Author Topic: BC of Airgun Slugs  (Read 629 times)

rsterne

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BC of Airgun Slugs
« on: July 01, 2021, 08:40:22 AM »
We are slowly learning that the G1 ballistics model is not the best match for airgun slugs, particularly the current designs of swaged slugs with a 2S tangent Ogive and about a 50% Meplat.... However, until a better model (eg. that proposed by Miles Morris) is established and becomes commonplace, it is the one most of us use.... I was looking at the HAM database of BC's this morning, and found something interesting in the section on .22 cal Slugs.... You can find it here....

https://hardairmagazine.com/ballistic-coefficients/

You can sort the table by any of the columns by clicking on the column heading.... If you sort by the "Form Factor" (and you can have low or high at the top), you will see something interesting.... The range of values only goes from 0.89 to 1.06, with the average being 0.97.... That means that if you use the Sectional Density as your BC, you will be pretty close for any of them.... The worst case would be an error of 11%....  8)

A lower FF means less drag for a given weight, and therefore a higher BC.... However, with the mismatch we see between the velocity during testing and what the actual drag is at the velocity you are using because the G1 profile doesn't suit our slugs very well, that method of estimating the BC should be close enough.... Interestingly, I have been doing that for years....  ;)

Bob
« Last Edit: July 01, 2021, 08:44:20 AM by rsterne »


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Alan

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Re: BC of Airgun Slugs
« Reply #1 on: July 01, 2021, 10:31:11 AM »
You now have a means of measuring (calculating) the BC. The question remains, have you done any testing yourself?
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Alan

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rsterne

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Re: BC of Airgun Slugs
« Reply #2 on: July 01, 2021, 05:48:19 PM »
I have done a limited amount, but nowhere near as much as I would like.... Currently I am waiting until I can cross the US Border to get some more lead, as I cannot cast any more because I have run out....  ::)

Bob
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JungleShooter

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Re: BC of Airgun Slugs
« Reply #3 on: September 05, 2021, 09:26:41 PM »
Someone send a lead truck over to Canada so Bob can make more slugs!!  😊


🔷 Bob, I'm not sure if I mentioned this, but there is a new ballistic calculator out.
It incoroporated Mile's experimental drag model for pellet and for slugs.



🔶 It's the "GPC Ballistics Applications".
It's a full suite,
for Windows, Android, iOS, Linux
I have it on my Android phone, where it installs as about a dozen separate apps. I don't know how it installs on a Windows computer, mabye all integrated into one program?


🔶It's very powerful, with a lot of very conventient functions that ChairGun offers for the Windows version — but that it cut out for the mobile version (phones and tablets).


🔶The developer of the ballistic caculator is active on UK forums, and did this project out of boredom when he was recovering from an injury he said.  🤣


🔶Here is the webpage of the developer where you can download the different apps/ versions:

https://GPC.fotosoft.co.uk/Home.html

Matthias


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rsterne

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Re: BC of Airgun Slugs
« Reply #4 on: September 05, 2021, 09:54:16 PM »
I have one called "Easy BC" which also has Miles' Drag Models in it....

Bob
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Alan

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Re: BC of Airgun Slugs
« Reply #5 on: September 06, 2021, 04:46:12 AM »
It has never ceased to amaze me, how such a tiny bit of drag and/or wind drift affects the path of any projectile. In my (airgun) case, 100 yard shots, at mid-summer temps (90s), amount to 1/2 a mil less hold over, than mid-winter (50s) shots. All this said, we now have the means to measure (via affordable doppler radar) rather small changes in BC during a projectile's path, regardless of the cause. Thus, I suspect a lot of older, tried and true BC programs, will fall out of favor.
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Alan

I have a Hill EC-3000 compressor. If you're a fellow Guild member, and you pass through Roswell, NM, I'll fill your tank as a courtesy (4,250 PSI limit).

Steelhead

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Re: BC of Airgun Slugs
« Reply #6 on: September 06, 2021, 07:53:05 AM »
I find the technical aspect of this fascinating, even though it's difficult for me to grasp. Sort of like watching documentaries on the universe (which I love) and trying to wrap my head around distances, densities, speed and numbers.

I'm curious on what, if anything air temperature in a gun has to do with bc and/or accuracy. I know that firearm ammo is very sensitive to temperature and shots with cold ammo that have been exposed to low temperatures (late fall weather while hunting for example) are vastly different than hot ammo (cartridges left in a hot car trunk, dashboard, etc.)  The difference between the two trumps all other factors like powder ratio, wind, outside temperature, etc.

Are airguns as affected by hot or cold temps of the air in the reservoir itself? Just a curiosity and I certainly don't want to to take away from the original thread, but airguns have a lot more (to me) variables than a firearm and understanding/limiting them only leads to more consistent results.
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Alan

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Re: BC of Airgun Slugs
« Reply #7 on: September 06, 2021, 09:01:07 AM »
The simple answer is, yes. Hot air flows a bit faster than cold (more dense) air. But, we also have to consider that the air cools as it expands, so a ton of other factors have to be considered. I'm sure Bob can address those.

Kevin, what I was alluding to in my previous post was the changing BC of a projectile, as it slows down. I think the most common misconception is, that the BC is the same no matter the velocity, and that simply isn't true!
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Alan

I have a Hill EC-3000 compressor. If you're a fellow Guild member, and you pass through Roswell, NM, I'll fill your tank as a courtesy (4,250 PSI limit).

Steelhead

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Re: BC of Airgun Slugs
« Reply #8 on: September 06, 2021, 09:17:10 AM »
Being a natural cynic and a person who sees a lot more gray than black and white, I always seem to have the response in my head..."Okay, but what about...?" Different airguns, barrels, elevations, temperatures, air pressures, fps, lubes, projectile designs, etc. can spin me for a loop trying to put the right pieces in the right spots. Granted, that's the challenge of this hobby and it's what makes it hard to reach to upper echelons of performance.
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Alan

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Re: BC of Airgun Slugs
« Reply #9 on: September 06, 2021, 11:58:24 AM »
I think you and I are in the same school of thought. if you need more power, get a bigger gun, rather than push the one you have to its limits!

I should add, that the type of hunting and where you hunt, also dictates what you shoot.
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Alan

I have a Hill EC-3000 compressor. If you're a fellow Guild member, and you pass through Roswell, NM, I'll fill your tank as a courtesy (4,250 PSI limit).