Anyone have good BC numbers for the JSB 15.89 pellets

[Pellgunfun]

Hi,

The only BC numbers I can find for the JSB 15.89gr pellet is 0.031.  It's not even listed in Chairgun, and some that I find in Chairgun have to be changed because I haven't really found them to be spot on.  Is 0.031 BC for the 15.89 pellet accurate, or is there a more accurate BC I should be using for that pellet.  I've been traveling a lot with work so my ability to scour the internet is a bit limited right now, but I can logon and check out the GTA forums.  I'd like to get all my data squared away before I get back home, and start doing some testing.  If anyone has accurate BC numbers for this pellet, "I'll be a thanking you".

Lol, can anyone name the movie that the above quote is from???  Come on now, It's a great movie and it comes on every year.

[BigTinBoat]

Here's a good post about BC from a little while back. He has the JSB listed as 15.9 once and then 15.7 in the chart, but I think that's the pellet you are looking for.

http://www.gatewaytoairguns.org/GTA/index.php?topic=14735.0

[rsterne]

The BC varies depending on velocity because the drag models used don't work well for pellets.... Using the G1 drag model (the most common), for the JSB Exact Jumbo 15.9 gr. it is about 0.032 at 700 fps, increases to 0.042 at 800-850 fps, and then starts dropping again, to about 0.032 at 950 fps, and then it really starts to tank above that.... The best way to find out your drop for your pellets in your gun is to shoot at various ranges and record them....

Bob

[Pellgunfun]

I'm gonna focus on keeping the muzzle FPS at 900 fps to about 910 fps at the most I think.  No sense going beyond that I think.

[MicErs]

JSB 15.89 Jumbo Exact in my D460:  BC measured at 0.0330.  The gun is a 24 fpe gun so whatever that works out at.  That agrees pretty closely with what Bob said.

[MicErs]

The BC varies depending on velocity because the drag models used don't work well for pellets....
Bob

I look forward, very much, to seeing your work in this area.  If I may help in some way, please let me know.  Have Crony will measure data =)

[rws45user]

I got this number from doing a few things . First from a  real life  start  to  finish o to 50 meters  crony test . At 0  out of the barrel the crony read 553.5 fps  and at  50 meters it read 416.4 .  watch this video   . You can see the numbers  at each distance . I then took this into chairgun . I entered the pellet weight  15.89  and the  nuzzle velocity  as 553.5 .  I set the end range as 55 yards  which is pretty close to 50 meters . I scrolled the table  down so I could see the 55 yard  fps  number .Then I started changing the BC numbers till it matched   the 50 meter  reading of the crony which is 416.4    The closest I could get was  416.5  thats pretty close and the other numbers  from 0 to 50  matched as well .  The BC number that made this work  was 0.0227   the gun used was a lgv master pro . So I would say  0.0227 would be a real life  BC number to use  in chairgun . 

[rsterne]

At the velocity you tested at....

HAM finds it to be 0.031 at just over 900 fps using the GA drag model.... https://hardairmagazine.com/ballistic-coefficients/

Bob

[rws45user]

So it appears  that BC as far as  air guns go is  that every gun with a different  fps  is going to produce a different BC . 553.5 fps  = BC  0.0227. I would like to see what other fps's  BC turn out to be . Maybe  we can figure out  a formula  if we start off with a  low fps  and go to the high fps . Has anyone  listed the BC for the same pellet  with every  fps  from lets say 553.5 and up ?  Lets not use a model . Lets do a real life  test of the pellet .  I just did a little chairgun   BC number changing   from 0.0227  to  0.0310  with a 912 fps  gun  at 50 yards  the drop difference is   .3  at 0.0277 the drop was 6.5  and with the  0.0310  the drop was 6.2  . At 100 yards  looking through the scope  its .8  of a mildot  difference or  4 inches . So at longer distances  it really makes a difference .

[rsterne]

Every pellet is different, and there is no formula that can be applied.... Part of the problem is that the "drag model" used to compute the BC (usually either G1 or GA) is not a great match for our pellets (nor could it be, with so many different pellet shapes)…. It is particularly inaccurate in the transonic range, over Mach 0.8 (900 fps)….

The very best way for one pellet in one gun is to set up targets at varying distances and make up a "range card" for your own setup....

Bob

[Horatio]

So it appears  that BC as far as  air guns go is  that every gun with a different  fps  is going to produce a different BC . 553.5 fps  = BC  0.0227. I would like to see what other fps's  BC turn out to be . Maybe  we can figure out  a formula  if we start off with a  low fps  and go to the high fps . Has anyone  listed the BC for the same pellet  with every  fps  from lets say 553.5 and up ?  Lets not use a model . Lets do a real life  test of the pellet .  I just did a little chairgun   BC number changing   from 0.0227  to  0.0310  with a 912 fps  gun  at 50 yards  the drop difference is   .3  at 0.0277 the drop was 6.5  and with the  0.0310  the drop was 6.2  . At 100 yards  looking through the scope  its .8  of a mildot  difference or  4 inches . So at longer distances  it really makes a difference .

I think the BC would also be different from different guns at the same fps.

Does the rifle achieve its 800fps from 1,000psi and a long barrel or 3,300 psi and a shorter barrel? Add in port size, valve duration, etc. and I would guess that the way the skirt is blown out would be different.

I think all that matters is 2 things. 1) Is the drop and accuracy acceptable to you with your gun. 2) Is there enough fpe/penetration at the target for your intentions.

[K.O.]

I am running the pc version of Chairgun and in toolbox it has custom profile and edit custom profile...so you can create a new profile... after you learn the Cd(drag coefficient at a particular velocity... it goes .2-.4-.6-.8-1-1.2 mach and so on... So we can create the drag curve for any pellet not just an overall pellet...

[rsterne]

Kirby, that is true.... However, you need the Cd for your pellet at the different velocities to create that custom drag profile.... This means a lot of Chrony work at all those different velocities, or using a LabRadar at a few different velocities, and then using that data to calculate the Average Cd for each Mach number that you can input.... When you are said and done, the number of input points are rather limited, so you end up with a crude approximation of what the actual drag curve is....

To make it worse, every pellet will produce a different drag curve.... The only company I know that does this is Lapua, and they have the drag curves, and the software to use them, to accurately predict the downrange velocity, energy and trajectory, but only for their own bullets, of course.... I dream of being able to do that someday, but until I retire in 2021 I sure won't have the time to even start.... At least the articles I am writing for Hard Air Magazine will pay for the LabRadar before then.... 

Sam, you are correct, the exact same pellet can have a different BC, even at the same velocity, depending on how much the gun used distorts the skirt....



Fired pellets on the left, unfired (but pushed through the bore) on the right.... Photo courtesy Lloyd Sikes....

Bob

[Doug Wall]

So it appears  that BC as far as  air guns go is  that every gun with a different  fps  is going to produce a different BC . 553.5 fps  = BC  0.0227. I would like to see what other fps's  BC turn out to be . Maybe  we can figure out  a formula  if we start off with a  low fps  and go to the high fps . Has anyone  listed the BC for the same pellet  with every  fps  from lets say 553.5 and up ?  Lets not use a model . Lets do a real life  test of the pellet .  I just did a little chairgun   BC number changing   from 0.0227  to  0.0310  with a 912 fps  gun  at 50 yards  the drop difference is   .3  at 0.0277 the drop was 6.5  and with the  0.0310  the drop was 6.2  . At 100 yards  looking through the scope  its .8  of a mildot  difference or  4 inches . So at longer distances  it really makes a difference .

I think the BC would also be different from different guns at the same fps.

Does the rifle achieve its 800fps from 1,000psi and a long barrel or 3,300 psi and a shorter barrel? Add in port size, valve duration, etc. and I would guess that the way the skirt is blown out would be different.

I think all that matters is 2 things. 1) Is the drop and accuracy acceptable to you with your gun. 2) Is there enough fpe/penetration at the target for your intentions.

Once the pellet leaves the barrel, it doesn't care how it got the 1000 fps. Other than a few "minor" things like twist rate and exact bore size, the gun shouldn't matter hardly at all.

[K.O.]

well actually twist rate may not be a "small thing" it may determine when the pellet becomes dynamically unstable... I think the new Maximus barrels are about 1:19 twist rather than 1:16... also the XL 725 barrels seem to be around 1:19 twist also rather than the 1:15 of the .25 Mrod...

I am pretty sure slower twist with most pellets helps long range accuracy and reduces spiraling which will have more than a small effect on B.C.

Bob I think I am the one who told you about Lapua using Doppler.. in 2015 It was back when the 200g .308 Whiteout BBT was being discussed while it was being developed...

and yep it would be a lot of work to get the model  for each pellet but it would be about the best "good"  B.C. we could come up with... We could them name it GP one thru what ever is needed..

[Horatio]

So it appears  that BC as far as  air guns go is  that every gun with a different  fps  is going to produce a different BC . 553.5 fps  = BC  0.0227. I would like to see what other fps's  BC turn out to be . Maybe  we can figure out  a formula  if we start off with a  low fps  and go to the high fps . Has anyone  listed the BC for the same pellet  with every  fps  from lets say 553.5 and up ?  Lets not use a model . Lets do a real life  test of the pellet .  I just did a little chairgun   BC number changing   from 0.0227  to  0.0310  with a 912 fps  gun  at 50 yards  the drop difference is   .3  at 0.0277 the drop was 6.5  and with the  0.0310  the drop was 6.2  . At 100 yards  looking through the scope  its .8  of a mildot  difference or  4 inches . So at longer distances  it really makes a difference .

I think the BC would also be different from different guns at the same fps.

Does the rifle achieve its 800fps from 1,000psi and a long barrel or 3,300 psi and a shorter barrel? Add in port size, valve duration, etc. and I would guess that the way the skirt is blown out would be different.

I think all that matters is 2 things. 1) Is the drop and accuracy acceptable to you with your gun. 2) Is there enough fpe/penetration at the target for your intentions.

Once the pellet leaves the barrel, it doesn't care how it got the 1000 fps. Other than a few "minor" things like twist rate and exact bore size, the gun shouldn't matter hardly at all.

But before the pellet leaves the barrel, I think it matters. I also wonder if the depth of the rifling effects the BC or If polygonal rifling is used.

Also, what if what we think of as a “minor” thing is actually quite a big deal due to the BC already being so poor?

[mr007s]

I tend to calculate BC from POI at range.
It can be found in Chair Gun toolbox

[rsterne]

That works well IF you can accurately determine the center of the group at 2 different ranges.... and the further apart the better....

Bob

[rws45user]

I was talking about  the jsb 15.89 pellet  not every pellet made .

 If you watch the  video  I post  above  .  Take the numbers  you see in the video   for the 15.89 pellet that he list  at the different  distances  for the 553.5 fps  and ending  at 416.4  at 50 meters they all are the same  in the chairgun program .  So his real life  shooting of the pellet through the crony  at the different distances  match what chair gun comes up with .

  So no matter what gun  chairguns formula  match the real  life  numbers  at the different distanced he listed .

 As I said in the above post  once I entered the pellet weight,   end distance of 50 meters and   muzzle  velocity   at 553.5  I scrolled down on the  table  so I could see the  fps at 50 meters  I changed the BC numbers  until the fps matched  416.4  the  crony's  fps at 50 meters . Once  the 50 meter   416.4 was meet by changing the BC  all the other  fps 's  matched  the real life  numbers  .

  Chair gun didn't know the  barrel twist  or what damage  the pellet went through  the barrel  but it came up with the same numbers as the real life  numbers . I think there is something  to this .

 If you can match real life  numbers  like the  muzzle vel and the end distance  velocity   you can just enter that into chair gun and  get its BC . Now if you change the  fps  of the muzzle vel  to 912 fps  and match that  to the 807 fps at 30 yards  like in the   link of BC that someone post   chair gun comes up with the BC  of  0.0363 . This BC matches  the 807 fps at 30 yards .  The  0.0310  BC .  in chairgun at   30 yard distance   showed 791.9   not 807 fps . 

  What I'm getting at is  chairgun   matched every distance  the same  as real life  flight   from 553.5 to 416.4    So if chair gun got it right  then  I would think  chairgun would get it right  from  912 fps to 807 fps  at 30 yards .   So for real life results  I would go by the chairguns BC  that  made the 807 fps  to match with its  BC number of  0.0363  because  chair gun nailed  the real life numbers  at the 553.5 fps  to 416.4  fps numbers .

  Now   lets just say chair gun has got it right  with the BC   and 553.5 = 0.0225  and  912= 0.0363     we can use chairguns  BC calculator velocity  box  to get the BC for any  FPS .

In chair gun once you entered  the BC number  you think is correct  if you right  click your pointer  over the BC number it will open up a  box where you can  choose  calclulate BC from : and then choose velocity change .  I entered  the  0.0363  bc number   and once the window is open you can change   velocity  from 912  to  553.5  and the range to 50 meters  and the  807  to 416.4 and then press calculate  and sure enough it gave me  the  same BC I had  that worked out in the video  of   553.5 = 0.0225 .  So I think the right BC for this pellet is  0.0363 at 912 fps .   

With this box in chairgun   once you have the correct  BC at one muzzle vel  you can have it for any  muzzle  vel   with the same pellet and  you can figure out  the BC for every  fps gun there is if  you know the  muzzle vel  and end  distance vel . Try it out in chair gun  and see for your self   0.0363  set at  912 v1  30 yards  and 807.0 v2  .  and you will see that BC puts the pellet at 807 fps at 30 yards  and it matched the videos  results  from the 553.5 fps gun .

[rsterne]

You are correct, ChairGun can correctly calculate the BC for a given set of numbers (starting and ending velocity, and distance between them)…. However, if you calculate the BC between 900 and 800 fps, don't expect it to be correct between 700 and 600 fps.... or 500 and 400.... The drag models used just don't match our pellets closely enough.... As a pellet slows down going downrange, it's Drag Coefficient changes.... The model used (in ChairGun you should use GA for pellets, and G1 for bullets) does this as well, but not in precisely the same manner.... Since the BC is the ratio of your Cd and the Cd of the drag model used, the BC can (and usually does) change with velocity....

  Now   lets just say chair gun has got it right  with the BC   and 553.5 = 0.0225  and  912= 0.0363     we can use chairguns  BC calculator velocity  box  to get the BC for any  FPS .

This statement is only correct if you already KNOW the near and far velocities and the distance between.... If you calculate a BC of 0.0225 from your 553-416 fps measurements and then change only the MV to 912 fps and click "Calculate", Chairgun will use the BC previously calculated of 0.0225 and give a MUCH lower result than the correct number of 807 fps....

Bob