Ballistics Coefficients Discussion

[rsterne]

Well I dragged out my Disco Double today, with the .22 cal barrel on it, and tethered it at 2900 psi, cranked up the hammer strike, and started blasting pellets downrange in my shop.... I used 1 yard for the muzzle velocity and 6 yards for the downrange (carefully measured) so the difference was exactly 5 yards.... I recorded the temperature, barometric pressure and altitude, and used those in ChairGun to determine the G1 Ballistics Coefficients.... I used all of the different weights of JSB Exacts in .22 cal, and maxed out the RS were over 1300 fps, and even the 25 gr. Monsters were Supersonic.... I started with the gun maxed out, shot 5 shot groups recording the near and far velocities and the ES for each set of five for each pellet, then reduced the hammer strike and repeated the procedure three more times at progressively lower velocities.... Here is the raw data.... If you click on it you should be able to read it....



Once the muzzle velocities dropped below 1000 fps, the downrange velocity was too close to the muzzle velocity, so data below 1000 fps will have to wait until later in the year when I can work outside and out of town, with a greater distance between the Chrony positions.... However, I think I got some really good preliminary data, which certainly shows that the G1 (and GA) models are useless with pellets in the transonic and supersonic regions.... The drag is MUCH higher than those models predict, which I knew, this data confirming it solidly.... Here are three graphs showing the data in different ways.... First the raw data plotted as points, with each run being a different colour.... They represent the Cd that each data point has, rising way above the G1 baseline....



The second graph breaks it down by pellet.... You will notice that the RWS Hobbies, which are a wadcutter, have a MUCH higher drag, and no sign of it coming down towards the G1 baseline, even at 1000 fps.... I could probably run another couple of data points at lower velocities with that pellet inside.... Also, the JSB Monster has a rather unexpected trend as well, not fitting well in with the rest of the Exact Series.... perhaps because of its more cylindrical shape or vastly higher SD....



In the third graph, I left out the Hobbies and Monsters, and the data is organized once again by run.... I have added a dotted line representing the general trend for the Cd, which I think may be a pretty good representation of what the Drag Profile for the Exact Series does in the transonic and supersonic range....



This is only preliminary data, but I think it gives a pretty good idea of what is happening with pellets at higher velocities.... I would go so far as to suggest that creating a new Custom Profile in ChairGun, starting from the GA profile, and modifying it as per the dotted black line above, might be a pretty good place to start in the creation of a Custom Airgun Profile....

Bob

[HPAJunkie]

That is some great work and stuff that I am quite interested in.  Thank you for taking the time to compile this data.  There is so much to learn that it can be overwhelming sometimes but I think it is worth the effort!!

[michaelthomas]

That is very interesting, Bob.  I'm very, very curious to see where things go once you get down into the 750-850 range.  In particular, I want to see those 25.3 monsters around 785.  That is the energy cutoff (35fpe) for the open class.  Not sure of anyone having much luck shooting them all that accurately.....but that may be a good project for me all by itself. 

Nice work, Bob.

Mike

[rsterne]

My problem now is that we have new neighbours.... I used to be able to shoot in my backyard, but not any more.... Now when I want to test at anything over 20' I have to pack up and go out into the woods.... I have an excellent place to shoot, with over 100 yards available and a bench, but no power, and I really like running my Chrony with the indoor light kit, even when outside.... It is just much more stable and reliable, which of course is critical for a situation where you have to move the Chrony downrange for half the testing.... Unfortunately, it is in a forest clearing on the north side of a mountain, and the snow doesn't melt until about April, in a good year.... I may have to resort to a generator....

If anyone wants to contribute data, *ahem* Michael *ahem* then you can send it to me at.... bob at mozey-on-inn dot com.... I need the near and far velocities (with ES if you wish), the distance between, pellet type and weight, plus the temperature, barometric pressure, altitude, and the humidity if you have it (minor effect, I didn't, and will use 50% if you don't have it).... I would suggest testing at 20-25 yards to get a large enough difference in velocity for accurate calculations, 5-10% will do.... If the distances are too close (for the velocity loss) the ES will be too large a percentage, reducing the accuracy.... If the distances are too far apart, the pellet will be slowing over such a large range that we won't get enough resolution to produce a good drag profile.... Shoot 5 shots groups (or more) and record the average velocity at both distances.... If the sum of your two ES numbers is greater than the velocity loss, you need to either increase the distance between your Chrony readings, or use a different gun or setup with a smaller ES.... as the data may be unreliable.... I am particularly interested in completing the drag profile for the .22 cal JSB Exact Series of pellets, as they consistently have one of the best BCs around, so it makes sense to me to use them as the benchmark to measure other pellets against....

Bob

[rsterne]

One thing to note, if you guys didn't pick up on it.... Even pellets starting out at over 1300 fps fall back subsonic by the time they have travelled about 10 yards.... a bit further for a round-nose, a bit closer for a wadcutter.... It sure shows the folly of trying to shoot pellets above Mach 1....

That huge velocity loss is why I tested (and in fact could and should test) inside at only a 5 yards distance change.... Even using 10 yards would have lost half the data points....

Bob

[michaelthomas]

Bob,

The only real data I have that would be relevant is my HV data shooting the AA 16.0's

I had more data in the note section of my iPhone.....but did a stupid move and erased it.  It seems there may be some way to recover it, but I haven't tried that, yet.  I had the .22 RS, Express, and the 18.1's.

Temperature was 68F, elevation is 5700, pressure is 30.37in......according to the airport.  The near measurement was 1yd from the muzzle, and the distance between was 23yd.

AA16.0gr  Near-752  Far 709
                Near-855  Far 796

If I remember, these were about 10% better (GA) BC than the JSB 15.9's.....weird, but very repeatable.

Sounds like going supersonic will be the ticket for game at 5 yards or less.......or I suppose you could use a baseball bat....

Mike

[rsterne]

Michael, thanks for those two data points.... I have calculated the BCs, using ChairGun and the G1 Profile, and corrected for your altitude, etc. and I get 0.0411 for the lower velocity and 0.0369 for the higher.... Your high altitude makes a huge difference.... ChairGun is not very friendly for correcting for altitude and pressure, you have a choice of one or the other.... I found a Density Altitude Calculator here.... http://wahiduddin.net/calc/calc_da_rh.htm .... When I input your data, I get an absolute pressure of 24.631" Hg, so that is what I used in ChairGun.... I used the same method for me here yesterday (2450', 30.07", 68*F) and got 27.504" Hg, which I used in ChairGun to calculate my data.... Anyway, here are the results of your two measurements (with the AA's) and my three (with the Exacts)....



Eventually I will fill in the data in the 900s, but the trend is becoming very clear.... The more data we collect, the better our Drag Model will be....

Bob

[michaelthomas]

Hi Bob,

I am going to CA for the weekend to shoot at the place that is hosting the Nationals this year (Open Grove).  I can't wait....it should be a great time.

When I get back, I can do some BC testing with the other .22 JSB's I have and give you the data.  It will be mid 700's to mid 800's.  I have a bigger valve that I can put in one of my guns and probably get up to 950 or so with the 18.1's.  I don't have any of the 25.3's on hand......but will see if anyone has some I can bring back from CA.

Mike

[rsterne]

Thanks, Mike, the more data the better.... Have fun in CA.... If you're ever up here, drop in....

Bob

[QVTom]

Bob, are you only interested in pellet data?  I can test 357 JSB and H&N from 800 to 1200 fps.

Tom

[rsterne]

I would welcome your results, Tom.... No reason not to have multiple drag profiles if the answers are radically different.... If you can test even lower velocities that is also valuable for downrange, after the pellet/bullet slows.... I hope to do the Exact series down to 500 fps, maybe even lower.... If it turns out the larger calibers fit into the same profile, that is what I will do.... I suspect that they might have similar form factors, making the difference in BC due to the difference in SD.... If you notice, that is mostly what is happening in the .22 cal from 13-18 gr.... pretty much following the same profile.... I want to also develop a drag profile for flat-based bullets and another for boattails, that are suitable for airguns....

Bob

[michaelthomas]

Thanks, Mike, the more data the better.... Have fun in CA.... If you're ever up here, drop in....

Bob

I will definitely stop in if I'm in the neighborhood.  Maybe even plan to be in your neighborhood....lol.  My wife and I like to see new things.  Never been up that way before.

Mike

[plinker81366]

Bob, BC is still .0303 with the 18.2 grain pellets..am i getting it right? lol

[rsterne]

Depends on the model you are using.... Using the most common G1 model, it was 0.0321 @ 1198 fps, 0.0369 at 1079 fps, and 0.0519 at 1003 fps.... The last number may not be very accurate because the sum of the two ES's was greater than the velocity loss over 5 yards (the distance is too short to get a reliable answer).... What we can say, with confidence, is that once you go over 1000 fps the drag skyrockets, so you should avoid those velocities....

If we manage to create a new drag profile that replicates the JSB Exact Series, then whatever the BC ends up being will only be usable with that drag profile (function).... This is no different than today.... You can't use a G1 BC with the G7 drag function, or vice versa.... it's like comparing apples and oranges.... If you end up with a drag function that is a perfect match for the pellet, the BC will be a constant over the entire range of velocities, and it will equal the SD.... Getting the correct data for the trajectory and wind drift then requires that you use that drag profile in ChairGun....

Bob

[rsterne]

I did some more testing today, still inside.... I ran the Hobbies down to a lower velocity, and tested the Predator Polymags and H&N Baracudas as well.... Here are all the results....



The first thing you will notice is that even down at 700 fps, the Hobbies have nearly 3 times the drag of the G1 model.... This is typical of a wadcutter, and shows how hopeless they are at retaining velocity and energy, and the faster you push them, the worse they get.... The Predators follow a more typical drag curve, similar to the Exacts, but their drag is higher at all the velocities tested.... This is typical of a pointed pellet, they are in between a wadcutter and a round-nose in drag.... Right at the speed of sound, they actually have more drag than the Hobbies.... If you are using them, stay below 1000 fps.... The Baracudas were pretty similar in overall drag to the Exacts.... That means they will have a better BC because they are heavier.... It will be interesting to collect more data on them when we do more testing at lower velocities and longer range increments.... The data on the Baracudas isn't great at the lower velocities because the DeltaV is small.... they need testing over more distance to increase the accuracy of the results....

I think the testing I can do inside, at high velocities, is pretty much done.... All the pellets have a LOT more drag when they go over 1050 fps.... It is also pretty clear to me that using the JSB Exact Series to establish a new Drag Model for pellets is a pretty sound idea.... They are available in a lot of weights, the shapes are consistent from weight to weight and caliber to caliber, and they exhibit some of the lowest drag of any pellets I have tested, not only in these tests, but previously.... If we develop our new Drag Profile (Function) around them, then pellets having more drag will have lower BCs than the equivalent weight Exact, just as they should.... However, comparing them to the JSB Drag Profile should be a lot closer to reality than comparing them to the G1 (or GA) profiles currently in use.... It will be interesting to see how much data we can collect and how close it all fits so that a true trendline can be drawn....

Bob

[Cal]

Would there be some reason to adopt a standard based on geometry rather than a "Manufacturers product"?

Round ball,  blunt cylinder etc.

After all,  why go to the trouble to generate a new reference only to need to qualify it as "from years 2001 through 2016" etc.  , assuming the pell maker could change offerings for any reason, at any time.

[rsterne]

Of course, and I would suggest the pellets that have the lowest drag, in other words round-nose designs.... not domes, but those where the nose is a hemi-sphere.... That leaves us with JSB Exacts, Crosman Premiers, H&N FTTs, and possibly the H&N Baracudas (although not a true hemi-sphere).... It excludes the RWS Superdomes and similar pellets where there is a significant angle or ridge at the back of the head.... they are known to have more drag.... and also pointed pellets or wadcutters (flat or semi-domed) or hollowpoints....

All it takes is for somebody to get their tail in gear and do the work.... Of course the further the design moves away from a hemispherical nose, the more the drag will move away from the model.... There are a zillion pellet designs out there, but only one "round ball" or "blunt cylinder".... so of course the more designs you include, and the wider the variety of those designs, the more compromise you have to make in the model.... Perhaps we should just go back to the GA or G1.... or how about a constant Cd model like was used in the first versions of ChairGun, where the Cd was simply assumed to be about 0.20 at all velocities.... They work just fine.... except that the BC changes so drastically with velocity that any trajectory and wind drift predictions are pretty much useless, because as the velocity decays with distance, the drag no longer matches the model.... However, when comparing different pellets, fired from the same gun (adjusted the same), a better BC will mean a flatter trajectory and less wind drift.... so in that respect, they do the job....

The other alternative is to choose ONE pellet, caliber, weight, and design, measure it, draw it out with dimensions, and declare it to be the standard airgun projectile, just like the Krupp model, or the G1, or the G7.... If that is what you are suggesting, how do you propose we decide on what to use?.... A target shooter might well prefer a wadcutter, for example.... IMO, there is no perfect solution, because pellets vary all over the map.... To me it makes sense to come up with a model that is much closer to the best pellets (ie the least drag) available today, as it will be a lot closer to reality with all pellets than what we have now.... If you have a better solution, let's hear it....

Bob

 

[rsterne]

I was working with ChairGun last night, and had a revelation this morning as I was laying in bed about 6AM.... Last night I was looking through the built in Drag Profiles in the latest version of ChairGun, and in the widget on customizing the Gx (custom) profile were two choices that didn't make a lot of sense.... The were constant drag, with the Cd = 0.50 or Cd = 0.19.... I didn't think too much about it, but did remember seeing one of the PB Ballistics Website mention that Airgunners sometimes used a profile where the Cd was constant with Cd = 0.20.... Over the last couple of days, when I was calculating BCs at supersonic and transonic velocities, I noticed that I was getting totally different answers to what I had 3-4 years ago, and chalked it up to ChairGun using a different Model.... but when I woke up this morning, I suddenly thought that the earlier versions of ChairGun may have used a straight-line, constant Cd model.... So this morning, I loaded that into ChairGun and re-checked my data from 4 years ago, and it was very close to the BCs I calculated then.... When I adjusted the Profile to a constant Cd of 0.20, the results agreed 100% with what I did then.... So, when I drew that graph in the first post in this thread, the BCs were calculated by ChairGun using a constant drag profile, where Cd = 0.20 over all velocities.... This was a fortunate piece of luck, or I never would have noticed that the drag was changing with velocity.... Here is what is happening....



The upper graph shows the G1 drag profile and the old constant drag profile used in early versions of ChairGun.... It also shows the data collected by Michael and I for the 16 gr. AA/JSB pellets over the last couple of days.... The lower graph shows the BCs as calculated by ChairGun using the two different profiles.... You will notice that they are almost identical at the lower velocities, because the drag profiles are almost identical there.... However, at above Mach 1 the calculated values of the BC are totally different, depending on what drag model you use to make those calculations.... As I said, it was fortunate that the early version of ChairGun used the "wrong" drag model, or I would likely not have noticed that the BC was changing with velocity....

This brings up an interesting point for somebody like Michael, who is interested only in determining what velocity the very best BC occurs at, at a constant range.... He might be better using a constant drag model to calculate his BCs, because it emphacizes the difference.... On the other hand, once he determines the best RANGE of velocities where the BC peaks, he should shift over to the best available drag model (what I hope to achieve) to determine the absolute best muzzle velocity to produce the least wind drift over the 25 yard distance, to compensate for the velocity loss over that range.... Of course he already knows the range of usable velocities, so which method he uses doesn't really matter, and in fact he should probably develop his own drag model, for the specific pellets he uses, over the specific range of velocities that is useful to him....

It is interesting how things change over the years.... It hasn't been that long since ChairGun was first introduced, and yet the drag calculations it uses to predict trajectory and wind drift over a wide range of velocities are now MUCH better than when it was first introduced.... If you assumed the BC was constant over velocity 3-4 years ago (as most did) then you could justify shooting at >1000 fps and expect less wind drift.... Now that ChairGun uses better drag profiles, that is no longer the case, even if you use a constant value for the BC.... If we manage to develop an even more accurate drag profile for Diabolo pellets, then we will have an even better understanding of the optimum velocities to use to overcome wind, our greatest nemesis, at longer ranges.... It is getting more and more likely that we will stop using velocities much over 900 fps.... exactly what the benchrest guys like Timmy Mac have known for a very long time.... His latest EBR win at 75 yards was done at ~880 fps.... Michael is gradually dropping his velocities for 25 yd. benchrest down under 800 fps.... Finally.... finally.... it is becoming clear to me why....

Bob

[rsterne]

I added the data for some more pellets today, I tested the other two types of round-nosed pellets, Crosman Premiers and H&N FTTs, and I also tested a pointed pellet, the JSB Straton.... Here is a graph showing all the results....



The Crosman Premiers were not too far off the drag of the JSB Exact Series, a bit higher in the 950-1050 fps range, otherwise fairly close.... However, the H&N FTTs had a lot more drag, and the JSB Straton pointed pellets even more, in fact in the 900s they have more drag than the one wadcutter I tested, the RWS Hobby.... I don't know what JSB knows that the other manufacturers are missing, but their pellets have consistently less drag in the 900-1100 fps range in particular, the only other pellet that is similar is the H&N Baracuda, so far.... The rapid increase in drag in the transonic region is delayed for that group of pellets....

I have added a potential candidate for a new Drag Profile (Function) running through the drag curves for the JSB Express, Exact, Heavy, and H&N Baracuda, and the supersonic values for the Crosman Premier.... It is shown as the black dotted line above.... As a simplification, we could use a Cd of 0.20 below 700 fps, 1.00 above 1300, and an "S" shaped curve forming the transition between the two.... The exact details of the shape, particularly in the area from 700-1000 fps, needs a lot more work.... When I do more testing in that area, I am going to concentrate on the JSBs, but will include the Baracudas and Premiers, and the Predator Polymags as well.... All the other pellets have significantly more drag in that range, so IMO are a poor choice to include in the modelling process any further.... While an argument can be made that they should be included, I am beginning to view the new model I am trying to create as a MINIMUM drag for Diabolo pellets, or nearly so.... at least one that represents what is current state-of-the-art in pellet design.... I think this makes a great deal of sense, as then pellets which are more "draggy" will consistently fall above the model, and have lower BCs, just as they should.... By creating a drag profile that is representative of the best, their BCs will match their SDs, which is the way a Drag Function is supposed to work.... If anyone disagrees, they are more than welcome to create their own drag profile for their pellets of interest, of course....

Bob

[Scotchmo]

that I have seen but not investigated in detail:

We already know that BC varies with muzzle velocity.

It also seems to vary with distance, even when velocity is accounted for.

Example:

#1) When shot with a muzzle velocity of 850fps, I might get a BC of .028 average over the 1-25 yard distance.

#2) When shot with a muzzle velocity of 1000fps, I might get a BC of .024 average over the 1-25 yard distance.

#3) When shot with a muzzle velocity of 1000fps, the velocity at 25 yards is 850fps, the same as the muzzle velocity of #1. I would then say that the measured BC of #2 from 26-50 yards should be the same as #1. Instead, I get a BC of .025 over the 26-50 yard distance.

A velocity of 850fps near the muzzle gives a BC of .028 over the next 24 yards (1-25 yards).
A velocity of 850fps taken downrange gives a BC of .024 over the next 24 yards (26-50 yards).

Speculation:
When fired at 1000fps, the pellet velocity drops to 850fps at 25 yards but the rpm of the rotating pellet remains about the same as when it exited the barrel. The excessive angular momentum tends to fight the drag stabilizing tendencies of the Diabolo shape. The result would be a slightly increased "angle of attack" as the pellet arcs downward, which would increase the drag and reduce the BC.

Any thoughts?