderate (20 ft-lbs) to low (5.5 to 12 ft-lbs) energy levels, the traditional rifling seems to be better suited to cope with the natural variations of pellet rifles.

Thanks for reading!





HM

Reply
RidgeRunner
1/30/2016 06:02:21

Hector,

This is a fantastic post! I have enjoyed reading your blog for some time now. It is great to get the "technical stuff" and not just the "fluff". I have learned much from you. Thank you.

Reply
HECTOR J MEDINA GOMEZ
1/30/2016 08:17:56

Thanks, Ridge Runner!

In the end, YOU are the cause of our existence.

Without readers, why would there be any writers? LOL!

Thanks for your kind words and for reading all my ramblings.

Un abrazo!





Héctor Medina

Reply
Kevin udy
3/17/2017 09:51:30

Another brilliant article....you write the best airgunning articles I think I've read.
I'm a 62-and-bleddy-counting Cornishman (UK) and have recently returned to using airguns more seriously after years of just occasionally using an old, slightly tired, Weihrauch HW85 to try and murder the local squirrels that have been destroying my attic, and the odd rat.
I had asked at my gunshop shooting range the other day about the effect of tighter/looser fitting pellets, as some I had tried were snug to quite tight fit in a new Walther Century I bought on impulse recently, thinking it had the same internals as the LGU/LGV. (Good bit miffed at finding I was wrong). They said it didn't make a lot of difference, and in general terms I didn't notice a LOT of difference, although the one pellet tat was a very easy-to-loose fit was a little less accurate. I had thought a tight fit would cause more drag in the barrel and loss of velocity, but now, reading this excellent article, I'm now think the tighter fit is a good thing?
Brilliant article and I know more now that I did on waking up this morning.
Thanks for taking the time and trouble, old son :)

Reply
Hector Medina
3/17/2017 19:19:37

Kevin,

Thanks for your kind words.

As I say: Let the barrel tell you what it likes.

Many times I've seen barrels that shoot 'loose' fitting pellets very well. And also barrels that shoot tight fitting pellets well.
Since the tightness we feel is only the leade/chamber , the information we get from the pellet insertion is very little.
You COULD run a pellet into the barrel with a cleaning rod and judge how tight/loose the barrel is throughout.

So, test all the pellets you can.

After all it's part of the fun.

Thanks again for reading and give my best to Penzance!

Reply
Mark
4/28/2017 21:40:31

This was really an amazing reading and a great explanation of the airgun shooting action. Great job! Sir, thank you very much!

Reply
Hector Medina
5/3/2017 08:36:30

Thank YOU sir!

Without readers, what would writers do? LOL!

Thanks again!




HM

Reply
dave hardan link
9/29/2017 21:14:38

Greetings. I'd like to know how you know the temperature (2-3000f.) Inside the compression chamber. Thank you.

Reply
HECTOR J MEDINA
9/30/2017 14:04:12

Dave;

We know the RANGE of temperatures achieved in most compression chambers of medium to high power guns by indirect means.

One indication that is visible to everyone is the discoloration of certain bluings in the joint where the compression cavity meets the solid face that then turns into the forks of breakbarrels.
When new, that joint is imperceptible, but with use it starts going "purplish", so that tells us there is a HUGE temperature spike that is not long enough to change the color to golden/straw, but is enough over time to change the blue to purple.

Other method is the introduction of carefully selected lubricants/mixtures (don't do this with a FINE airgun!), with KNOWN flash temperatures. If it "diesels", then you know you have reached/exceeded the instant flash temperature of the "witness" compound".

Another indication still is the old experiments where glass/quartz windows were cut into the compression chambers and the color of the spark/flash was described in some precision.

Lastly, add carefully notched beech seals to determine which materials are most suited to resist the flame cutting (how plasma cutters work) effect.

With all the above indications, we can, with certain confidence, rely on a RANGE of APPROXIMATE temperatures that CAN be achieved in the compression chamber of a springer.

The reality is that what is important with modern lubricants (like Ultimox, or Krytox), is that it is VERY difficult to detect any dieseling at all; there is no smoke, no smell, and no residues (if you have removed ALL the fossil-derived fuels), but we know the temperature spike is still there, so other means would have to be undertaken. The field of SEALS is one where there are still oportunities for large improvements in airguns. And in this respect I am happy with the HMR breech seal design, that has a metal insert that protects the plastic element that actually creates the seal and uses the Diana made silicone rubber material that creates a good seal at the pistons also.

I hope we have provide an answer to your question.

Thanks for reading!





HM

Reply
Wade Sutherland
10/24/2017 22:45:18

Hector, I'm sure we've met and it may have been in Texas at one of those Nationals.... Anyway, this was a masterpiece. A great read and done in such layman's terms.

Reply
Hector Medina
10/25/2017 12:59:24

Hello Wade!

Yes, we've met several times, probably the first time at GOB's in Tennessee.

And then, of course the bunch of Nationals.

Thanks for your kind words, coming from one of the "Deans" of airgunning in the USA, they mean a LOT to me!

Keep well and shoot straight!





HM

Reply
Paul Brooksbank
10/25/2017 17:14:26

I love the reply to the "how do you know the temperature" question. That nails that one!

Reply
Hector Medina
10/25/2017 17:43:20

Paul,

Thanks for your kind words. Not all credit is on me.
Some of those experiments go way back to the Cardews' research, and even for some of my original research I couldn't have gone too far without the help of two great friends, both passed away; one of them a chemist with over 45 years of experience in the field of petrochemicals, the other an airgun tinkerer extraordinaire.

We are all the results of our past experiences and "if we can see far, it is because we are standing on the shoulders of giants".

Again, thanks, my friends would have been delighted to read your comments.

Keep well and shoot straight!





HM

Reply
Russell Best link
10/26/2017 05:41:21

Hector..just wanted to thank you for another well written article. I used to make custom gun barrels ( mostly rifle), and it's refreshing to see someone so knowledgeable posting good info these days. Well done!

Reply
Hector Medina
10/26/2017 16:15:47

WOW!

I'm humbled Russ.

Coming from you those words will always stay in my memory.

I have always respected your work, you know that. You have been one of the lights that illuminate the path in the world of spring-gunning for over 2 decades and more, and so, your comments bear a lot of weight, thanks for your kind words.

Un abrazo. my friend!





HM

Reply
Russell Best link
10/26/2017 17:45:09

You are too kind Hector, but thank you for the props. Carry the torch forward my friend!




Leave a Reply.

    Hector Medina

    2012 US National WFTF Spring Piston Champion
    2012 WFTF Spring Piston Grand Prix Winner
    2013 World's WFTF Spring Piston 7th place
    2014 Texas State WFTF Piston Champion
    2014 World's WFTF Spring Piston 5th place.
    2015 Maine State Champion WFTF Piston
    2015 Massachusetts State Champion WFTF Piston
    2015 New York State Champion WFTF Piston
    2015 US National WFTF Piston 2nd Place
    2016 Canadian WFTF Piston Champion
    2016 Pyramyd Air Cup WFTF Piston 1st Place
    2017 US Nationals Open Piston 3rd Place
    2018 WFTC's Member of Team USA Champion Springers
    2018 WFTC's 4th place Veteran Springer
    2020 Puerto Rico GP Piston First Place
    2020 NC State Championships 1st Place Piston
    2022 Maryland State Champion WFTF 
    2022 WFTC's Italy Member of TEAM USA 2nd place Springers
    2022 WFTC's Italy
    2nd Place Veteran Springers

    Archives

    August 2022
    May 2022
    April 2022
    February 2022
    August 2021
    July 2021
    June 2021
    May 2021
    April 2021
    March 2021
    February 2021
    October 2020
    September 2020
    August 2020
    July 2020
    June 2020
    May 2020
    April 2020
    March 2020
    February 2020
    January 2020
    December 2019
    November 2019
    September 2019
    August 2019
    July 2019
    May 2019
    April 2019
    January 2019
    December 2018
    November 2018
    October 2018
    September 2018
    August 2018
    June 2018
    May 2018
    April 2018
    March 2018
    February 2018
    January 2018
    December 2017
    November 2017
    October 2017
    September 2017
    March 2017
    December 2016
    October 2016
    September 2016
    July 2016
    June 2016
    March 2016
    January 2016
    December 2015
    November 2015
    October 2015
    September 2015
    August 2015
    June 2015
    May 2015
    April 2015
    March 2015
    February 2015
    January 2015
    December 2014
    November 2014
    October 2014
    August 2014
    July 2014
    June 2014
    May 2014
    March 2014
    February 2014
    December 2013
    November 2013
    October 2013
    September 2013

    Categories

    All
    Events
    Gear
    Hunting
    Tests

    RSS Feed

Proudly powered by Weebly