Ammo Ballistic Coefficiency at lower speed🤔

[Redick]

I was wondering, would the BC of a given choice of ammo be the same at a lower speed? For example, I have a Mk3 wildcat with the power adjustment knob. For power 7 I'm at 900fps and on power 1 I'm at 675fps for the ammo I'm using. Are my BC numbers going to be the same for both speeds?

 

[Odoyle]

Sometimes BC goes up if shot below 700fps.

https://www.airgunsofarizona.com/Calcs.html

 

[Odoyle]

NO! Silly Wabbit...

That's just a SCREENSHOT for the example to use...

CLICK LINK ABOVE...

 

[nervoustrig]

The BC varies with velocity. For example, for domed pellets the optimum BC will generally occur somewhere between a 800 - 900fps muzzle velocity. It’s less for a wadcutter.

 

[dc8cappie]

Ok, let the truth come out! Someone is wrong here, me thinks!

 

[nervoustrig]

Odoyle’s statement is correct for a wadcutter. No disagreement. 

 

[JungleShooter]

BC changes with velocity (well, drag changes, but because our BC is calculated based on a generic drag model instead of a custom drag model specific for the pellet you're using, BC also changes).



I'd love to experiment and see what the BC/ drag pellets is at the low end, say 500fps and less (because a lot of HP and wadcutters will get into the low velocity range fairly quickly after leaving the muzzle).



I'll attach a graph that Bob Sterne (rsterne over at GTA) made a couple of years ago, measuring the BC the JSB line in .22cal.

It would be cool to have these kind of charts for other pellets as well. Surely helps in selecting the optimum velocity (providing that the pellet shoots with good precision, of course).

Matthias

 

[Macdoc18]

It looks like the best coefficients for the lighter pellets are around 700 and the heavier pellets at around 800. I guess that is the best best md-flight speed or is it the best muzzle speed?

 

[JungleShooter]

The velocities in the chart are average velocities.

For example, you shoot with a muzzle velocity of 900fps.

And you're shooting out to 65 yards. There, your pellets have 700fps of velocity left.

The BC for this shot would then be found at the 800fps marker in the chart.

Hope this helps,



Matthias

 

[tsmith]

i always shoot my pellets at 2 different distances and use a calculator like AOA provides. for example. the crosman 14.3 grain pelets are usually listed as a BC of .019. every gun that i use them in comes out somewhere in that .027 to .031 range. so velocity can vary the bc of a pellet. (IMO)
ps. when i do this the numbers in a ballistic calculator it comes very close to my ACTUAL holdovers.
so never trust a list of BC’s from a chart.

 

[Berserkeley Mike]

Hmm, that chart shows a trend:
13.4 estimate BC peak at under 600fps
14.3 around 650?
15.9 around 700
18.1 around 800
25.4 estimate over 800
So not only do heavier pellets have generally a better BC (higher peak), but the peak BC happens at a higher velocity, or the more "right-shifted" the curve is on that plot.
Interesting...
Mike

 

[Ballisticboy]

If the correct reference drag law is used, BC should be a constant, or near constant, at all speeds. There will be some changes, as a reference drag law will never be an exact match to the true drag law for each projectile design. The problem is that most quoted BC's do not say what reference drag law they are based on, or if corrections have been made for the atmospheric conditions that the tests were carried out in. Use of the wrong reference drag law will give larger changes in BC with velocity.

Just because a pellet has its highest BC at a certain speed does not mean that is where its minimum drag occurs. It just means that is the speed at which the reference drag law and the pellet specific drag law have the biggest difference. It is basically a fake value which will not apply at other speeds, which is often quoted by manufacturers for sales purposes.

As for the Airguns of Arizona calculator, you should never use that one now, as it is based on a constant drag coefficient and will be completely erroneous at higher speeds. It was OK as a start many years ago, but has been replaced by much more appropriate methods and reference drag laws.

For round head pellets, you should be using the GA or GA2 reference drag laws. For wadcutters there is a new drag law available in MERO called WC0, which needs to be proven in use. For slugs there are the SLG0, SLG1 or RA4 drag laws, you should not be using the G1 drag law at higher speeds. If you use a BC for a slug based on G1 which has been calculated at high speeds, it will be optimistic at lower speeds.

Any quoted BC is fairly useless unless the reference drag law used is also quoted.

Custom drag laws would be the best way to go, unfortunately there are very few people around who can calculate one or produce one from tests. It would get rid of all the problems associated with using BC's.

 

[Berserkeley Mike]

Ah, then as I understand you, that shape of the _curve_ IS the reference drag "law" or model as you state, GA, GA2, RA4, G0, G1 or whatever.
Thank you Sir, I think I understand a little better what those Reference models mean.
And it helps me understand this other thread on an "other" board, particularly about the different Reference law/models used for particular BC values:

Cast Bullet Database - A Work In Progress - Airguns & Guns Forum

Cast Bullet Database - A Work In Progress, Air guns, Airgun Forum

www.gatewaytoairguns.org

where in my infantile understanding, different BC values could be used for different Reference models.
As Mr. Bob said, "NOTE, YOU CANNOT USE THE G1 BC WITH THE GS DRAG MODEL, OR VICE VERSA".
So, which is most accurate? I guess I'll have to test this!
And, wouldn't it be great if the pellet designer/manufacturer would quote their assumed drag model?
Thanks,
Mike

 

[JungleShooter]

▪For round head pellets: GA or GA2
▪For wadcutters: there is a new drag law available in MERO called WC0
▪For slugs there are the SLG0, SLG1 or RA4 drag laws

Miles,
you did WHAT?!

You gave us a wadcutter drag model?!!
➠ That is so cool. So helpful!
Because wadcutters (and their cousins, the wide hollow point pellets) —
live ballistically in the shadow of round heads and slugs.

I'll head over to the MERO ballistic application right away!

GPC Ballistics Applications for Windows, MacOS and Linux

A few ballistics applications for Windows, macOS and Linux written in the PureBasic programming language.

gpc.fotosoft.co.uk

Thank YOU!
Matthias

 

[Squack]

The trouble with low speeds is the amount of things that change during the flight.
There was an old saying: the novice studies BC charts, and the expert studies the wind.