Bullet Hardness & Ballistic Coefficient

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Paul Pollard posted this 05 June 2019

Here's some information from a few days of collecting data. This is my first look at bullet hardness and relation to ballistic coefficient. The sheet lead heat treat didn't take, but decided to shoot a few to see if I really needed harder bullets. The 6 to 8 inch groups said that something harder than soft lead was needed. With sheet lead at 30 clicks and a BC of 0.198, it looked pretty bad. At 32 clicks the BC degraded the even more to 0.177.

It's too soon to say, but the lube may make a difference with equal powder and different lubes. The attached table may make this as clear as mud.

BC Table

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John Alexander posted this 06 June 2019

Paul,

Very interesting.  Two questions.

How are your measuring the BC?

What do you think the physical mechanism is connecting bullet hardness to BC?  Distortion of soft bullet changing shape to less efficient shape?  Distortion or lower velocity causing excessive yaw?  Other?

John

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Paul Pollard posted this 06 June 2019

John,

1. The Labradar chronograph tracks the bullets and records velocities reliably to 60 yards, sometimes to 80 or 100 yards. By using the muzzle velocity and the 60 yard velocity, those values are plugged into the JBM calculator for B.C. using the velocity choice. The numbers have varied somewhat, maybe because of the alignment on the target. The higher numbers, above, were lower than those taken in April. In the April tests, the BC recorded a high of .247 and a low of .237. These were with LinoWW alloy. Two of the groups were not heat treated; the other 18 groups were heat treated. The numbers were higher at first, then declined a little. That's what make me wonder about alignment. The two un-heat treated groups were the worst of the day, but the first group had a BC of .245; the second non-heat treated group had a BC of .231 (lowest of the day).

2. If the bullets are stripping or slipping or whatever they do, the soft ones are probably unstable and have a low BC. It could be they get bent during the shooting. There was lots of lead scraped out to the muzzle when cleaning. I could have put it back in the pot to remelt. The velocity on the sheet lead was 2017 fps for one group. The second one was 2117 fps which really degraded the BC and accuracy.

3. The original reason for getting the Labradar was to see if stability of the bullet could be determined by the higher BC. It's easier just to look at the target and see if they're stable.

4. I plan to keep collecting data and trying to see what it means.

Paul

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Ross Smith posted this 06 June 2019

Paul: Did you experience leading from the softer bullets?

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Paul Pollard posted this 07 June 2019

Yes. See 2. Above. There was bunch of leading.

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tedly posted this 09 June 2019

I'm kinda puzzled here.

According to what I recall from Hatcher's Notebook, the BC is just the sectional density, multip[lied by a bugger factor to account for the bullets form.

Hardness should have no effect, since it doesn't affect the form nor the bullet's weight. Notice the change from 18 to 25 BHN, which doesn't appear to be significant. However, lead and Lino have different densities, which would affect the sectional density of the bullet, and therefore, the BC.

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John Alexander posted this 09 June 2019

tedly wrote:

"Hardness should have no effect, since it doesn't affect the form nor the bullet's weight. Notice the change from 18 to 25 BHN, which doesn't appear to be significant. However, lead and Lino have different densities, which would affect the sectional density of the bullet, and therefore, the BC."

=================

Excellent point that softness itself shouldn't affect BC directly but the different densities would affect SD and thus BC.  However the softer alloys would have the higher densities and thus should have higher BC, not lower.

John

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Larry Gibson posted this 09 June 2019

The Ballistic Coefficient is a form factor describing the bullets efficiency and moving through air.  It is derived by determining the TOF (Time Of Flight) between the muzzle and some distance. Several pieces of information must be entered into a formula to correctly determine the BC.  There are also several different form factors for different bullets used.  The old Hatcher/Dupont charts get you close......sometimes.

 

If you can actually measure the BC of each shot (as the OP is doing) you can indeed determine the stability of a bullet.  The higher the BC given the same load and test firearm the more stable the bullet is.  The OPs BC results also indicates, with cast bullets, that something is happening to the cast bullet causing a lower BC.

What is indicated by the lower BC of the lower BHN bullets is that some deformation during acceleration in the barrel is occurring. I brought this topic (the measurement of the BC to observe the degree of stability) up during HV cast bullet testing over the last 8-9 years.  Some choose not to believe it but the actual results prove it as fact.  The OP is demonstrating that the harder BHN bullets are withstanding the acceleration better and holding theiir form....the result is they have, given the same load, the higher BC.

LMG

Concealment is not cover.........

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Ken Campbell Iowa posted this 10 June 2019

...   don't forget that harder castings are also lighter ... therefore the difference detected should be modified by some factor allowing for density.  

that is >> the harder, lighter bullets would have a smaller bc even if not distorted.

************

edit :  oh, i have already received mail from my english friends imploring me to " bugger off " .. anybody know what that means ?

ken

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Paul Pollard posted this 10 June 2019

Ken said, ... don't forget that harder castings are also lighter ... therefore the difference detected should be modified by some factor allowing for density.

that is >> the harder, lighter bullets would have a smaller bc even if not distorted.

Ken...If these were monotype and Linotype bullets, the weights would be lighter for the harder bullets. These bullets are the SAME alloy. The 18 BHN are not heat treated. The 25 BHN ARE heat treated.

The sheet lead bullets are 3 grains heavier, but the heat treat didn’t take. The sheet lead, if hardened, should have been the densest and therefore, the highest BC. Since they didn’t harden, they provided a test for too-soft bullets.

Thank you, Larry.

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tedly posted this 10 June 2019

Ken and John got it right, I had got my mass and BC's backwards. Antimony expands on cooling, and harder bullets from the same mold will get lighter as the antimony content gets higher.

The Ballistic Coefficient is a correction (bugger factor, Finagle's constant) to compare a bullet's trajectory to that of a standard projectile used in firing trials. It gets bigger with increasing length, and with mass for diameter (these two are a consequence of each other, given solid bullets). For a given twist, a longer, higher BC bullet is less stable. For a given load, a longer bullet will travel slower; I don't know if that makes it more or less stable. (Just went to JBM calculator - it's less stable.)

The ballistic equations will give you drop, time of flight, and change in velocity from tables, or on-line calculators. If you measure any of these you can determine the BC that will 'correct' your trajectory to the measured (or modeled) one,

I went at this from the back door; my 45-70 was sighted at 200 metres (we call it 40 rods ) and due to rain, and curmudgeonliness I shot it at 50 and 100 yards. I could then go to JBMs external ballistics, and found my trajectory and velocity most closely matched a BC of .4 . Then I calculated my sight settings, and found that my 3" tall tang sight should get me out to 1000 yds, and I'm only going to 800 so I should have some sight stem in hand.

Paul, your method is really ingenious and interesting, you're wringing out that Labradar. I can't find my copy of Veral Smith's book, but I'm sure he annealed, as well as hardened WW (antimony, arsenic, and tin lead alloys). That would give bullets of the same density from BHN 6-9 right up to 25 or more. Your method would be capable of sorting this out, where measuring drop lacks precision.(At  least with the groups I can shoot, it does.)

 

 

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frnkeore posted this 11 June 2019

TOF, is misleading, regarding BC. If you increase the starting velocity (MV) of a bullet, it will always, decrease the TOF but, can in some cases, decrease BC. You can see that in Lymans BC charts.

It is the lose of velocity, between the muzzle and target, that dictates the BC of a bullet. For instance, if the MV is 2000 fps on all shots and the 100 yd vel is 1800 fps (as a base), if on the next shot your 100 yd vel is 1810 fps, the BC is higher and the following shot, at 1790 fps, that BC is lower.

A bullet at 1000 fps, can have a much higher BC than a bullet starting at 2000 fps but, the 2000 fps bullet, will always have a shorter TOF.

In addition, a higher BC will deliver you, less wind drift, no matter the velocity! Those that try to increase the velocity of a cast bullet, thinking that it will give them a advantage at the target, are wrong. The thing that gives us one of the biggest challenges is wind drift and that increases with range. Paul is right to study this, as that is the key to reliability shot to shot.

Increasing BC and decreasing BC variability, is one of the best ways to increase accuracy.

Frank

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joeb33050 posted this 11 June 2019

The most fascinating thing that I know about the ballistic coefficient is that with bullets of the same shape, BC varies with length. Only the length dimension. A 1" 17 cal bullet has the same BC as a 45 cal bullet 1" long. All same-shape bullets of a given length have the same BC.

(BC also varies with MV and delta V, of course.)

Softer bullets probably change shape, ?and length? on firing, so BC changes.

Zounds!

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frnkeore posted this 11 June 2019

BC is a product of sectional density / Form Factor. The actual form factor can vary with velocity. Depending on sub vs supersonic and the diameter of the meplat.

Designed form factors, need to be tested against actual form factor but, the SD remains constant.

Frank

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Larry Gibson posted this 12 June 2019

The point the OP is making is if the TOF, for a given load is slower then the BC is lower.  Ergo, given the same bullet of the same alloy but with different BHNs something must be adversely affecting the bullets form as evidence by the lower BC.

All bullets that are the same exact shape will have ballistic coefficients with respect to that standard projectile that are simply equal to their sectional densities. Other, more or less aerodynamic shapes, will have a form factor (that drag coefficient ratio) to correct them for the fact they fly farther or shorter than sectional density alone would indicate in comparison to a standard projectile.

LMG

Concealment is not cover.........

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joeb33050 posted this 12 June 2019

The point the OP is making is if the TOF, for a given load is slower then the BC is lower.  Ergo, given the same bullet of the same alloy but with different BHNs something must be adversely affecting the bullets form as evidence by the lower BC.

LMG

 

BC is a function of form and SD. SD cannot change on firing. Form can.

There's 2 choices, change in form or change in sectional density/length. But wait! Form can't change without SD/L change, and SD/L can't change without form change.

Then shooting softer bullets shortens them AND changes form. The polar case is, the bullet turns into a cylinder with max SD/L and crappy form. 

Don't BP guys count on this bumping happening? Civil War soldiers, Minie Ball? Bore-size soft bullets?

 

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Paul Pollard posted this 13 June 2019

I don't have any experience with black powder. The bullets I'm using are not minie balls or soft bore-size bullets. They were soft throat-size bullets (by mistake).

The soft bullets caused leading which is a form of fouling. Can powder fouling cause lower BC? If not, why do we clean the barrel? Can too much lube cause a loss of accuracy and lower BC? 

I'll keep testing and track the results.

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