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Ballistic Coefficient for Cannon

This is a discussion on Ballistic Coefficient for Cannon within the Military History forums, part of the Armed Services category; This is definitely a bit off the normal topics on this board but it is Ammunition related and hopefully I can get some feedback. Besides ...


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Old March 6th, 2017, 07:03 PM   #1
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Ballistic Coefficient for Cannon

This is definitely a bit off the normal topics on this board but it is Ammunition related and hopefully I can get some feedback.

Besides my M1A and other stuff I also shoot a reproduction civil war 3-inch Ordnance rifle with the 5th Mass Battery, a New England CW reenacting unit.

We will be competing at 900-1000 yards this June in upstate New York and need to make some prediction as to where at least start with the elevation at this range. This firing is going to be with full powered military loads (one pound GOEX 1F and 10 lb projectile) We have plenty of shooting experience with lighter lead target loads up to 200 yards with about 2.5-3 MOA accuracy. Not so much with the longer range.

Based on the following specs and photos of our intended projectile what would be a reasonable theoretical BC for the load? BTW this projectile is an approximation of the actual 10 pounder CW Parrott bolt. Difference is this one is cast and machined zinc instead of iron and the obturation cup is bolted to the base instead of cast into it.

Projectile data:
Closest dimensionally and proportional to the G1 standard shape
Diameter 2.982"
Length (less brass obturation cup) 6.05"
Length (including cup) 6.8"
Weight 10.25 lbs
MV 1,250 fps
Length ogive 1.55"
Radius ogive approx 6"
Nose diameter 2.15"

Seriously grateful for any informed opinions/estimates regarding what the BC would be for this round. There does exist original military ranging data but it will not work in our case since the weight and material of the projectile is significantly different from the original CW ammo. Also the powder granulation is different now versus then.

Regards,
Badger
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Old March 6th, 2017, 07:07 PM   #2
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I ain't never shot a real cannon but this is pretty cool! 10.25 pound 1250 FPS just sounds cool! Keep posting some pics of this badger thanks!

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Old March 6th, 2017, 07:29 PM   #3
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No problem I'll post a few more here for general interest. Still hope to get some feed back regarding my original post.
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Old March 6th, 2017, 07:36 PM   #4
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I'm not that smart but have you thought of looking up old artillery manuals? If that doesn't fair well, I would contact Brian Litz.....

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Old March 6th, 2017, 07:51 PM   #5
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The old CW ordnance department data does not apply to our case. The projectiles are different metal (zinc vs cast iron) and therefore differ in weight by about a pound. Also we use modern GOEX black powder granulations which were not used for CW artillery in the 1860's. This will also change the ballistics from the original data.

We have chronographed our loads at 1,250 fps and know the drop at 100 yards and 200 yards. With a reasonable BC estimate we can get a hack at the degrees elevation we'll need at further ranges by using a simple ballistic computer app.

Regards,
Badger

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Old March 6th, 2017, 11:08 PM   #6
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XXIV Corps, forum administrator, may be of some help??

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Old March 7th, 2017, 03:25 AM   #7
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The B.C. is an experimentally derived number, based on velocity drop over a fixed distance then compared to the velocity drop of a known projectile over the same distance.

Your projectile is not a full ogive forebody, comparison to a G1 projectile would not be very accurate in any case.

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Old March 7th, 2017, 04:15 AM   #8
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https://en.wikipedia.org/wiki/Ballistic_coefficient

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Old March 7th, 2017, 05:10 AM   #9
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Quote:
Originally Posted by POLACK View Post
i = Coefficient of form.

and

Cp = drag coefficient of the actual test projectile at range

These are only accurately found experimentally.


As to Wiki's calculation of the drag coefficient by the formula:

Cd = 8/[(density of projectile) x (velocity at range, squared) x (pi) x (diameter of projectile, squared)]

Note that that formula will give you the same drag coefficient for ALL bullets of the same average density and diameter. So, according to that formula, all copper jacketed , lead core .308 bullets have the same drag coefficient regardless of weight and shape.

Is that a reasonable assumption? Would you think a copper jacketed, wadcutter shaped, 130 grain .308 bullet going 2500 fps, would have the same drag coefficient as a .308, 175 grain Sierra Math King going the same speed?

Hardly, not even a good thumbnail estimate.


As to Wiki's formula for i

i = [2 / n] x square root of ([(4n) - 1] / n)

This has been a standard rule of thumb formula, It shows up in Hatcher's Notebook, and is a fairly good estimate, but only for tangent ogives. It breaks down with wadcutters and semi wadcutters, conical forebodies, (obviously, these don't have an ogive), secant ogives, etc.

According to this formula, a .308 147 grain, M80-type projectile has a B.C. of .560 . . . that's a good bit better than a 175 gr Sierra Match King. That formula gives .470 for the 175 Match King, much closer to the experimentally derived B.C.s given by Sierra (.505, .496, .485)

Aside from Sierra Match Kings, very few projectiles use tangent ogives.

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Last edited by lysander; March 7th, 2017 at 08:22 AM.
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Old March 7th, 2017, 05:17 AM   #10
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Since you already know the MV and actual 100 and 200 yard trajectory info, you should be able to 'reverse engineer' an approx BC by tinkering with a trajectory calculator's BC input to obtain the traj 100 and 200 yard 'drop' values with the 1250fps MV.

My guess is that BC and trajectory will be similar to a 22cal 40grain rimfire.

Jay Kosta
Endwell NY USA

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Old March 7th, 2017, 03:57 PM   #11
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Note that BC is usually for a supersonic projectile. You are starting out just barely supersonic, so will likely be subsonic for most of the trajectory.

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Old March 7th, 2017, 04:18 PM   #12
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I'm curious as to your sight in range and the drop between 100 and 200 yds in inches.

BTW that's a pretty impressive group in your first post. Nice shooting !!

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Old March 7th, 2017, 04:21 PM   #13
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Quote:
Originally Posted by JayKosta View Post
Since you already know the MV and actual 100 and 200 yard trajectory info, you should be able to 'reverse engineer' an approx BC by tinkering with a trajectory calculator's BC input to obtain the traj 100 and 200 yard 'drop' values with the 1250fps MV.

My guess is that BC and trajectory will be similar to a 22cal 40grain rimfire.

Jay Kosta
Endwell NY USA
I was thinking along these same lines. But it is also possible that you will end up putting numbers into a computer ballistic program that were never anticipated by the software designers, and it will output gibberish or just "error."

here's another idea. I have always found it's amazing what you can get from folks with a friendly phone call or email. Especially if you are talking to folks who are passionate about the same things you are.

You might try to get a name of someone in military ordnance, say at Aberdeen Proving Ground, some senior ballistics guy, and tell him what you are trying to do. There's a very good chance if you find the right person, that his eyes will light up, he'll say "oh wow, that's cool! I can help you figure that out..." and it will make his day.

Just out of curiosity, what are your drop figures for 100 and 200 with your modern load? Do you know the muzzle velocity of the original load or did they just publish drop charts?

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Old March 7th, 2017, 04:34 PM   #14
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BTW, just an off topic historical note... Ballistic programs are not some new idea, we have all these computers and someone decided to make a ballistics program. It's the opposite.

The computer was invented for one reason: to run ballistics programs. The first electronic digital computer was built for the DOD to calculate ballistics.

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Old March 7th, 2017, 04:47 PM   #15
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hey,i can help,but after counting to 10,i have to take my shoes off

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