Ballistics Coefficient Variation with Velocity

Ballistics Coefficient Variation with Velocity

Joined: February 17th, 2010, 3:30 am

March 23rd, 2010, 9:04 pm #1

This is a repeat of a post I made on another forum:

As many of you know, I have done quite a bit of research on Ballistics Coefficients.... After I did my second round of pellet testing at a higher velocity than the non-PAL round I did originally.... I began to realize that BCs were not constant but varied with the velocity.... Wadcutters tended to get worse as the velocity increased while round nosed pellets tended to improve (over the two speeds I tested).... Hollow points, domes, and pointed pellets were mostly in between.... Since I now have a couple of PCPs where it is very easy to adjust the velocity.... and since JSB have recently introduced two new pellets in .22 cal (the 13.4 gr. Exact Jumbo RS and the 25.2 gr. Exact Jumbo Monster).... I decided to spend a day shooting all five .22 Exact series pellets over the Chrony at various power settings at two known distances.... and then calculate the BCs using Chairgun.... I started with the RVA cranked to the maximum, then used 3, 4, 5, and 6 turns out.... plus added some data from my non-PAL testing for a couple of the lower velocities....

This is NOT an exact science.... just a few fps variation in the velocity can change the BC by 10%.... I shot 5-shot groups over the Chrony at 1 yard and at 26 yards and used the average velocities to calculate the BC.... Then I plotted the data on graph paper for each pellet and tried my best to fit a curve to each set of data.... It wasn't easy, but this is the results:

[/img]

The first thing that becomes obvious is that there is an optimum velocity for each pellet.... in terms of where the BC peaks.... Secondly, the heavier the pellet, the higher that optimum velocity is, and the higher the peak BC.... Thirdly, by the time you get into the transonic region, pellet performance suffers dramatically.... and these efficient, round nosed pellets are down to BCs typical of wadcutters, or at best, domed pellets.... Velocities in the region of 1050 fps yielded BCs of only 0.0130 -0.014.... I couldn't test in the supersonic region in consideration of my neighbours.... but one would expect the deterioration to continue....

Possibly the biggest revelation, at least to me, is that published BCs are only a rough guide.... Stating that "Pellet X" has a BC of "Y" means little unless you also specify a velocity range.... In fact, a pellet will actually have a different average BC depending not only on the muzzle velocity, but also on the distance to the target.... as it is constantly slowing down along the way.... The only true way to find out the trajectory of your gun.... or how much of it's muzzle velocity and energy it delivers at a given range.... is to test it....

Please don't try and get actual BC numbers from the above chart.... Its only purpose was to try and show the trends that I found in todays testing.... :roll:

Bob
Quote
Like
Share

Joined: February 17th, 2010, 3:30 am

March 23rd, 2010, 9:06 pm #2

Using the information above, I decided to test some JSB Exact Series pellets at the maximum power level I currently have available to see just how much the air resistance slows a pellet over the first 5 yards relative to it's velocity.... Here are the results:

[/img]

It's pretty obvious that in the transonic region the BC takes a dump.... and show how important pellet selection is in a high powered gun.... :shock:

In case anyone cares.... I tested a Hobby and it was right at the speed of sound.... it lost ~100 fps in 5 yards (over 16% of it's energy).... and the initial BC was only 0.007.... :roll:

Bob
Quote
Like
Share

Joined: February 11th, 2007, 10:27 pm

March 26th, 2010, 2:23 pm #3


Very good info, thanks a lot for that.
I have found that in .22 up to 950fps in the 16gr range is deadly accurate and gives good down
range energy.(JSB Jumbo 15.8)
I usually try to tune the 850's in 22 to run at around 915fps average using 14.5gr pellets.(RWS SD's)
That power level seams to provide a good balance between power, accuracy and shot count with most
pellets.


The 850 Store
<a></a>
Quote
Like
Share

Joined: February 17th, 2010, 3:30 am

March 26th, 2010, 4:54 pm #4

I realize that everyone has a tendency to want to see the high velocity numbers.... and it certainly is a great advertising point as well.... What I have learned with PCPs is that since they have a preference for heavier pellets anyway (more efficient, longer shot strings, more FPE generated).... why not take advantage of that by using pellets more suited to their performance....

IMO, the whole issue of long range performance is dependant on Ballistics Coefficient.... The wind drift is less, the retained energy is greater, the benefits of a high BC outweigh almost everything else with the exception of velocity.... The JSB Exact series of pellets have some of the best BCs in the AirGun world.... and they are now available in five different weights in .22 cal.... from 13.4 gr. to 25.2 gr.... All of them peak at a BC of over 0.030, and the heavier ones exceed 0.040 at optimum velocities....

If you check out the graph in my first post, and draw a horizontal line across at a BC of about 0.025.... you will find the following velocities for the various pellets:

13.4 gr. RS : 850 fps (~22 FPE)
14.3 gr. Express : 900 fps (~26 FPE)
15.8 gr. Exact : 950 fps (~32 FPE)
18.1 gr. Heavy : 1000 fps (~40 FPE)

What that means is that if you exceed those velocities, your losses in the first few yards are HUGE.... Not only do you lose a lot of energy in those first 5-10 yards.... but your trajectory after that point is a lot worse than you would expect.... Add into the equation that lighter pellets tend to be noiser and less efficient (more wasted air) and I think you will quickly decide as I did that heavier pellets are the way to go.... My favourite pellet in the 30-35 FPE range is the JSB 18.1 gr. Heavy.... It has a wonderful BC (I've measured as high as 0.044 at ~ 800 fps).... it's VERY accurate in any gun I have tried.... and it hits a TON !!!....

Don't be afraid to try heavier pellets.... Once you get into the 900+ fps range.... there are more gains to be had by increasing pellet weight than velocity.... Although waisted pellets and .22 rimfire behave quite differently.... you may find the following chart interesting.... It shows how quickly the BC drops in the region between 905 fps and 1047 fps....

http://www.exteriorballistics.com/ebexp ... imfire.cfm

Bear in mind.... waisted pellets perform even worse in the transonic region.... ie above Mach 0.8 (900 fps)....

Bob
Last edited by rsterne on March 26th, 2010, 5:49 pm, edited 1 time in total.
Quote
Like
Share

Joined: May 25th, 2008, 4:38 am

March 26th, 2010, 7:24 pm #5

I've been shooting my Dominator HPA .22 (938 FPS) using JSB 15.8 GR in 50 yard benchrest...it was doing ok...but after last match,I tried JSB 18.1 GR and they did better in the wind...probably 30% better on groups..I'm guessing they are shooting around 920 FPS or so at muzzle...(got to test across my chrono...)

With my 850 .177 HPA,been shooting JSB 10.2 GR at 960 FPS...it did well in 50 yard benchrest as well...but wind affects pellet more than the .22 Cal..

Quote
Like
Share

Joined: February 11th, 2007, 10:27 pm

March 26th, 2010, 7:55 pm #6

I realize that everyone has a tendency to want to see the high velocity numbers.... and it certainly is a great advertising point as well.... What I have learned with PCPs is that since they have a preference for heavier pellets anyway (more efficient, longer shot strings, more FPE generated).... why not take advantage of that by using pellets more suited to their performance....

IMO, the whole issue of long range performance is dependant on Ballistics Coefficient.... The wind drift is less, the retained energy is greater, the benefits of a high BC outweigh almost everything else with the exception of velocity.... The JSB Exact series of pellets have some of the best BCs in the AirGun world.... and they are now available in five different weights in .22 cal.... from 13.4 gr. to 25.2 gr.... All of them peak at a BC of over 0.030, and the heavier ones exceed 0.040 at optimum velocities....

If you check out the graph in my first post, and draw a horizontal line across at a BC of about 0.025.... you will find the following velocities for the various pellets:

13.4 gr. RS : 850 fps (~22 FPE)
14.3 gr. Express : 900 fps (~26 FPE)
15.8 gr. Exact : 950 fps (~32 FPE)
18.1 gr. Heavy : 1000 fps (~40 FPE)

What that means is that if you exceed those velocities, your losses in the first few yards are HUGE.... Not only do you lose a lot of energy in those first 5-10 yards.... but your trajectory after that point is a lot worse than you would expect.... Add into the equation that lighter pellets tend to be noiser and less efficient (more wasted air) and I think you will quickly decide as I did that heavier pellets are the way to go.... My favourite pellet in the 30-35 FPE range is the JSB 18.1 gr. Heavy.... It has a wonderful BC (I've measured as high as 0.044 at ~ 800 fps).... it's VERY accurate in any gun I have tried.... and it hits a TON !!!....

Don't be afraid to try heavier pellets.... Once you get into the 900+ fps range.... there are more gains to be had by increasing pellet weight than velocity.... Although waisted pellets and .22 rimfire behave quite differently.... you may find the following chart interesting.... It shows how quickly the BC drops in the region between 905 fps and 1047 fps....

http://www.exteriorballistics.com/ebexp ... imfire.cfm

Bear in mind.... waisted pellets perform even worse in the transonic region.... ie above Mach 0.8 (900 fps)....

Bob
Like I said, 15.8 gr. Exact : 950 fps (~32 FPE) seems to perform best.
I tune the 14.5's at 915fps average, which is also pretty close to the sweet spot for that weight and it allows you to
use a slightly heavier pellet without loosing too much velocity.
The up side is that no matter what power you end up tuning for, there is bound to be a pellet that is happy 
with your set up. We are blessed with such a good selection of shapes and weights these days, it's really great. .
The 850 Store
<a></a>
<img alt="Photobucket" src="http://i158.photobucket.com/albums/t99/rbva/NTAMs.jpg" border="0">
Last edited by rbva on March 26th, 2010, 8:11 pm, edited 1 time in total.
Quote
Like
Share

Joined: May 25th, 2008, 4:38 am

March 26th, 2010, 8:16 pm #7

Dominator .22 on chrono:

JSB 15.8 GR..
Lo..944.0 FPS
Hi..951.1
AV..947.5
ES..7.12
SD..2.44

JSB 18.1 GR..
LO..902.5 FPS
HI..909.1
AV..905.3
ES..6.6
SD..2.8

Think the JSB 18.1 will be my "wind" pellets..lol..
Quote
Like
Share

Joined: February 17th, 2010, 3:30 am

March 27th, 2010, 1:16 am #8

If you chrony those two pellets at 50 yards I think you will find they will have almost identical velocity remaining at that distance.... based on your muzzle velocity and the initial loss with the 15.8 gr....

Bob
Quote
Like
Share

Joined: August 26th, 2008, 9:21 pm

March 27th, 2010, 3:55 pm #9

Bill

when you say you are "chrony them at 50 yards" is your chrony placed at 50 in front of the target / pellet stop ?

or are you testing them twice once as the pellet leaves the barrel and once when its impacts on target?
Quote
Like
Share

Joined: February 17th, 2010, 3:30 am

March 27th, 2010, 4:38 pm #10

I do a 5 shot group with the chrony 1 yard from the muzzle and record the average velocity of the group.... Then with no changes and as soon afterwards as possible (and with about the same air pressure in the tank) I set up a target at 50 yards and find my POI.... I set up the Chrony immediately in front of the target making sure that the center of the "sweet spot" for chrony reading is right in line with the POI of the pellets.... I fire another 5 shot group and take the average.... then using ChairGun you can calculate the Ballistics Coefficient over 50 yards for that pellet and muzzle velocity....

I have a Lexan guard (2 layers of 1/8") about 1" in front of the display to protect it from an errant pellet.... I would suggest that you also have a guard in front of the rear sensor housing as I have heard of people shooting that up as well.... If you have a unit with a remote display, you can make the guards out of 1/8" steel.... One thing to beware of is that if you are testing several different pellets the POI can be far enough off (especially at 50 yards) to hit the Chrony.... Ideally, you should have another target a couple of feet off to the side to check the POI of the first shot....

It is actually VERY interesting to take these measurements.... and find out just how much of your hard earned velocity and energy is left downrange.... You will be AMAZED at how quickly some pellets shed velocity even though they look quite similar.... My biggest surprise was the difference between domed pellets (such as a Gamo Hunter or RWS SuperDome) and a true round-nose pellet like a JSB Exact series.... and also how poor most pointed pellets are....

I haven't tested all pellets at all velocities of course.... but I have definitely seen some trends.... Based on one series of tests at ~500 fps (our non-PAL limit in Canada) and another series in the ~850 fps range.... I found the following trends for the Ballistics Coefficient (tests done at 25 yards):

Wadcutters got worse as the velocity increased.... the BC dropped from 0.010-0.015 to 0.009-0.013....
Round Nose pellets (JSB Exacts, Crosman Premiers, Kodiaks) clearly got better as the velocity increased.... the BC increased from 0.020-0.024 to 0.032-0.044.... Crosman Premier (& Benjamin) Hollow Points were in this group (they are round nose pellets with a very small hollow point)....
Domed, Pointed and Hollow point pellets had no clear trend.... although Crow Magnums and Predators improved slightly.... Otherwise some improved, some were worse or stayed the same....
Heavy pellets (EunJins etc.) improved significantly as the velocity increased.... this trend is in addition to the above trends for the shape....

If you are interested in how one particular pellet reacts to velocity changes.... the only way is to test it.... I have found that most pellets seem to have a "speed limit" above which their performance falls off a cliff....

Bob


Quote
Like
Share