AusVarmint

This testing was done last weekend in The Snowy Mountains. I want to share it with you as I think it would be a first in air rifle data collection at this range and the results, if supported by further testing, are interesting and probably counter-intuitive. I hasten to write they are specifically related to the very dynamically stable pellets as used in this rifle, and though it is quite possible other pellets behave similarly, there will be many that probably do not.

The purpose of this study was to collect data at three different ranges 50, 100 and 200 yards under consistent ambient conditions for the derivation of the respective BCs for input into the Chairgun ballistic programme . It was intended to then compare the results in order to perhaps find out more about BC changes with range or perhaps time of flight TOF and or velocity.

Rifle BSA .25 cal Hornet prepared by John Bowkett; pellets the new 0.25 cal JSB 25.4 gr Kings. This combination is known for consistency having shot a 25 shot string with just 6 fps extreme spread; and for accuracy having shot 0.573 MOA at 100 yards and very close to MOA it at 202 yards (1.08 MOA / 2.30 inches ).
Temperature 23 C or 73.4 F
Altitude was 4,035 ft or 1230 m. (The BCs below have also been corrected for “sea level” NTP for others to relate to. … Very good conditions for the most part ( nothing is ever perfect).

One chronograph was used with steel angle iron protection. It was moved from 1.5 ft to 200 yards, to 100 yards, to 50 yards for the tests. Range was established with a Bushnell range finder. As I could not see the chronograph, or read it if I could, a spotter was required. See pics below.

Minimum time was wasted in barrowing the equipment to set up between ranges. The sky was overcast throughout and the light was even thus making for good chronograph functioning.
The chronograph had a fresh battery. The rifle was kept charged well above the regulator set pressure.

RESULTS:

889.9 fps at 1.50 ft; ( 888, 891, 890, 889, 892 rounded) therefore MV 891.5; ME 44.84

Retained velocity at 50 yards:
762.0
760.2
760.0
764.3
763.1
Average 761.9; fpe 32.74. BC 0.0398 or 0.0337 corrected for NTP.

Retained velocity at 100 yards ( rounded ):
662
669
663
668
670
Average 666.4; fpe 25.05. BC 0.0430 corrected 0.0364

Retained velocity at 200 yards:
511
510
514.8
Average 511.9; fpe 14.79. BC 0.0451 corrected 0.0382

Only three successful shots passed over the chronograph's sensors at 200 yards before the soft breeze made shooting for this purpose impracticable at that range, but the three are very close together. .

Using Steve Woodward’s calculator this gives BCs as follows:

0 to 50 yards : 0.0398 or corrected to NTP 0.0337
0 to 100 yards: 0.0430 or corrected 0.0364.
0 to 200 yards: 0.0451 or corrected 0.0382.

And:

50 to 100 yards: 0.0467 or corrected 0.0395.
50 to 200 yards: 0.0471 or corrected 0.0399.
100 to 200 yards: 0.0474 or corrected 0.0401.

I am thinking that the progressive improvement in BC from 0.0337 average to 50 yards to 0.0382 average to 200 yards, as the pellet moves down range, may be a direct reflection of the progressive damping out of precession / nutation ( albeit of minute magnitude in these very dynamically stable pellets ).

The pellets may require as much as 0.40+ seconds, or 100+ yards, to achieve this damping. When one considers the time taken by a spinning top to settle, then at least 0.4 seconds may seem appropriate for a pellet. It is interesting to note that the .50 cal BMG round has been shown by Doppler to improve its BC out to 800 yards then decay again after 1300 yards or so. This may be happening here scaled back to air rifle ranges.

The implications, of this data, point to the further refining of range tables for long range shooting. For example if the average BC from muzzle to 200 yards is 0.0382 at sea level, that should be used rather than the 0 yard to 50 yard figure of 0.0337 in this instance. I have had other data to suggest this can be extrapolated back to closer ranges. In “normal” air rifle shooting to 40 or 50 yards a ballpark BC figure is close enough to use but as the range stretches out things seem to change.

I must point out that all pellets are not equal and as you all know, all rifles do not shoot them the same. Some pellets are definitely dynamically unstable and would not profile their BC like these JSB King .25s do. One glaring example is the Beeman Crow Magnum which is very dynamically unstable ( great for whacking things at close range as it gyrates at about 45 degrees and more. ... So each pellet would have to be tested to see its particular characteristics. … My interest is in long and extreme range precision shooting, thus I have no use for pellets that will not perform to sub MOA out to extreme range in excellent conditions.

Pictures:

The Moffitt Farm Range with Rob Moffitt way down there at the 200 yard target on the flat through the tree gap waiting to spot. He stands beside his formwork board screen with the target stand to his left.



Close up showing the Chronograph hidden behind the steel angle iron; a 100 L oil drum and paint tin used to show evidence of the 14+ fpe remaining pellet KE at 200 yards; and Rob with his barrow and form work protection shield.



The paint tin showing POI entry and exit ( 200 yards ) of the initial ranging shot and the result of near 15 fpe measured at 200 yards.



Likewise three serious dents in the 100 litre oil drum from the JSB Kings at 200 yards from the next three ranging shots. You will have noted that the 25.4 gr Kings had 512 fps retained for 14.79 fpe at 200 yards:



100 yard target showing the narrow band (lines drawn after ) between which the pellets had to pass in order to "ring the bell" Those to right or left did not register on the chronograph. I was aiming off laterally shot to shot to beat the light breeze 3 o’clock breeze ( which obviously beat me for those two to the left and over compensation for the nice little group to the right of centre ):



It is not easy to get data at these ranges and the more complex the experimental design, the more time in logistics, therefore the more chance of conditions changing. This was the best I could do on the day and the mountain weather was very kind and as good as it gets for such a session.
It is an 11 hour round trip from home to this location.

I hope some have read this far and of course it is open for comments.
Kind regards, Harry in OZ.

Harry you have done it again.It is very interesting to see the BC change over that distance.I was thinking it would decrease as the range was extended and the velocity decreased.The manufacturers claimed BC for some projectiles could be questioned.Food for more thought.Thank you Harry

Excellent write up Harry. Good idea about a plate in front of the chrony, they are a tad delicate to in coming, but I can vouch for the good spares service from Chrony Canada.

neil

i see you even claimed a rabbit in the background great effort for that shoot interesting write up thanks

Interesting post.
You have done a lot of good work to get all this information. I am sure that you are on the right track and will come up with a usefull set of B.C. data for those pellets.
It is not surprising that you are getting different B.C. figures at different ranges as the velocity is different and so the dynamics of the air flow will change over the form of the pellet.

What B.C. standard projectile are you using? G1? if so maybe you should research some of the other standard projectile forms to get a better match to the pellet that you are using. This may require you to write a program to predict your trajectory.

If not then Multi B.C. numbers based on velocity would provide a suitable trajectory when used with a program that can handle multi B.C. numbers for the one projectile.

I don't think that just using an average B.C. is the way to go as with long range shooting you would need to use a different B.C. for each range.

wadcutter wrote:Interesting post.
You have done a lot of good work to get all this information. I am sure that you are on the right track and will come up with a usefull set of B.C. data for those pellets.
It is not surprising that you are getting different B.C. figures at different ranges as the velocity is different and so the dynamics of the air flow will change over the form of the pellet.

What B.C. standard projectile are you using? G1? if so maybe you should research some of the other standard projectile forms to get a better match to the pellet that you are using. This may require you to write a program to predict your trajectory.

If not then Multi B.C. numbers based on velocity would provide a suitable trajectory when used with a program that can handle multi B.C. numbers for the one projectile.

I don't think that just using an average B.C. is the way to go as with long range shooting you would need to use a different B.C. for each range.

WC the BCs used in the "Chairgun" air gun ballistic programme are calculated directly from velocity changes over two (or more) known ranges. I am using the calculator written for that programme to derive the BCs I displayed for the data above. The following is a recent quote from the author of the programme:
September 24 2009,

"The BC definition (BC = Sectional Density / Form-Factor) is specifically referenced to the Ingells / Krupp standard projectile (1 Pound weight, 1 Inch diameter, Form-Factor defined as unity giving a BC of 1 by definition). In order to glean a meaningful BC value - by substituting a pellets weight and diameter in expression means that you have to know (or be able to calculate) the Form-Factor for the pellet compared to the Krupp projectile. The form-factor can also be defined as the measured Cd (Drag Coefficient) of the pellet divided by the Cd of the Krupp projectile at the same velocity. We can find the Cd for the Krupp projectile from tables and we can calculate the pellets Cd by hiring a wind-tunnel for a while . . . not really a practical proposition.

The alternative is to observe (in the real world) the change of velocity over a known range - using two or more chronos - and simply calculate the BC directly.
This is not to say that the BC=SD/FF expression is completely useless. Since the Sectional Density can be calculated from the physical properties of the pellet, we can make fairly accurate estimates of the BCs of other pellets by direct comparison provided that we assume the Form Factor is unchanged.
That is (assuming that the FF remains constant):
SD1/BC1 = SD2/BC2 = SD3/BC3, etc. .............................
Dave ... " End quote.

And this from Steve Woodward who wrote the BC calculator programme that supports "Chairgun";
September 24 2009,
"The formal definition of BC is based on actual measurement of time of flight... ...(or - equivalently - retained velocity) versus range.
Therefore, by definition, velocity measurement is the gold standard, and any attempt to calculate BC from the dimensions (e.g., sectional density) of the projectile is the approximation.... " End quote.

You may find this chart of interest. Using the data from the test, I have constructed it with an overlaid quadratic polynomial to predict BC at intermediate ranges. I intend testing the predictions at a future date. The "Y" BCs should be read as being relevant to each particular range "X" . eg., a BC of 0.0375 would be used for the range of 130 yards to derive scope elevation clicks, POI displacement from a default zero and velocity, KE, TOF, momentum etc .



I am not sure that a second order polynomial is the best treatment but have had excellent success using it in the past in conjunction with correlating scope elevation clicks against point of impact vertical displacements at various ranges.

This has blown out to book proportions and is probably of little interest to most folks so I should stop.
Kind regards, Harry.

nt

You may find this chart of interest. Using the data from the test, I have constructed it with an overlaid quadratic polynomial to predict BC at intermediate ranges. I intend testing the predictions at a future date. The "Y" BCs should be read as being relevant to each particular range "X" . eg., a BC of 0.0375 would be used for the range of 130 yards to derive scope elevation clicks, POI displacement from a default zero and velocity, KE, TOF, momentum etc .

Yrrah,
There is nothing wrong with what you are doing and using a different B.C. for each range as you descride above will work quite well. I just thought that using a different ballistic program that allows you to input more than one B.C. would make life easier for you. That way you could use the one drop chart over a range of distances and still be accurate as the B.C. would be calculated on the predicted velocity range that the pellet was in at that time.

The method of determining the B.C. based on time of flight is also the best way to do it so your results should be quite accurate. I think that the Standard projectile for blunt nosed lead projectiles is called the GL standard. Form factor would change with bullet weight but would be easy to calculate with this type of projectile.

It looks like the software that you are using is based on the G1 standard. Not a very good standard to use when trying to calculate the trajectory for a lead pellet as you are finding out based on your field tests.

I did a bit of searching and there may be better software that you can try. Check out the link.


http://www.snipercountry.com/ballistics/index.html

Check out the FreeWare....... McTraj4 with McCoy original Drag Coef tables G1, G2, G5, G6, G7, GS, GL

This may give you a better standard to use that will work out a single GL based B.C. number that will predict your pellet trajectory quite well just from your muzzle velocity.

I had a similar problem with some of my extreme long range shooting that I have been involved with. The G1 standard is just hard to work with in some situations. I chose the G7 standard in my case and it worked much better.

[quote="wadcutter".....................................

I did a bit of searching and there may be better software that you can try. Check out the link.
http://www.snipercountry.com/ballistics/index.html
Check out the FreeWare....... McTraj4 with McCoy original Drag Coef tables G1, G2, G5, G6, G7, GS, GL
.[/quote]

Using JBM my data sort of fits somewhere between G1 and G2 . The others as such were not close.
However I shall follow up on your suggestions and check out the above reference ... thank you for that.

The "Chairgun" predictions have stood me in good stead so far out to "Minute of Rabbit" to 150 yards. but I am looking for better.
This reference will show where I was at 5 years ago with another air rifle and JSB .22 pellet in terms of verifying the validity of the program to 100 yd:

http://www.network54.com/Forum/79537/th ... d+shooting.

Harry. Edit : It looks as though the programme McTRAJ4 is not supported by Widows 7

I am not sure if you used this http://www.jbmballistics.com/cgi-bin/jbmgfa-5.0.cgi

It is a useful tool to convert and find the best drag function. This is worked in velocity bands so you can input your 0-50, 50-100, 100-150, 150-200 calculated G1 B.C.'s and compare them against other drag functions.

Then you can input the results into the JBM trajectory calculator http://www.jbmballistics.com/cgi-bin/jbmtraj-5.0.cgi and compare your observed trajectory V's the various options that can be calculated.

Interesting stuff.

Harry,
at $25 per 350 the JSB Exact Kings are slightly more expensive than the Kodiak Match. Do you reckon they are worth the extra cash? Or are they only really superior at 100m+ ranges?
Rgds

wadcutter wrote:I am not sure if you used this http://www.jbmballistics.com/cgi-bin/jbmgfa-5.0.cgi
It is a useful tool to convert and find the best drag function. This is worked in velocity bands so you can input your 0-50, 50-100, 100-150, 150-200 calculated G1 B.C.'s and compare them against other drag functions.
Then you can input the results into the JBM trajectory calculator http://www.jbmballistics.com/cgi-bin/jbmtraj-5.0.cgi and compare your observed trajectory V's the various options that can be calculated.
Interesting stuff.

I have re-visited JBM and delved deeper at your prompting WC. I have found two variations which do a fair job of staying within 10 fps of all three of my data points. Both G7 / 0.022 BC and RA4 / 0.043 BC will do this. If I bias the BCs to favour the longest ranges then velocity at the 50 and 100 yd still show within 10 fps of the data. The POI displacement error that that would cause at up to 100 yards, when resolved into clicks or holdover, would not compromise missing a bunny's kill zone especially as the difference is known. ( using my data and rifle etc of course ).
So ... that was a fruitful exercise and I have some print-outs to keep me interested on the next trip to the farm along with the other stuff outlined above.
Thank you for that WC ......... Kind regards, Harry. Late edit: Using GL /0 .0395 BC gives spot on for 512 fps at 200 yd; just 3 fps higher than the 666 at 100 and 8 fps higher at 50 yd. That may be the single scale BC best bet. I will see how that works out when it is all put together on the farm range in a few week's time. ... H.
PS Edit: I now have dovetailed three tables to reflect each segment using the best BC for each to match the raw data.... H

trapper wrote:Harry,
at $25 per 350 the JSB Exact Kings are slightly more expensive than the Kodiak Match. Do you reckon they are worth the extra cash? Or are they only really superior at 100m+ ranges?
Rgds

That's a hard one to answer for another shooter Trapper.
If the rifle was a springer or lower powered PCP then I would say definitely yes. Springers almost always shoot lighter pellets to higher energy as well as velocity.

For a higher powered PCP in the 45 fpe zone, the difference in trajectory over 50 yards is not great when zeroed for each at that range. As the distance reaches out the JSBs are advantaged by higher velocity and the BCs appear very similar. With a 50 yard zero the Kodiaks drop 3 to 4 inches more at 100 from my rifle.
At 50 yards the difference in grouping is negligible if bunny hunting is the game. From 70 yd on the difference shows. So I guess it depends upon purpose. I will not be buying more Kodiaks but those I have will get used. I bought a sleeve of JSBs from Rob Potter and maybe will get more as the dies are fresh and I know how good these pellets shoot for me. They are still not much more than half the price of anyways decent rimfire bullets and I will get 400+ shots for the price of a box of 50 top dollar bullets.
Perhaps get one tin and draw your own conclusions based on your own rifle and its use? That is always a good plan ....... Not very helpful I guess.
Kind regards, Harry.

Hi Harry,
thanks for the response. My Rapid runs at 45fpe. I tend to use it at ranges of less than 50m so it sounds like there's no real advantage in using the JSBs. Having said that, I will definitely get a couple of tins to try out as you suggest.
Cheers