Well there just doesn't seem to be enough discussion happening here lately;so I thought I'd bring up something I've wondered about for a long time.But first,I think the hollow point pellet was misnamed.If it is hollow,then it doesn't have a point,and if it has a point ,then that's not hollow.Anyway...
When I was a school kid I took the family vacuum cleaner hose off and put it on the discharge of the machine.Air blew out of the end of the hose,and I held it vertically.Then I put a pingpong ball into the column of air and watched it ride up and down.It stayed in the column and did not fall to the ground.I probably saw it on a Julias Sumner-Miller film,or something.And Captain Kangaroo probably would have been proud of me too.But I've always thought that air spilling around the round shape and correcting as the pressures shifted ,was the same dynamic that helped the domed projectile to be so accurate.(setting gyroscopic effect aside for simplicity)
What about the hollow point pellet?I would think that the air would fetch up on the walls of the cavity and push it off the aim point very easily.But they can be very accurate.So I theorize that when the projectile moves through the air at such a high rate of velocity,that air quickly fills the cavity and becomes pressurized until no more air can enter.At that point,the air acts as the full dome would act;causing air to spill around it .And I'm thinking that the friction of the air on air would be less than it is on the metal of a full dome.Now the surface presented to the air by a projectile is a factor in figuring the ballistic coefficient.So the hollow point projectile could have a really good B.C. for a time.
Inaccuracy usually increases over distance.But I have noticed,in my shooting,that after about 60 yards the accuracy of my hollow point pellets falls off much more radically than my domed pellets do.That could be because the extra lead mass in the dome puts more mass forward than in the hollow point,but I have felt that the change is too acute, and too remarkable.
At 50 yards the average pellet has lost enough velocity to drop it's energy down to at,or about 1/2 it's original muzzle energy.If my other assumptions are correct,then maybe after my hollow point pellets pass the range of about 60 yards,the pressure in the cavity decreases.Then it can no longer keep side winds,nor forward wind effects from the dropping nose(as it follows the trajectory curve downward)from forcing it off it's former path.Then, from that point on ,the effective B.C. may be so low that the pellet's trip is over quickly-it's accuracy badly damaged.
What do you think?
EricinMaine-
Here I thought you wanted to know how a Hollow Point works,the work doesn't start until it hits its target and it works by expansion.Most people use them for hunting....
A Fat round does not have to go deep...just deep enough,um that sounds sexy,butt then I am a Fat Head,lol.
Well, for starters, the tendency of a ball to stay inside a column/jet of fast moving air
is more a property of the jet than of the ball -- specifically the "Bernoulli effect" of pressure reduction caused by velocity. The motion of the air inside the column makes the pressure there lower than ambient pressure outside, which literally sucks the ball into the column and keeps it there, even if the column is tilted off vertical.
I'm not sure how - or if - that's reflected in any aspect of the exterior ballistics of airgun pellets.
I think you are correct in your understanding of exterior ballistics of a HP pellet. The velocity creates a high pressure "bubble" in the HP nose cavity, effectively creating a tip of the pellet. When it impacts the bubble likely improves HP expansion. I've extracted numerous HP pellets from critters and the mushrooming is very minimal, especially with harder alloys such as Crosman.
In long range rifle shooting, most all of the high BC bullets are hollow point. When a bullet goes supersonic for a long enough period of time the tip becomes very hot and would melt a plastic tip. Recently some polymer tipped bullets have come out that can withstand this heat.
I think that your "bubble" theory is pretty close, but at sub, or trans-sonic speeds, the "bubble" doesn't really extend significantly in front of the pellet to improve the BC.
"the accuracy of my hollow point pellets falls off much more radically than my domed pellets do." That just goes to show that pellet quality varies between different types of pellets, and that each gun has it's preferred most accurate pellet.
The air in the HP doesn'tdoesn't really contribute to expansion. A non-compressible "fluid" in the HP, like fluids from the target, or something like wax, will act to initiate expansion. (I've done a lot of experimentation to confirm that). With powder burner sub-sonic bullets, if the hollowpoint gets plugged with a non-fluid material, it severely restricts expansion.
For high velocity HP rifle bullets, the tiny HP results from the bullet jacket being drawn from the base toward the tip. This allows a very smooth boat tail base on the bullet that really improves the BC and accuracy, more so than the tiny HP on the tip detracts from it. For varmint bullets of this type, they don't expand on impact, they "explode".
The study of ballistics is divided into 3 areas; interior,exterior,and terminal ballistics.Most of you saw a terminal ballistics question in what I wrote.That is the area I have given the least amount of thought to .You brought some interesting information and ideas.Early study of terminal ballistics involved SPLAT THEORY.I don't know if I remembered the name correctly ,but it involved gauging the final velocity of a projectile by the splat size when it hit a hard object.And then the angle ,or direction of approach, from the shape and orientation of the splat.So I wonder...could the expansion of different hollow points and various fillers in hollow points be studied that way?You could use the same solid object.Shoot from the same distance.And keep the momentum constant.Remember momentum is mass times velocity,and velocity is a vector involving speed and a direction.You would always want the angle ,or direction to be the same.--??
Steve in NC,
Thank You for the video ,that was cool (well cool USED to be a cool word).
I thought that they used stationary solid models and blew fast moving air at them in wind tunnels to study how similar objects ,at different velocities ,would pass through relatively static air. And when I considered that the dome is pretty much the same "leading"shape as the ball;so I thought their situation must be related.When I think about what you wrote,it makes more sense to me if I picture the following situation.Enlarge the size of the vacuum hose,or the hair dryer,to a 3foot diameter.Now we have a 3 foot column of air.Put the pingpong ball in the middle and it would bounce up and down and all around across a 3foot circle.Now the 2 situations don't look quite so closely related.
EricinMaine-
I've always thought the pellet going through the air was creating a sort of vacuume around it. Like riding behind a tractor trailer, and drafting it. I thought that same empty space around and behind the pellet, might be contributing to stability?
First of all forget the air bubble idea, it simply does not work like that. If it did there would be no difference in the drag between a hollowpoint and a round head pellet. At best the area of the hollow acts like a flat face hugely increasing the local drag force but in practice the total effect is worse as there is some element of flow reversal. A very deep hollow may contain some static air at its base but it will be very little and will still be circulating in some way. The hollow in a hollow point is purely for terminal ballistic effects. You can get some idea of the aerodynamic effects of the hollow from Hoerner's Fluid Dynamic Drag book much of which can be found on the internet.
There are actually four different areas in ballistics, internal, intermediate, external and terminal. The intermediate area is extremely important for precision from your rifle pellet combination.
Finally, the flare produces a lot of drag at the back of the pellet, worse than a cylindrical base, but the drag is not used to stabilise the pellet. The flare stabilises the pellet by producing lift the same as fins would. Wind tunnel studies have shown that much of the flare is not actually seen by the airflow but there is still sufficient to produce a stabilising lift force and to deflect the flow producing a reduction in the base pressure and thus an increase in base drag.
Miles,I'm glad to see that you made it over to AIRGUN WARRIORS.And it is good to hear from you again.
My conjecture is based on the behavior of the Crosman Permair hollow,and domed .22 cal.pellets that both weigh 14.3 grain.I expected the domed pellets to be the more accurate at all ranges,but especially as the range increased.What I found was that the hollow points were much more accurate,at least till the 60 yard mark.After that point their pattern grew radically.And this through more than 1 rif
le.(sorry-computer acted up.)In light of what you wrote,maybe I should view them as more like the wadcutter design.Wadcutters tend to be very accurate to 25 or 30 yards,and then the accuracy seems to fall away fast.In the hollowpoint design,quite a bit of the dome shape still remains in the head.Maybe this allows them to maintain accuracy for a bit longer.
As for the domed pellets,maybe the head size is not the same ,or varies too much.I can't measure head sizes.But I have weighed a tin each ,of the hollow point,and the domed pellets.The hollow points varied from 13.6 grain to 14.8 grain.The domed pellets varied from 13.7 to 14.8.For both pellets,the greatest amount were within a tenth of a grain.More of what was left were on the light side for the hollow points.More of what was left of the domes were on the heavy side.
I just lost some more of what I wrote.Website,or computer acting up.That's it for now.
EricinMaine-
As I understand it, the actual ballistic coefficient (BC) of pellets per testing data ought to tell us something, moving the 'ball of air at the front of a hollow-point pellet' concept out of the area of theory and into the area of fact, either yea or nay. Immediately, the test data at www.straightshooters.com came to mind. I remembered that they show test data for various pellets shot out of various rifles, and the hollow point pellets have always come away with a lower BC than other shapes (like especially the round-nose shape) in the test data on their site. Here's an example based on pellets of all sorts and shapes shot out of a Beeman R9:
http://www.straightshooters.com/beeman-r9-.22-beech.html
Now, something that is very-important when it comes to that information regarding the BC of various pellets is that I seem to remember that the velocity data listed there is based on actual measurements over a chronograph on at least some of the distances in their tables, not all of it being calculated velocities as determined by the use of a ballistic program. But, although I swear that statement was on their site at one time in the past, I just can't seem to find it there right now at all. Maybe someone else can, or can at least testify to the fact that they also remember reading it there.
I'd like to add a footnote: While the Crosman and Benjamin pellets with the little dimple on the front are described as "Hollow Points" on the package and/or lid, that dimple is so small I don't know if it really makes a difference between the performance of the "Hollow Point" and 'regular' or domed pellets of the same caliber and brand as far as BC. IMO, the much-larger 'hollow' dent in the RWS Super Hollow Point or Beeman Crow Magnum is really more of a true example of a hollow-point design, and thus field testing them for BC should provide more-definitive data.
Re: Crosman and Benjamin pellets with the little dimple on the front
Several years back, James Perotti and I measured the BC of .22 CPHPs off his back porch down his backyard range -- came out exactly the same as .22 CPs: 0.024
So you're absolutely right: The dimple is merely for cuteness, at least as far as exterior ballistics are concerned.
Miles,I'm glad to see that you made it over to AIRGUN WARRIORS.And it is good to hear from you again.
My conjecture is based on the behavior of the Crosman Permair hollow,and domed .22 cal.pellets that both weigh 14.3 grain.I expected the domed pellets to be the more accurate at all ranges,but especially as the range increased.What I found was that the hollow points were much more accurate,at least till the 60 yard mark.After that point their pattern grew radically.And this through more than 1 rifle.(sorry-computer acted up.)In light of what you wrote,maybe I should view them as more like the wadcutter design.Wadcutters tend to be very accurate to 25 or 30 yards,and then the accuracy seems to fall away fast.In the hollowpoint design,quite a bit of the dome shape still remains in the head.Maybe this allows them to maintain accuracy for a bit longer.
Thank you for the welcome.
If we assume my idea about the increase in group size at longer ranges and higher speeds being down to dynamic instability then it would be logical that wadcutters would be the worst, followed by hollowpoints and then domed pellets. This assumes that the ratio between the spin rate and the forward velocity will increase fastest on the wadcutters and least on the domed with the hollowpoints somewhere in between.
As for comparing BC values for hollowpoints and domes there are a number of problems to watch out for. At least 99% of the BC values I have seen quoted have taken no account of the atmospheric conditions during the tests. Thus the air density during the different tests could have been different, or the winds blowing in different directions, both of which will affect the BC value. The other problem is what reference drag law was used to produce the BC? Very few writers state clearly what basis they have used to calculate their BCs. It does not matter if they have used Steve's constant Cd method or any of the drag laws in something like Chairgun (including mine) as long as a consistant basis is used. Otherwise no comparison is valid.
The problem in using different pellet designs, even those from the same manufacturers with the same names, is that they are seldom identical designs. Even small changes in the flare length or design can give a large change in the drag. Different flare wall thicknesses will give different degrees of flare wall distortion when fired which again will change the drag. One way around this would be to take a domed pellet and drill a small hollow in the front thus retaining the shape of the rest of the pellet. However, getting the hole in the middle consistently would be a major headache and the change in mechanical properties (inertias) could change the flight behaviour again giving different drag and BC values.
Thus practical testing is very difficult if meaningful results are to be obtained.
Here's another database of BC's. These were generated using a single, regulated gun, 10 shots for each pellet. Velocities were measured every 10 yards using Labradar.
Doug, I drew a blank for a couple of seconds, but then I got it. I was looking for a link address in your post. If it's not obvious to someone else, the page you describe with more BC info opened when I clicked on the icon in the upper left hand corner of your post, located immediately under the capital 'B' in the words "Ballistic Coefficients". Thanks for the information.
For all that we might think goes on during the flight of either type of pellet,turning to records of real world tests was wise of those who brought examples.It is all we may be ahle to have at this time.For most of our uses,we only need to know the general relative difference between a handful of different pellets. Gleaning from one tester, and sticking with them ,would be the most meaningful.But in all the tests that we draw on,different sets of variables were left uncontrolled.That is not always wrong.For example,HAM ballistic testers took their pellets straight from the tin.That leaves a lot of variables uncontrolled.But like Tom Gaylord ,in his regular tests,they wanted to show what the airgunner is likely to find when they get their airgun product.--My ideas did not pass the scrutiny of my peers.In science,after observation and theorizing,that is the next step.I am satisfied with the answers I got with out the need for strictly controlled testing.
Mostly we are not scientists,but many of us like to dabble.We all want to know why-if it isn't too complicated.Miles outlined what we really need to do to put more meaning into this aspect of the hobby.There is a standard of atmospheric conditions.If we don't want to convert our findings to the standard;we should at least write the conditions down,and others will be able to make a better comparison to the data.Wind direction,speed and a good estimate of how much of a projectile's trip is effected by that wind,can be important,and our shooting direction.Pellet weight can be much more important to the pellet's flight than just a change in the height of impact.If it is too light-where is the lead missing from?Too heavy -where did the extra lead get placed?Just because a pellet weighs in properly does not guarantee that the lead was placed well and accrding to design.But if the weight is off, that almost guarantees that is isn't right.So in testing and experiments ,we should weigh 'em.Head size and skirt size can really matter for uniform accuracy and velocity;we should measure when we can.Please add any other variables that should be controlled to get better data.
After the physical part of the test;we need to use the same way of calculating results-check each other;s work.Miles mentioned drag coefficient.If we don't use the same coefficientt ,then of coarse we can't compare or relate the results.Another one I'm aware of is the 450240 used as a devisor in calculating the muzzle velocity.It uses Newton's findings for the acceleration due to gravity (32 feet per second,per second).He was really close,but there is a corrected figure now ,and hense,a new divisor.Who knows what any of us is using.This is a dark part of what we do because we usually don't mention how we got the results we ended up with.Many of us can't make it through the calculations .But we could include them.It HAS been done some on the Yellow.
Well I'm preachin' to the choir.Many of you taught me some of what I now know,IT IS WHAT IT IS-BUT WHAT IT COULD BE-IS WHAT WE COULD MAKE IT
Phew-you see what happens when I drink coffee too late in the day? EricinMaine-
For all that we might think goes on during the flight of either type of pellet,turning to records of real world tests was wise of those who brought examples.It is all we may be ahle to have at this time.For most of our uses,we only need to know the general relative difference between a handful of different pellets. Gleaning from one tester, and sticking with them ,would be the most meaningful.But in all the tests that we draw on,different sets of variables were left uncontrolled.That is not always wrong.For example,HAM ballistic testers took their pellets straight from the tin.That leaves a lot of variables uncontrolled.But like Tom Gaylord ,in his regular tests,they wanted to show what the airgunner is likely to find when they get their airgun product.--My ideas did not pass the scrutiny of my peers.In science,after observation and theorizing,that is the next step.I am satisfied with the answers I got with out the need for strictly controlled testing.
Mostly we are not scientists,but many of us like to dabble.We all want to know why-if it isn't too complicated.Miles outlined what we really need to do to put more meaning into this aspect of the hobby.There is a standard of atmospheric conditions.If we don't want to convert our findings to the standard;we should at least write the conditions down,and others will be able to make a better comparison to the data.Wind direction,speed and a good estimate of how much of a projectile's trip is effected by that wind,can be important,and our shooting direction.Pellet weight can be much more important to the pellet's flight than just a change in the height of impact.If it is too light-where is the lead missing from?Too heavy -where did the extra lead get placed?Just because a pellet weighs in properly does not guarantee that the lead was placed well and accrding to design.But if the weight is off, that almost guarantees that is isn't right.So in testing and experiments ,we should weigh 'em.Head size and skirt size can really matter for uniform accuracy and velocity;we should measure when we can.Please add any other variables that should be controlled to get better data.
After the physical part of the test;we need to use the same way of calculating results-check each other;s work.Miles mentioned drag coefficient.If we don't use the same coefficientt ,then of coarse we can't compare or relate the results.Another one I'm aware of is the 450240 used as a devisor in calculating the muzzle velocity.It uses Newton's findings for the acceleration due to gravity (32 feet per second,per second).He was really close,but there is a corrected figure now ,and hense,a new divisor.Who knows what any of us is using.This is a dark part of what we do because we usually don't mention how we got the results we ended up with.Many of us can't make it through the calculations .But we could include them.It HAS been done some on the Yellow.
Well I'm preachin' to the choir.Many of you taught me some of what I now know,IT IS WHAT IT IS-BUT WHAT IT COULD BE-IS WHAT WE COULD MAKE IT
Phew-you see what happens when I drink coffee too late in the day? EricinMaine-
I think that you are drinking too much coffee!
First of all, I am a scientist! I spent 34 years working on Kodak's Research Laboratories. One of the things that a scientist (especially an analytical chemist like me) looks at is statistics. We also look at the realistic testing that can be done. I was involved in the HAM BC tests, and quite a few of the things that you mentioned were addressed. To get really good statistics you need a sample size (n) of about 40. For bulk testing, this isn't really feasible due to the time required, so we settled on n=10. Atmospheric conditions were recorded, and plugged into the calculations. Hand selecting pellets for head size and weighing them probably wouldn't give better data. A 0.1 grain difference in 10 grain pellets is only 1%. BC also varies with velocity, but it wouldn't be very practical to try to adjust the gun to give the same velocity for each type of pellet. We'll leave the weight placement in the pellets up to the designers at the company that makes them- not something that we can measure or control.
We could go on all day, but testing that any of us do needs to be transparent and reproducible, and we need to try to stay away from things that really don't make much difference.
While the HAM figures may include atmospheric effects they still do not say what the most important parameter was in calculating the BC which is the reference drag law. Without knowing that, none of the published figures can be used with certainty as no one knows if they are using the correct law with the measured BC. It does not matter how the pellet speeds are measured, be it by chronographs or by LabRadar (about which I have some serious doubts having used far more capable radars for velocity measurements) if different drag laws are used to produce the BC and calculate trajectories then innacurate answers will be obtained. A serial of 10 pellets is sufficient, but it should be fired on three different occasions under different met conditions for an overall figure. This is what we do when testing small arms bullets. I do applaud the HAM tests as an attempt to produce a consistant set of figures.
It should also be noted that if the correct reference drag law is used the BC will be the same no matter what the pellet speed. This is the whole basis of the BC system. It is only if the wrong reference drag law is being used that BC values will vary with speed.
As for the 450240 figure used in calculating muzzle energy it is indeed incorrect using modern figures. I calculated the figure for the UK using the avaerage value for measured acceleration due to gravity and it came out very close to 450750 (it did not take very long as all the data is available on a spreadsheet so I am not that much of a nerd). However, the difference in calculated muzzle energy is very small and probably less than measured errors in pellet weight and velocity. I think I found out once that 450240 is correct if you happen to live in Salt Lake City.
Miles,
You are wrong about the BC being velocity independent. Our data, and others, have found significant BC differences with velocity. "Drag laws" can't be idealized for each pellet, so all we can realistically do is estimate the drag models. And as I have said before, the practicality of doing things "properly" (shooting on three different occasions) just isn't there. You do the best that you can, with the time and tools that you have.
http://www.exteriorballistics.com/ebexplained/4th/45.cfm
BC variation with velocity was well documented in posts on the old yellow forum and others.
Here's another reference:
A little "reverse-engineering" of the HAM BC numbers reveals they were calculated using a constant-Cd BC expression I derived and posted on the Yellow over 16 years ago...
https://www.tapatalk.com/groups/yellow/viewtopic.php?f=79537&t=13759&p=67980&hilit=bc+8000#p67980
All the HAM BCs (but one) are within rounding error of the result calculated by that formula.
Which isn't too surprising since said formula has been incorporated into numerous ballistic calculators during those intervening years, usually (as here) with the original source being uncited and (probably) long forgotten -- the sincerest form of flattery, I hear. ?
"Drag laws" can't be idealized for each pellet, so all we can realistically do is estimate the drag models.
Here's another reference:
Idealising pellet drag laws for each pellet design is exactly what we shoulld be doing and getting away from BCs especially for long range shooting. The small arms bullet industry has now "discovered" Mach number/Cd curves while in the ballistics of large calibre weapons we have been using them for the last 60 years. This is why software able to use Mach number/Cd curves is the way to go for pellets as well as everything else.
Quoting one of my own early articles at me is hitting a bit below the belt Doug. ? I was assuming a constant Cd drag law or at best the G1 drag law in that article which was what was commonly used in those days. Pellet suitable reference drag laws are now available, at least for domed pellets (there are two in Chairgun), so there should be little change in BC with velocity. As I said before that is the whole point of using the correct reference drag law.
I have produced drag laws for wadcutter type pellets as well, but that is more complicated and none of them has been used as yet. But, even if they were used in producing BCs, without the software able to use them in trajectories they will still be useless and, due to the complications, you may as well go straight to the Mach number/Cd curves.
The hollow point is for short range use. Most ppl try and use them for 35yrds. plus in which they are not accurate at those ranges in most but not all cases. But when used at closer ranges they are effective and work as designed opening up an creating more surface area upon impact with energy.
