Reply With QuoteYes that was the old US Army test. I think it had to penetrate a 1/2" board of certain density at 30 or 50 yards, not so sure about the distance I am afraid.Originally Posted by krisko
A.G
Registered User
Watched this video earlier in the week from a link on another forum. As was commented there and here much of what is said or concluded is not relevant when you have an energy limit. There are also some things he says (when he is talking about spin and velocity) which are a little worrying in respect of his understanding of external ballistics. We will see what his next video is like as it is supposed to be on pellet flight and ballistics.
Registered User
Yes that was the old US Army test. I think it had to penetrate a 1/2" board of certain density at 30 or 50 yards, not so sure about the distance I am afraid.
A.G
Guest
regarding how safe it is: i dont see much difference between 12fpe and 30fpe, in the wrong hands or if you have a bad day, both are/can be dangerous.
The legislation sucks, just like the tasers, battons, pepper spray is outlawed in the UK, there are places where all these are perfectly legal as long as it for self defence ONLY. And the daily news are not really full of abuse not at all.
back to the topic:
have you seen graphs of ballistic coefficient as a function of pellet speed? There is a relationship, this has a maximum. There the pellets perform best in a specific barrel.
Typically it is over UK's legal limit. Basically all UK legal airguns are performing sub optimal. UK users are simply crippled!
i have no idea how to post an image here so here is the link
http://www.network54.com/Forum/58323...+with+Velocity
if we are talking .22 here are the speeds
18.1 optimum 800fps
15.9 optimum 725fps
14.3 optimum 650fps
13.43 optimum 615fps only this last one seems to be "optimized" for UK. the trouble is with this one:
if you set your gun up to shoot this like it wants to, you may go over the limit with heavier jsb heavy or monsters.
Seems retarded isn't it.
In the US people run them more like these:
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)
not higher since the performance will also suffer if they exceed these numbers.
Right. so where are we then?
Last edited by krisko; 26-02-2017 at 10:56 AM.
Registered User
These results are typical of those assuming an unrealistic constant Cd (~ 0.204) drag law.
Back in 2010, the use of the Constant Cd drag law was commonplace. If the correct drag law is chosen (ballisticboy's drag law or Chairgun's GA which are basically the same thing) then the the BC v. Velocity curves will be close (within the bounds of experimental error) to being flat, horizontal lines. i.e., the BC value for each pellet is constant and independent of velocity.
Short version: you can safely ignore the linked thread and its conclusions in their entirety.
George
Last edited by GPConway; 26-02-2017 at 03:48 PM.
Guest
that is experimental data. not theoretical my friend
Registered User
Sounds good to me, some 0.5" ply-woods would require some serious power to drive an airgun pellet through it... 👍
Registered User
We're not seeing his experimental data (velocities at ranges). Instead, the graph is depicting the results of some method that takes his raw data and outputs a BC value. In this case, the method is based on an invalid assumption (constant Cd) resulting in an incorrect and therefore variable BC value.
If it's still not obvious, then I'll reverse engineer his graph - best I can - to show how it should have been interpreted.
George
Last edited by GPConway; 26-02-2017 at 03:52 PM.
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.
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
Well, they certainly considered the lowly .22 RF which is treated as a toy gun in the US as quite lethal at 50 yards. The air rifle is a different story. I have had the unpleasant task of answering back to a few US posters degrading our sub 12 guns on the NET but TBH and deep down they are not wrong. But that is how it is and we shall adapt.
I still think that the OP is probably best served with a decent heavy pellet in .177 as it has the speed advantage over a .22 pellet of the same BC. The JSB Heavy in .177 has near enough the same BC as a .22 16grs pellet but travels a lot flatter, gets to the target much quicker so there is less chance of the target moving off the kill zone and has a pretty decent energy transfer on impact.
A.G
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.
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