Ballistics Coefficients Discussion

[rsterne]

It's possible, I suppose.... I have never investigated the BC from 26-50 yards.... If, as you say, the BC is lower than starting from the same velocity at the muzzle, then your explanation of a nose-up attitude increasing the BC may well be valid.... It should then be less noticeable if the twist rate is lower, and more noticeable if the pellet is spinning faster than required.... Overstabilzed bullets have a tendency towards precession (spiraling), caused by a nose high attitude, which increases downrange from that very effect.... On the other hand, recent tests have also shown that the BC decreases if the SF is less than 1.5 as well, because the bullet has a very slight wobble, so that the "ideal" stability for the very best BC seems to occur in a narrow range around an SF of 1.5-2.0.... Even disregarding their "shuttlecock" stability, most pellets are overstabilized by that measure.... many have an SF > 4.0....

Bob

[Scotchmo]

By SF, I assume that you mean Spin Factor.

If the behavior that I saw is common, it gives us another reason to avoid much above 900fps. At 1000+fps, the velocity falls off fast, but you are left with an abnormally high SF. And that may hurt on the really long shots.

I saw that tendency with the 8.44gr JSB when shot at over 1000fps. Accuracy was good but velocity fell of fast. I've since switched back to 10.34gr JSB. Most of my shooting is in competition with a 20fps limit, so not much concern with shooting the 10.34gr too fast.

[rsterne]

SF = Stability Factor.... under 1.0 is unstable, the Military use 1.5 - 2.5, and over about 3.5 - 4.0 may be overstabilized.... All that really means is that when you spin a bullet too fast, it emphasizes any imperfections in the bullet, particularly an off-center CG, and the groups open up.... As mentioned, it can also cause a bullet to remain nose high at long ranges, which causes it to generate a lift force, which in turn causes it to yaw and/or spiral.... There was a tendency for a while for Benchrest PB shooters to push the stability down to 1.3 or even 1.2, but current thinking is that when you do under 1.5 the bullet may be stable enough to still group well, but wobbling enough to show a reduction in BC.... so my understanding is that most feel that trying to get as close to SF = 1.5 is the optimum.....

I am constantly amazed at how quickly pellets lose velocity when pushed to and above 1000 fps.... The 14.3 gr JSB Express starting out at 1294 fps is down to 1210 in 5 yards.... It then drops from 1210 to 1137 in the next 5 yards.... Therefore it has gone from ~1300 fps to under Mach 1 in just 32 feet.... which when you consider the amount of air wasted, and the amount of noise generated, trying to get it to 1300 fps is a complete and total waste....

As I work on this problem, I am beginning to realize more and more how little point there is to push pellets much past 900 fps, as you say.... If and when we can nail down the new Drag Profile, I think people will be amazed at how low the optimum velocities are for minimum wind drift.... which after all, is the toughest thing to overcome in the field....

Bob

[HPAJunkie]

Is there any chance of someone using a real high speed camera to film the attitude of a pellet at different ranges? Would that help explain different BC as velocity degrades?

On a different note; I was tired of trying to "fit" the hawke pellet path model to my actual tested pellet drops so I used the empirical method and shot groups out to 125 yds with 25 yard increments beyond 45 yard zero mark.  My results were basically zero drop at 45, 4.5" at 75yds, 13.5" drop at 100yds, and 28.25" drop at 125 yds.  The muzzle velocity was averaged to be 907 fps using JSB Diablo Exact Heavies at 18.13 gr. I am willing to put my chrony out to different ranges since there is a 300 yard field just outside of work and the boss likes that I take care of the mice and starlings on the business property.  Bob, what ranges and velocities would you like for your data model? I can oblige over the next few days to provide some for you. I can also provide the local airport altitude densities, temp, pressure, and actual altitude as well.

[rsterne]

I think that pellet "attitude" may be closely related to spin rate, so although interesting, would be hard to work into any model.... By keeping the distances short, it will not be an issue, IMO....

As I mentioned earlier, using long distances between the two Chrony locations, while providing good data to see what the AVERAGE Ballistics Coefficient is over that range, won't really do us much good in developing the model.... We need to break the velocity down into smaller pieces, so that we can plot the entire drag profile.... I have done the work that I can for over 1000 fps, because the velocity degrades so quickly I could do that indoors.... We now need the data from 1000 fps and slower, and for most pellets (certainly the JSBs Exacts) that can be done at 20-25 yards between the Chronys.... Wadcutters you might be able to use 10-15 yards until you get down under 700 fps, but I don't really care, because I can't see how they can be included in the model.... It looks like only round-nosed pellet can be be included, and perhaps not even all of those.... Assuming that your data has a relatively small ES (under 1% is ideal) then the DeltaV (difference between the two Chrony readings) I would like to see is 5-10%.... Under 5% the accuracy in the BC calculations will be falling off (due to the ES), and over 10% we may be looking at too large a piece of the drag profile.... We need data at velocities in about 100 fps increments from 1000 fps all the way down to under 500 (even down to 300, if anybody cares) so that we can plot the entire drag curve.... That data must include, eventually, all the JSB Exact Series pellets, in all calibers.... in other words, a lot of shooting.... Any data you can provide is most appreciated....

Bob

[HPAJunkie]

I can start on gathering data Tuesday. Today and tomorrow I am away from my gun, sadly. I do have crossman premier hollow points that I can use but I just might keep with the JSB's. I will tether the gun to my scuba tank so there is more consistent pressure and record muzzle velocities at 1 yd and at 26 yds and see how far I get down the line of velocities with the available time.

[rsterne]

Sounds good....

Bob

[sixshootertexan]

If one only has one chrony what would be the best way to gather data. My .25 is reg'ed at 1400 and shoots a good consistent fps.

[rsterne]

Set the Chrony up at 1 yard to the first sensor and shoot 5-shot groups with each pellet.... If you know you got an ERR, take an extra shot, so you are averaging 5 shots.... Record the average velocity and the ES for each group.... Move the Chrony to 20-25 yards (make sure you know the distance from the first sensor to the first sensor, and it's easier if it is exactly "X" yards), and shoot and record 5 shots with each pellet again.... Bring the Chrony back to the 1 yard point, adjust the velocity down about 100 fps and repeat the process for each pellet.... Record the temperature, altitude, barometric pressure, and if you can the humidity.... I only have one Chrony, and this is what I do....

You can either PM me the data, or email it to me at bob at mozey-on-inn dot com.... Please include the pellet type and weight for each set of data....

Bob

[rsterne]

I dragged out my Grouse Gun today, and it is quiet enough I can do some limited testing outside.... It is set up for ~ 21 FPE, and I tested several pellets.... The G1 Model BC's I got today may be of interest to you....

JSB RS 13.4 gr. - 0.027 @ 818 fps (20 FPE)
JSB Express 14.3 gr. - 0.030 @ 806 fps
Crosman Premier 14.3 gr. - 0.026 @ 806 fps
JSB Exact 15.9 gr. - 0.032 @ 778 fps
JSB Heavy 18.2 gr. - 0.035 @ 732 fps
H&N Baracuda 20.3 gr. - 0.027 @ 702 fps
JSB Monster 25.3 gr. - 0.035 @ 626 fps (22 FPE)

The pellets that had a BC that came the closest to their SD were the JSB RS, Express, Exact, and Heavy, they all had a Form Factor of about 1.5.... The other pellets were worse, with the FF of the heavier pellets pushing over 2.0 (twice the drag of the G1 model)....

Bob

[HPAJunkie]

I forgot my allen wrenches so I was unable to get different velocities from where the gun is set already, but here is what I was able to get.
1 yd                                   26 yd
912.2                                 852.4
922                                    857.6
903.9                                 859.4
913.7                                 859.1
911                                    865.7
916.6                                 857.1
907.6                                 851.8
907.0                                 861.8
910.2                                 845.2
915.4                                 869.6
Pellet was JSB diablo exact heavy at 18.3 gr
temp was upper 48
27% dew point
44% humidity
altitude density 30.18 and falling

[rsterne]

What was your altitude above sea level.... I assume the 30.18 is corrected for sea level?.... I will use average velocities of 912 near and 858 far, 25 yard separation....

Bob

[HPAJunkie]

I was about 4700ft above sea level.

[rsterne]

Your absolute pressure is 25.403" Hg, which means a conversion factor of 0.882 for the BC.... That gives a sea level BC of 0.0569 using the G1 Model.... That results in the first calculation for a Form Factor of less than 1.0, at 0.970.... At first I though that might not be a good fit, but when I plotted it with the rest of my data for the 18.1 gr. JSB Heavy, in fact it works out really well.... I am concentrating on the JSBs, with the Baracudas thrown in, as they are also a long range candidate....



You will notice that the drag of the Heavies seems to go through a minimum at right about where your data is, an average velocity of 885 fps.... This should be refined further as we add more data points, but thanks for that, it seems to fit well.... BTW, the little zig-zag in the data for the Exacts is because the bottom two data points are for AA pellets, not JSBs, and according to Michael, they have a bit less drag.... which seems to be showing up there.... Again, more data will fill in the blanks.... A possible new Drag Profile (Function) is shown as the black dotted line....

Bob

[Slavia]

Is the stability factor of the Greenhill formula the same as that of the Miller formula, or is a conversion factor needed to reconcile them?

[rsterne]

The Stability Factor is the same, but the Miller formula has additional factors not found in the Greenhill formula, which only used length and caliber, then added bullet density, and eventually velocity.... Even better is the stability calculations done by the McGyro program from Robert McCoy, and available in an online calculator by Geoff Kolbe at   http://www.geoffrey-kolbe.com/barrel_twist.htm

Bob

[Tomg]

I shot 10 shots with the JSB Exact 18.1grain (.22)
I took the median in at 1yd, and 36yd.

1yd: 956fps
36yd: 854fps

[rsterne]

I would need the altitude, temperature, barometric pressure, and if possible the humidity.... to do a BC calculation consistent with our other data and include that in our database....

Bob

[Tomg]

1400ft (Phoenix, AZ) Temp 80F, Barometric Pressure 29.98, 15% humidity.

[rsterne]

Your absolute pressure is 28.495" Hg, which means the BC correction factor is 0.933.... That gives a corrected sea level BC of 0.0451 using the G1 model.... That puts that data point a bit higher in drag than what HPAJunkie found the other day over a shorter distance increment (25 yards instead of 35).... However, both sets of data are still within an acceptable zone to fit with the data I did.... We can't ever have too much data, IMO....

Bob