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ballistic coefficient data

December 24 2009 at 3:46 PM
  (Login Teryx)
YF

An earlier conversation got me thinking about this. I don't see pellet manufacturers listing the ballsitic coefficients for their pellets. There is lots of data out there concerning muzzle energies from various setups. With many guns capable of reaching out 100 yards, there could be huge differences in terminal energy depending on pellet design. While something like a Predator and a Silver Bear might seem comparable at the muzzle, what about at 50 yards? 100 yards? Maybe pellet choice makes more difference than you think.

Does anyone know of a database that contains BC values for common pellets?

Teryx

 
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(Login pneuguy)

You're right. Pellet BC does make a huge difference in...

December 24 2009, 3:57 PM 

...not only energy retention, (what's possibly even more important) but also real-world accuracy (e.g., resistance to wind drift). Pellet BC can vary from one type to another by a factor of 4 to 1.

This database complied by Patrick H. contains a few typos and blank spaces, but is still one of the best available: http://www.airgunexpo.com/airgundb/pellet.cfm

Meanwhile, the one and only pellet manufacturer that publishes velocity retention data (from which BC can be easily calculated) is poor old downtrodden and much-derided Gamo.

Nobody else - not even the much hyped brands.

Steve

 
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(Login Teryx)
YF

WOW worse than I expected!

December 24 2009, 4:12 PM 

Or better depending on how you look at it. That's a great database. It really shows the folly of choosing a pellet based on how cool or wicked it looks. At 10 meters who cares, but at hunting distances you can lose much of your guns potential with the wrong choice of pellet. VERY interesting!

I'll admit, I'm guilty of Gamo deriding happy.gif

Teryx

 
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Yrrah
(Login Yrrah)
YFOT

Gamo ... if this is a transcription of Gamo's BC data

December 24 2009, 6:50 PM 

then its value is extremely suspect to say the least:

Gamo Hunter 15.30 Pointed 0.220
Gamo Magnum 15.60 0.220
Gamo Master Point 16.20 0.220
Gamo Match 14.10 Wadcutter 0.220

Maybe the typewriter had the stutters?

Best wishes for The Season and Kind regards, Harry.

 
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(Login gubb33ps)
YF

Have to use the plug-in calculator...

December 24 2009, 6:55 PM 

..but this seems more reasonable (and it's Gamo's data):
http://www.aceros-de-hispania.com/gamo-pellet-characteristics.htm

 
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(Login pneuguy)

Yup. And when you do, you get...

December 24 2009, 10:36 PM 

[linked image]

Steve


    
This message has been edited by pneuguy on Dec 24, 2009 10:37 PM


 
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(Login gubb33ps)
YF

thanks...nice to see.

December 25 2009, 1:57 AM 

Haven't tested those pellets for BC, but the match WC's and .177 ball seem a bit too high... and the TS10 and TS 22 kind of low (but those pellet's suck so bad in anything I've shot, who cares?).

An aside:
Gamo has been and still is making two .25 pellets...if their other calibers are a guide, the Hunter should be at least acceptable in accuracy, and if they could keep the pellet cost as proportional to the other caliber Hunters, believe they'd sell. As they already make them, just need to get them from point A to point B.


    
This message has been edited by gubb33ps on Dec 25, 2009 2:16 AM


 
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(Login pneuguy)

I agree. Gamo's number for the .177 ball is 50% higher than...

December 25 2009, 10:13 AM 

...my experience with round shot, and some of the other numbers look screwy as well. Maybe Gamo's test range is on a mountaintop somewhere?

However, the fact remains that Gamo is unique in publishing any data for their pellets at all. So just for fun I converted their numbers to BCs with a spreadsheet.

Steve

 
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Yrrah
(Login Yrrah)
YFOT

Thanks Steve ........ and it clearly shows

December 25 2009, 5:27 PM 

why the .22s don't cut the mustard in the real world.
The reason for companies not publishing is probably because BC varies so much from one to another rifle.

Merry Christmas .. Kind regards, Harry.

 
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(Login Parallax7)
YC

The trouble is...

December 24 2009, 4:49 PM 

BC will vary many times between different rifles shooting the same pellets from the same tin. Even if you take 2 of the same model of rifle and grab pellets from the exact same tin and shoot them out of both of those rifles, often they will yield different BC data. So there is obviously more going on here to it, possibly due to slight differences in barrels, slight power plant differences, etc...

 
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(Login gubb33ps)
YF

Folks don't want to hear that...

December 24 2009, 6:31 PM 

..it's just so easy to look it up on the chart and believe that's what you're going to get; why complicate it with reality?

Usually not all that far off, but on occasions will find that there is a whole lot more drop than the calculations indicate (and if you put a chronograph down range, may find it's a lot slower than predicted). One good indication of a pellet that's unhappy (and will give a lower than expected BC) seems to be to check vel. variations at the muzzle, then check again out at 20 or 25yards. IF you were getting 10 shot strings with 5 or 10fps variation at the muzzle, but find 20-25 fps variation at 20yards, then the pellet is adding some motions besides the predicted straight flight into the equation.

The good news is that when a rifle shoots way off the predicted BC of that pellet, the accuracy usually suxs and you'd not want to use that pellet anyway.

 
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(Login Parallax7)
YC

Good points Robert

December 24 2009, 9:24 PM 

I do the same with my rifles. Once I find its 'favorite' top 2 or 3 pellets, i'll chrony them at the muzzle, at 25 yrds and again at 50 yrds as well as shoot groups at various ranges. I'll let a combination of accuracy, BC, and smoothest shot cycle(least hold sensitivity) determine the numero uno. wink.gif

 
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Teryx
(Login Teryx)
YF

I haven't read where anyone said

December 24 2009, 11:01 PM 

That they were taking the numbers as gospel. Unless they are a pure fabrication they should serve as at least as a rough basis for comparison.

 
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(Login Parallax7)
YC

I wish it were that easy man

December 25 2009, 1:40 AM 

the numbers are just off so much, so many times that wouldn't you rather test your own rifle with it's own favorite and then you'll know for sure? A couple hundredths on a BC can mean a lot on a long range shot. It's the same thing with pellet trajectory in a software program like chairgun they are indeed fun to play around with on a cold, rainy or snowy day when you can't get outside to shoot. But your own results vary and if it's you putting lead on your FT winning shot or a rabbits head, it's better if you know for sure, a rough estimate is useless if you miss the shot or injure the quarry.

Just trying to help here, i've been down that road more than twice with BC charts, software programs, etc... I would just take those 'references' with a grain of salt as compared to what really happens when you shoot your rifle.


 
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(Login pneuguy)

What might be more helpful (and would certainly be more interesting)...

December 25 2009, 9:51 AM 

...would be for you to post some of your personal BC measurements, for comparison.

Got any?

Steve

 
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(Login gubb33ps)
YF

Know it wasn't directed at me...

December 25 2009, 10:56 AM 

but thought i'd give an idea of the contortions you'd go through.

We make it sound easy, but it's certainly not.

Ideal would be to have two working chronographs set at measured (exactly measured) distances, lined up for each shot to give an accurate reading. What you'll find is that each individual shot gives a different BC, usually not a drastic differences, but a measrueable one.

Problems:

1. Damned few of us have two working chronogrpahs. As mentioned before, an embarrassingly large number of us have two chronographs, a good one and and a dead one that got shot... and many of the ones that got shot, got that way from putting it far down range for BC tests.

2. At short ranges, small errors in vel. readings produce surprisingly large changed in BC.

What most of us do have is one working chronograph. To use just one, need to work by averages. Take a pile of readings at one range, and a pile at another, and figure the average overall BC. Like any average, it may not reflect the true BC of any individual shot.

The other odd thing is that fast pellets slow down quicker than slow ones, which means that BC tends to change with distance. A pellet launched at 1030fps may earn a BC of .025 over the first 50 yards, then earn .03 over the next 25 yards.
-----
Got to testing a Sumatra 5mm. Could toss the 23gr. EJ pellets to 1030fps. Used the one-chronograph-for-average type of testing.

2 yards: 1030fps
25 yards: 913fps
BC: .025

BUT there was a 10fps varaion in the strings used for testing. So call it 1035-1025 at 2yards and 918-908 at 25 yards. Pairing up the best and best of those averages gives a .0276 and the worst-to-wrost gives a .0232.averaged it comes out to .0254, which is close enough to .025.

Slowed the rifle down to 825fps with those 23gr. EJ's (which will keep most 14.3gr. pellets under the speed of sound):
2 yards: 825fps
25yards: 746fps
BC: .03

Because pellets tend to change BC as they lose speed (and really change them for the worst if they start to "wobble" of "spiral"), would be better to test over a long distance...50 or 60 yards... or two test twice, once at 0-15 and again at 25 - 50yards to get an idea of how they change with vel. decay.

-----
What I suspect, even with super accurate pellets, is that they all "wobble" a bit right out the muzzle; that blast of air that powered them over takes them right off the muzzle, and they tend to wobble a little bit. good pellets straighten out quickly, but suspect they lose more speed in that first few feet of travel than we suspect. So for all my personal use, have taken to only testing vel. at 5 yards.
-----

OR... can just read the numbers on a chart and take them as BC-law.



    
This message has been edited by gubb33ps on Dec 25, 2009 11:02 AM
This message has been edited by gubb33ps on Dec 25, 2009 10:57 AM
This message has been edited by gubb33ps on Dec 25, 2009 10:57 AM


 
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(Login Parallax7)
YC

Robert, you nailed it again! nt.

December 25 2009, 11:27 AM 


 
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(Login pneuguy)

Good example, Robert. Let's explore it for a minute.

December 25 2009, 11:50 AM 

Suppose one took that BC = .03 you found for the EJs at MV = 825fps and (erroneously) used it to calculate a 1030fps trajectory.

At 50yds, the calculated drop (of course) would underestimate reality because the estimated rate of velocity loss would be 20% less than reality. An interesting question to ask is: How big would the actual error be.

The actual error (real vs calculated drop) would be 5.02" - 4.85" = 0.17" = 0.3MOA = (slightly more than) one typical scope turret click (1/4MOA = .13").

Steve

 
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(Login gubb33ps)
YF

Actually...I kind of gave up.

December 25 2009, 8:35 PM 

Find that shooting them, finding their real-world drop at various ranges does me more good. Can make a pretty good prediction of drop at longer ranges once you know the real-world drop at 25, 50, and 75 yards.

but like you mentioned, the real difference betwen .025 and .03 is just lost in all the other factors... it's not an amount of difference you can really put to use.

 
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(Login Parallax7)
YC

Sure do

December 25 2009, 11:18 AM 

Here are the BC's for the 3 top performing pellets for my .177 caliber R9. The R9 was/still is in stock form. After sampling several pellets, these were chosen based on the fact that they provided the absolute best accuracy in my gun compared to all the others.

Chrony was then used at the muzzle, 25 yrds and 50 yrds to determine muzzle velocity and energy, terminal velocities and energies and BC's of each pellet. Since all of this testing, I choose FTS's as the best pellet because they rival JSB Express as the tightest grouping pellet (0.32" 5 shot grouping at 40 yrds and 0.5" 5 shot grouping at 50 yrds, they provide the smoothest shot cycle in my gun and have almost the exact same trajectory at 40 and 50 yrds as do the lighter JSB Express 7.9g)



Pellet muzzle 25yrds 50yrds BC

JSB Express 7.9 879 avg 848 687 0.0242

FTS's 8.6 832 avg 801 656 0.0255

Kodiaks 10.6 766 avg 743 633 0.0318



I have other guns, such as the HW50S, R7, TX200, HW97K all of which shoot these pellets at much different BC's. Some shoot the Express with a lousy 0.019, others at 0.027. Same goes for all the others, they are different in each rifle I own.


Hope this helps.

 
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(Login classicalgas)
YF

Robert..one of your offhand comments deserves more attention,IMO

December 25 2009, 3:22 PM 

Doesn't it seem likely that pellets showing BC's very different from published data have been badly distorted by the gun, and/or are wobbling badly? That would account for the association between odd BC's and poor accuracy....

 
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(Login gubb33ps)
YF

Could be distortion... (edit)

December 25 2009, 8:17 PM 

..or it could be that they "dance" at the muzzle (sorry...that's the way i think of it...they do a little wig-wag type of fishtail, which tends to rapidly self-correct from the built in butt-drag of a pellet's shape/weight distribution.)

Can that happen? think there is a reason that shroud baffles need to be a bit larger than pellet diameter, even when perfectly centered to the bore. Probably several shooters reading this have found the same thing, it seems a .22 pellet should pass through a .23" or .24" hole with room to spare, but until you get to about .25", will find some pellets clip. Once you get to .25-.26" , will get occasional clipping. By sooting the inside of the baffles, can find where that contact is, and the surprise is that they don't all clip at the same point. on the bushing's passageway.

When the bushings are close to the right size, will find some pellets that shoot fine with no clipping issues while others show the occasional "way off yander" shot. Now if pellet X can pass though unharmed, but pellet Y clips, have to suspect a little "dance".

Now if the pellet as a butt-wiggle-dance of only .03", as far as air resitance is concerned, you've gone from a 14.3gr. .22 pellet to a 14.3gr. .25" pellet. May well self correct that odd-ball notion and go back to being a 14.3gr. .22 pellet.

IF the above was the case, then the BC from readings taken at 25 to 50yards would give a much higher BC than readings taken from 1 - 25 yards. Will have to look for that when I get unexpectedly low down range vel numbers.
---------
Have caught enough undamaged pellets that took off at over 1000fps to see that most are changed by being smacked in the arse by 3K pressure then squeezed down a bore smaller than they started off.

Will look for the picture, but have a nice before/after shot of those 5mm EJ pellets, showing a well expanded skirt, a bit shorter from that expansion, and pretty deep rifling engravement... all of which would detract from their theoretical BC.

Found it:
[IMG][linked image][/IMG]

Which brings up your distortion comment. While I don't have a picture, when those pellets were LAUNCHEDF at 825fps, they were less deformed...and earned a higher BC. PROBABLY higher than the pressure deformed pellet that was shot at 1030fps would earn even once it slowed down to 825fps.


    
This message has been edited by gubb33ps on Dec 25, 2009 8:49 PM
This message has been edited by gubb33ps on Dec 25, 2009 8:24 PM


 
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(Login classicalgas)
YF

Thanks Robert,...very interesting data ...

December 26 2009, 11:59 AM 

I'm convinced the BC variation is related to the two factors you mention.

 
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(Login Teryx)
YF

Bottom line

December 25 2009, 4:07 PM 

There is no question that BC is an indefinite number by the very nature of how it is calculated. Pellets are more prone to problems because unlike typical bullet designs they are more aerodynamiccaly complex. Add to that the fact that they are distorted to one extent or another by the rifle. They are designed to distort so that the skirt can seal and the choke can compress the diameter. Anything that changes the airflow over the pellet will affect the final value recorded.

Still, like any data, the question of statistical reliability has to be considered when looking at the numbers. I have not actually spent any time acquiring BC data, but it is my gut feeling that if you eliminated the extreme numbers due to extremely tight chokes, extremely low and high velocities, pellets that become destabilized in flight, etc, that you would end up with at least a 20% confidence in the numbers. In other words, a .008 pellet does not magically become .025 unless something like I mentioned is at play. Do the people who have worked with these numbers believe that to be an incorrect assumption??

What struck me from the chart that Steve referenced is that the range of recorded numbers are as high as 3X to 4X within a given caliber! Typical numbers for conventional bullets of a given caliber across a range of weights and profiles might vary by 50%- 60%. The numbers recorded for any given pellet have to be taken with a grain of salt, but the fact remains that a careful consideration of pellet is warranted. Unless the numbers are horribly flawed, the need for vigilance in pellet choice is much greater than one would expect who deals with conventional bullets.

The chart could certainly serve as a guide and then the final tests done with the particular gun. When all the pellets are fired from the same gun, much of the variability is cancelled, and even such aspects as velocity variance due to pellet weight are at least "real" for that gun.

Teryx

 
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(Login Parallax7)
YC

Good points also Teryx. nt.

December 25 2009, 5:17 PM 


 
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