Registered User
If you use the Chairgun software, it has a range of pellets in its data base that you can put in at the various ranges, including your expected muzzle velocity. From this you will be able to click on the chart shot curve at the different ranges to see what the retained energy is. It's down to coefficient and weight of the pellet overall, but will give you a rough idea.
Guest
i have not got his raw data but he speaks about chrony work so to me it is experimental
here is more to read if you are interested in this
http://www.gatewaytoairguns.org/GTA/...?topic=14735.0
Registered User
https://www.youtube.com/watch?v=3YrZqR-Ak0c&t=317s
guy tests BC in this video, over 12ftlbs though.
Registered User
Of course it's experimental and I'm sure the data he's collected is honest and good and worthy.Originally Posted by krisko
Pity we don't know what it was ...
It's his method for deriving BC values from it that I have problems with.
That's just the same again 16 months later. The words may be different but the graph's the same and still just as wrong.
Still no data and still the same problems. What a waste of bandwidth.
George
Last edited by GPConway; 26-02-2017 at 08:40 PM.
Registered User
I have found his values to be near what is stated in Chirgun but I find the Chiargun values closer to the POIs that I get with my rifles.Although I must say that ambient temperature does make a lot of difference.
A.G
Registered User
... and the ambient pressure even more so since air density is proportional to absolute pressure/absolute temperature.
The average altitude in the UK is 531 Ft and in the USA: 2493 Ft. according to Wikipedia.
If the ambient pressure isn't measured explicitly at the time of testing then those average altitudes correspond to average errors in measured BC of ~2% and ~9% respectively before the effects of ambient temperature, relative humidity and equipment accuracy/repeatability are considered. Do these tests on a hot summer day in Colorado (or a mid-winter day in Norwich) without all of the necessary compensations and the calculated BC values could easily be 25% or more in error.
George
Last edited by GPConway; 27-02-2017 at 10:36 AM. Reason: Grammar
Registered User
Just for giggles - and since I had nothing better to do - I had a go at this last night.
For the sake of my sanity, I've only considered his curve for the 0.22 JSB Exact 15.9 Grain since we know that the particular drag law for this pellet (GA) is well matched.
First the excuses ...
1) I don't have access to the original velocity/range data so interpolation of the BC v. velocity graph is necessary. Obviously not an ideal scenario.
2) The said BC v. Velocity curve has been 'smoothed' by the original author but he doesn't mention how this was accomplished. i.e., by guesstimate or by polynomial regression, although I suspect the former as he seems to have a point to prove.
3) No atmospheric data is available and it's not clear if all of the original data was collected under similar atmospheric conditions.
The closed-form expression used to generate the graph assumes a constant Cd = 0.204 regardless of velocity so the BCs must the same at the velocities where the GA and constant Cd curves intersect.
That intersection can be seen at ~335 Ft/s and ~800 Ft/s so, by inspection and at 800 Ft/s, BC = ~0.036 from the graph.
To expand this a little, the underlying equation is:
BC1/Cdc = BC2/Cdv or, rearranging BC2 = BC1 * Cdv/Cdc
where BC2 = Ballistic Coefficient (GA drag law)
BC1 = Ballistic Coefficient (from graph)
Cdc = assumed constant drag coefficient at all velocities = 0.204.
Cdv = reference GA drag coefficient at any particular velocity.
So at v = 800 Ft/s, BC2 = BC1 * 0.204/0.204 = BC1 = 0.036 as above.
Applying this to values interpolated from the graph (BC1) with appropriate GA Cd values gives:
v = 1000 Ft/s, BC1 = 0.022, BC2 = 0.032
v = 900 Ft/s, BC1 = 0.029, BC2 = 0.035
v = 800 Ft/s, BC1 = 0.036, BC2 = 0.036
v = 725 Ft/s, BC1 = 0.038, BC2 = 0.035
v = 700 Ft/s, BC1 = 0.037, BC2 = 0.030
v = 600 Ft/s, BC1 = 0.030, BC2 = 0.031
So, according to the above, the average BC value amounts to 0.033 (GA) with max = 0.036 and min = 0.030 against the normally accepted value of 0.031.
The dispersion is about what you'd expect (see excuses #1 and #2 above) and the average BC value is surprisingly close considering the unknowns of excuses #2 and #3.
Evidence enough that, given the correct drag law, the Ballistic Coefficient can be considered constant and doesn't vary with velocity.
If a significant BC variation is to be seen, then the wrong (or at least an inappropriate) calculation method has been used.
George
Last edited by GPConway; 27-02-2017 at 08:09 PM.
Guest
honestly i have no clue how to calculate a BC "accurately", for it to be correct in value. I don't care since calculations like that always base on some mathematical model with all kind of fancy constants to make it right.
personally i would just take the chrony MEASURED velocity at your range lets say 25yards and divide it by the chrony MEASURED muzzle velocity, to get the idea about the loss of speed over the distance at certain speed for a certain barrel/pellet combo.
It is a shame i cant do the experiment for JSBs here legally. But if you look at tables in my second link, he does list his chrony results for various pellets for both low power PAL versus 25fpe+, and indeed the ratio between the speed at 25yards versus muzzle: IS interestingly better for high power for JSB diabolo! so there is a sweetspot in speed.
RWS hobby on the other hand is better at lower energies, at medium and high speed it looses way too much speed way too fast.
i am sure when he was drawing his curves based on his experimental data he just connected the dots like kids do, hence the smoothing.
Last edited by krisko; 28-02-2017 at 07:03 AM.
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