Please help me find G1 Ballistic Coefficients for JSB Exact .177 pellets

[Earl]

I shoot JSB Exact .177 pellets in the 500 pellet tins, round nose, 7.33, 7.87, 8.44, 10.34 grains.
I understand the G1 Ballistic Coefficients vary with muzzle velocity.
I shoot muzzle velocities of 300 to 1000 fps.
There should be charts available but I cannot find them.

[Motorhead]

using CHAIRGUNS default BC values will get you real close

[rsterne]

While the BC does vary with velocity, the one in ChairGun is reasonably close, and the GA drag model there is even better.... I think the BC values in ChairGun for pellets use the GA model, they certainly should.... At over about 900 fps, the BC will likely be lower than what the GA (or G1) models indicate.... but the pellet spends little time or distance slowing from 1000 fps to 900 anyways.... so if you gun is sighted at 20 yards or beyond the trajectory should be pretty close....

Bob

[subscriber]

Earl,

Straight Shooters have published measured "velocity decay" data for a range of pellet weights (including JSB) from 0 to 50 yards, with calculated ballistic coefficients.

First few links below show data for more powerful air rifles. Lower links for less powerful air rifles:

Follow link; then click on "Our Take" tab to see velocity data:
http://www.straightshooters.com/hw-1852-weihrauch-hw100-.177-walnut-thumbhole-rifle.html
http://www.straightshooters.com/air-arms-s510-extra-fac-super-lite-.22-hunter-green.html
http://www.straightshooters.com/rws-350-magnum-.177-beech.html
http://www.straightshooters.com/weihrauch-hw90-.177-beech.html
http://www.straightshooters.com/Beeman-R1-177-Beech.html
http://www.straightshooters.com/weihrauch-hw50s-.177-beech.html
http://www.straightshooters.com/beeman-r7-.177-beech.html

[rsterne]

I have never found the data supplied by Straight Shooters to be very good.... horrendous differences between the BC in different rifles, and not consistently over the pellet tested.... That would be my last place to look.... JMO....

A much better resource is the testing done recently using a LabRadar by Hard Air Magazine.... https://hardairmagazine.com/ballistic-coefficients/

Bob

[Taso1000]

Earl,

If you can get two chronographs you can figure out your ballistic coefficient.   

Taso

[subscriber]

I have never found the data supplied by Straight Shooters to be very good.... horrendous differences between the BC in different rifles, and not consistently over the pellet tested.... That would be my last place to look.... JMO....

Either SS took poor data, or the fact that BC varies with velocity is reflected in their data for different airguns at different velocities:

To quote your preferred source:

Because the BC changes with the pellet’s velocity, there is actually no single Ballistic Coefficient for any airgun pellet!

I suggest the OP plot all of the SS data for JSB pellets and throw out the obvious outliers.  That is, unless you have evidence that the data is based on outright lies? 

Also, I would compare the start and end velocities given by SS with those published by Hard Air.  Even better, would be to go and collect actual data with a chrono at ranges the OP cares about, using his actual air rifle.

One thing that will affect the BC for a given pellet is how much it is reduced in diameter by the choke and mangled by the action of loading.  Also, how stable it is when released from the muzzle; and how well it tracks the trajectory (as in effective spin rate). 

An unregulated PCP pressurized to 3000 PSI may "blow out the skirt" of a given pellet more than the same pellet fired from an airgun regulated to 1500 PSI.  This will affect BC too.  Ditto, the difference between springer and PCP pellet condition after firing.

Lots of reasons why pellet BC will vary between airgun models.  That does not make the data wrong; just subject to  interpretation with respect to pertinence...

[subscriber]

Earl,

If you can get two chronographs you can figure out your ballistic coefficient.   

Collecting your own data is absolutely the best thing.

You can use the same chrono at multiple ranges by shooting a large enough sample of the same population of pellets.  For instance; shoot at 1 yard and then at 50.  Then go back and shoot at 1 yard, and then again 50 yard; under the same conditions (PCP charge pressure; ambient temperature & pressure, humidity, wind).  If the two average velocities sets from 1 yard do not agree withing 2 %, start over.  If the 1 yard averages agree, you can trust the 50 yard data also.

[subscriber]

No wonder the Hard Air BC data seems very consistent, and does not agree with the BC data collected by Straight Shooters for multiple different airguns.  HA use only one FX Impact (with interchangeable barrels for different calibers), set to full power.  For one thing, those barrels would have an effective twist rate much slower than the plethora of conventionally rifled airgun tested by SS:

From: https://hardairmagazine.com/ballistic-coefficients/

But the keys to our reliable, accurate BCs are consistent, comparable test methodology and high quality equipment. For this reason, all HAM airgun Ballistic Coefficient testing has been undertaken by the same experienced, dedicated testers, using the same FX Impact air rifle, set to full power, with all FPS measurements recorded using the same Labradar Doppler Radar system.

As is well known, the regulated PCP power system of the FX Impact air rifle provides very consistent Muzzle Velocities. The Impact also uses interchangeable barrels and bolts of different calibers. This allowed the same Impact air rifle to be used for BC testing in all four calibers. This eliminated any variation in BC from using different air rifles.

See also: https://hardairmagazine.com/news/ham-publishes-most-comprehensive-database-of-airgun-pellet-ballistic-coefficients/

I would conclude that while useful and probably very accurate, the HA BC data is not necessarily "much better" than the SS data, unless you own an FX Impact.  If you are shooting an RWS 350 for instance, the SS data may actually be more representative...

[Earl]

using CHAIRGUNS default BC values will get you real close

I downloaded ChairGun4 and this is what I think it shows:
JSB 7.33 is not listed
JSB 7.87 BC=.0210
JSB 8.44 BC=.0210
JSB 10.34 BC=.0304
JSB 13.43 BC=.0280
Are these numbers correct?

[subscriber]

Hard Air's numbers for .177 JSB pellets appear in the screen capture below.  Chairgun's BC numbers seem a bit high...

From: https://hardairmagazine.com/ballistic-coefficients/

using CHAIRGUNS default BC values will get you real close

I downloaded ChairGun4 and this is what I think it shows:
JSB 7.33 is not listed
JSB 7.87 BC=.0210
JSB 8.44 BC=.0210
JSB 10.34 BC=.0304
JSB 13.43 BC=.0280
Are these numbers correct?

[Earl]

My purpose is to tape pellet drop charts to my guns using the most accurate pellet for that gun.
I will shoot 10 shot groups at 18 yards and 49 yards and use the BC that makes the chart match the actual drop.
I will use 10 yards and 30 yards for the 400 fps guns.
I shoot targets in my home and back yard between 10 and 50 yards.
Suggestions and advice are appreciated?

[rsterne]

I am not suggesting that SS at lying in their data, but the BC values vary over such a huge range something very unusual is going on.... It could be that such different guns are being used that some pellets are having their skirts "blown out" and changing shape while others are not.... but I have never been able to make their data make sense to me.... Averaging their BC for your pellets of interest.... or selecting a gun the same as yours.... may provide acceptable results....

There are two things that most tables of BCs do not state.... First, they often don't state what drag model is used to calculate the BC (ie G1, GA, or other).... Secondly, they often don't state if the BC has been corrected to NTP or ICAO atmospheric conditions.... I could not find either in the HAM data (they did mention their altitude, but not if the data was for that altitude or corrected, at least I couldn't find it).... A lot of older BC data for pellets was calculated using a "constant drag model" using a Cd of 0.2, which produces horrible results when the velocity enters the transonic (over Mach 0.8, ie 900 fps).... Some of the older trajectory calculators, and BC calculators, also used that drag model.... The G1 model is reasonable for many of our bullets, and the GA model is better for most pellets.... but neither has a fast enough rise in drag over Mach 0.8 to be accurate in the Transonic....

I applaud HAM for attempting to give us a modern BC table, using the LabRadar.... but I wish they told us which drag model they were using, and corrected the data to ICAO conditions.... Of course having a BC calculated using one drag model, and not using that model in your trajectory and drift calculator.... is a matter of GIGO (garbage in = garbage out)....

Bob

[subscriber]

Earl,

My suggestion is that, beyond simple calculation, you start constructing a drop table for just one airgun and one pellet type with actual test data; rather than trying to generate tables for five guns and five pellet types all at once.  Do this, after establishing which pellet is most accurate in that airgun at the maximum range you care about.  Precise drop tables for the less accurate pellets makes about as much sense as jumbo shrimp

I think that drop calculation may be useful as a starting point.  If you are just shooting groups, rather than scoring a target with a small 10-ring, then calculated drop tables may be good enough.  If your target size is a constant 1/2" in diameter, then your drop tables will need to be a lot more accurate.  So, if you are going to shoot at multiple ranges anyway, I suggest you extend that concept to be fully predictive of your airguns, using your pellets under your conditions.

One of the less glamorous, yet important factors in constructing drop tables is the height your sight-line is above the bore axis.  This can make all the difference in the world, if you are trying to keep your shots inside the brain of a squirrel at 47 yards.   

Now, you can either measure the sight-line above the bore axis (something that actually changes as you increase sight elevation) with a ruler; or you can capture it by shooting the gun at multiple ranges.  If using Chairgun, you will need to measure it or assume it.

My advice is that you shoot all you guns at more than two ranges.  Start with the closest zero range predicted by Chairgun (perhaps 8, 10 or 12 yards) for your intended zero at target range (perhaps 25, 35, 45 or 50 yards; depending on the power of the gun and you preferences).  Then shoot at intermediate ranges that accumulate calculated increments of 1/2" vertical error; or just pick 20, 30, 40 and 50 yards; if the particular gun would be used up to that range.

When you plot the trajectory from these data points (average of 10 shots with outliers thrown out), you will see that the point of impact crosses the line of sight twice.  Also, the highest point in the trajectory appears to be closer to the further range at which the pellet crosses the sight-line.  My suggestion is that once you think have a your zero settings figured out, you shoot at exactly those two ranges to confirm the pellets land on your aim point.  Then, you can calculate (interpolate) the intermediate ranges that accumulate increments of perhaps 1/4" vertical error and add them to your drop table.

Once you have constructed your drop table, test it by shooting at "odd" ranges, such as 23, 27, 33 and 48 yards.  You might also shoot some groups at 5 yards to see how much you need to hold over close targets.  If you are using a high mounted scope the hold over required at short range can be surprising.

If you use a 1/4" vertical increment drop table you will find that you might start with a 12 yard zero that is also a 50 yard zero (or a 10 yard zero that is also a 37 yard zero).  With some 1/4" increments of hold-under and hold-over landing on odd intermediate ranges, such as 18 and 32 yards. 

What I find useful is a single zero that would allow me to hit within perhaps +- 1/2" of may aimpoint at anywhere between 10 and 50 yards.   All I need to remember is to add or subtract, all, some or none of that 1/2".  This can be estimated fairly easily, but would only be possible with more powerful airguns.  Anyway, these are your tables and you can set them up any way that suits your needs. 

[Earl]

I shot 10 shot groups at 18 and 49 yards with JSB 7.33 and JSB 8.44 pellets and adjusted the BCs to match the actual locations on the targets relative to the location of the cross hairs.
7.33 BC = .015
8.44 BC = .019

JSB 8.44=603         
DistElevWind   
yd   in   5.0   Vel
0   -1.7   0.0   603
5   -0.9   0.0   585
10   -0.3   0.1   567
15   0.0   0.3   550
18   0.1   0.5   540
20   0.1   0.6   534
25   -0.2   0.9   517
28   -0.5   1.1   508
30   -0.8   1.3   501
35   -1.7   1.7   486
40   -3.0   2.3   471
45   -4.7   3.0   456
50   -6.9   3.8   442
55   -9.5   4.6   428
 2550-1500=90shots         
avg 8.44 vel=603         
adj BC 8.44=0.019         
Crosman Challenger   

JSB 7.33=647         
DistElevWind   
yd   in   5.0   Vel
0   -1.7   0.0   647
5   -0.9   0.0   622
10   -0.3   0.2   599
15   0.1   0.4   576
18   0.1   0.5   563
20   0.1   0.7   554
25   -0.1   1.1   533
28   -0.3   1.3   521
30   -0.6   1.5   513
35   -1.4   2.1   493
40   -2.6   2.8   474
45   -4.2   3.6   455
50   -6.2   4.5   437
55   -8.6   5.6   420
 2550-1500=90shots         
avg 7.33. vel=647         
adj BC 7.33=.015         
Crosman Challenger         
May 20, 2018         
      

         

[K.O.]

There are many variables that will effect your very low fpe round when out in real conditions that may make it very hard to get reliable info...

The lower the fpe the more the very small things like a barely noticeable 2 mph air movement from different direction on differing days/times...I have not done the math but I bet even a change of  temp and ambient psi matter at 40+ yards with less than 7 muzzle fpe...

also the weight variation from pellet to pellet may have an effect...

so what is my point...

your 10.34g 950 fps ~ 20fpe info will be the most useful...  at low fpe I just learn to recognize just what I can generally do at 30  yards or 20 yards with my Daisy 953 on a still to near still day...

https://www.gatewaytoairguns.org/GTA/index.php?topic=93523.0