Does velocity affect ballistic coefficient?

ButterBean

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Nobody has commented yet about MagnetoSpeed chronograph accuracy:

View attachment 294988

So at velocities around 3,000 fps, your chronograph could be as much as 15 fps lower or higher than the reading. At 70 degrees F, sea level, 2" high scope, 200-yard zero, 0.284 bullet diameter, 145 grains, and a G7 BC of 0.230, the Berger calculator says the difference will be about a half-inch of vertical at 500 yards. That's not your three inches, but it's something. Depending on your ballistics calculator's features, a variety of other minor environmental factors (e.g. the Magnus effect, whereby a crosswind out beyond 200 yards could pull your shot upwards) might contribute another inch or two that your calculator doesn't account for. Litz's book (cited above) spells them out. You might want to figure out how much of the three inches that surprised you came from the environment, and how much from the gun and load. I would also be careful (1) not to hastily attribute all three inches to a single effect, and (2) to distinguish adjustments for muzzle velocity from adjustments to BC assumptions. In his Modern Advancements in Long Range Shooting, Vol. II, Litz says (p. 227), "A concept that many long-range shooters are familiar with is truing or calibrating the ballistic solver. . . . This process is necessary . . . where shooters may not have good information on their bullet's muzzle velocity or BC. . . . It is possible to determine MV with reasonable accuracy based on observed drop. However shooting to determine BC or drag is a very different thing which is much more difficult to do accurately." If you want precise (+/- 1%), empirically determined, velocity-dependent BCs for a given bullet, you'll usually find them in Litz's book, Ballistic Performance of Rifle Bullets, 3rd Ed.
That all figures in but really has nothing to do with this thread, Are you kin to Mr. Litz or Mudrunner by chance ? and are you shooting any Hammer Bullets ?
 

MarshMello

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I loaded up a load for my 7 rum using 145 grain hammers. Average velocity of this round is 3635 FPS over 12-15 rounds fired over a magnetospeed. I zeroed the load at 200 yards and entered all the data into my ballistic calculator and went out to shoot steel and verify drops. I used a bc of .230 as indicated on hammers website, which they said was verified by drops, not estimated.

At 415 yards, I was about 1 1/4” high. Not too bad, so I backed off to 525 yards and was 3” high. When I got back home, I started making adjustments to the bc in the calculator. When I entered it as .320, it’s dead on with the results I saw on the steel plate.

I know my velocity numbers are good, as the SD of my load was single digits and the ES was around 13. I also measured the sight height, so that’s not giving me any issues either. The only thing “unknown” is the bc. Is it common to have to adjust the bc that far from the advertised bc? I know this is a pretty fast load, so maybe that has something to do with it? What are y’all’s thoughts?
Your plan to take it out to 800+ yards is a solid one. Make sure to zero it again in those conditions first to get the most accurate data. If you are prone especially, be aware of mirage. You'll find the BC adjustments will be most telling past 600. Don't discount mirage and wind as well. Mirage can make the target appear higher and cause you to hold higher. Wind can have some interesting effects on elevation as well at distance. I switched the 4DOF app and it's been nice to play with potential effects on elevation from wind. I read a comment somewhere in this string about stadia marks and hold overs..I agree this can simplify elevation adjustments however that doesn't fix bad data..if you don't know what your correct data it doesn't matter if you dial or reference stadia, you'll miss by the same amount, but probably a little faster if you're holding instead of dialing. Your 415 data is not to worrisome. That's 1/4 MOA off. Stick with measuring your variations in MOA, not inches. It's more representative for what you are doing. I would not sweat +/- 1/4 MOA as you play with your BC settings to get everything to line up.
 

Blackdirt Cowboy

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Black just so you know all of the Hammers do this when you wick them up, I shot the 101 HBO's out of the RUM last week, I zeroed at 100 and dialed what was called [email protected] 400 and it shot .5Mil high, as I said I have seen this with all the Hammers I have shot, Speed drastically changes BC and I use the ballistic app's as a guideline and that's it, there's no app that I have seen yet that will accurately calculate for the Hammers , It's been conclusive with our test group
That’s interesting. So do you think with adjustment to the BC in my calculator, it will hold true out to 1000 now? I know the only way to tell for sure is to shoot. But I only ask because I do t have a dirt backstop. I put my steel plates in front of hat bales and that makes it really difficult to spot misses. I also usually don’t have a spotter to help me see misses as well.
Your plan to take it out to 800+ yards is a solid one. Make sure to zero it again in those conditions first to get the most accurate data. If you are prone especially, be aware of mirage. You'll find the BC adjustments will be most telling past 600. Don't discount mirage and wind as well. Mirage can make the target appear higher and cause you to hold higher. Wind can have some interesting effects on elevation as well at distance. I switched the 4DOF app and it's been nice to play with potential effects on elevation from wind. I read a comment somewhere in this string about stadia marks and hold overs..I agree this can simplify elevation adjustments however that doesn't fix bad data..if you don't know what your correct data it doesn't matter if you dial or reference stadia, you'll miss by the same amount, but probably a little faster if you're holding instead of dialing. Your 415 data is not to worrisome. That's 1/4 MOA off. Stick with measuring your variations in MOA, not inches. It's more representative for what you are doing. I would not sweat +/- 1/4 MOA as you play with your BC settings to get everything to line up.
I agree, 1/4 moa isn’t something to get worked up about. But at 525 I was a little over .5 moa high. Realistically, I probably won’t shoot at game past those ranges, so it’s still a dead animal. But I’m always pushing for perfection. It will be interesting to see what happens at 800+ yards. Thanks for everyone’s help. This is really my first foray into long range shooting. It’s been a blast so far, and I’m happy with the results so far.
 

ButterBean

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That’s interesting. So do you think with adjustment to the BC in my calculator, it will hold true out to 1000 now? I know the only way to tell for sure is to shoot. But I only ask because I do t have a dirt backstop. I put my steel plates in front of hat bales and that makes it really difficult to spot misses. I also usually don’t have a spotter to help me see misses as well.

I agree, 1/4 moa isn’t something to get worked up about. But at 525 I was a little over .5 moa high. Realistically, I probably won’t shoot at game past those ranges, so it’s still a dead animal. But I’m always pushing for perfection. It will be interesting to see what happens at 800+ yards. Thanks for everyone’s help. This is really my first foray into long range shooting. It’s been a blast so far, and I’m happy with the results so far.
Yes Sir, you may have to tweak a bit as it will be slowing down considerably but your all over it
 

CapeEsperance

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I am now going to upset a lot of people with this next statement; "throw your calculator away". Get good scope with stadia lines and verify your range drops. Who has time to calculate and evaluate and estimate when hunting of at the range. I know several military trained snipers and most now use stadia lines to correct shot placement even before the first shot. Seems that the old ways come back around. (new military practice is to use ACOG, less chance of mistakes and less time) The new fads are just that, fads, learn how to use your MOA stadia lines to estimate yardage, correct shot placement and round adjustment of different lots. You will be happier and less frustrated. Complication causes doubt and doubt causes missed shots by low confidence. Do not overthink this, everything that can be done with the app can be done better by simple logic. I remember when I started seeing these things show up at ranges and some people would get angry at their poor shooting. One guy I was shooting next to was shooting a 300 WM if I remember correctly, I could tell he was getting pretty upset and I asked him what was wrong? He told me that he was off at least 36" at 800 yds. I asked him if he had stadia lines on his scope, he replied that he did. I asked him why he didn't just aim with that stadia that matched his drop? He started using this and I never saw him with an calculator again. Eventually I saw him take 4th at Whittington. He just shook my hand and thanked me. I don't think a person could have slapped the grin off of his face. Times like this are the ones we live for. Happy shooting.
Apologies, but I am new to this, Ehat is a “stadia line”
 

FEENIX

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Apologies, but I am new to this, Ehat is a “stadia line”

stadia_mark.jpg
 

jraulsten

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Copy that, It wont work with Hammers
I need to try some Hammers. So as not to get any more off track than this thread already is, please PM recommendations for a 300 PRC, 9 twist, using H1000 and N570. Need to get off the 215 Hybrid habit😄
 

Tommo64

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Not wanting to sound like too much of a simpleton here but you say hammer advertised .230 and you found .320 to be accurate. Could be as simple as a typo on their part. Punched in 23 instead of 32. Easy enough mistake to make….
.230 does seem to be a bit on the low side, especially for a 145gr mono bullet.
 

JTH

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What scope are you using and ballistics calculator?

BC is velocity dependent and all ballistic calculators need to be trued
Not looking to hijack the thread, but I think the answer on this might be helpful to the OP. I’m in the same boat, working on truing up strelock for a 131 hammer running over 3500 FPS, verified by lab radar. If you strictly adjust the BC to get strelock to show the actual adjustment required at 600 yards, will this bc hold true for calculating wind drift as well?
 

purpledragon

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  1. Uh, no.
  2. It's different when a published expert says it with engineering precision.
  3. I gave the audience a pointer to the technical literature, so they can learn more from a real expert if they want to.
Freedom of speech is a fine thing.
Published Expert, Wouldn't a fellow with hundreds of thousands of rounds fired and studied data, be an Expert as well??????? Just askin?? At 63yo, and shooting rifles since 5yo, and studying effects for 40+yrs, make someone kinda expert and not just some yayhoo?? Not upset,just asking a somewhat simple question.......
 

Veteran

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Here's a video from Rex that is maybe too elementary for some of you, but is very interesting as it presents the history of how G1 and G7 standards evolved since the 1870's. It's a very good basic explanation of what G1 and G7 are, where they came from and how they differ. He does go into in some detail on the same information presented by Brian Litz concerning BC variation with velocity.

I think its good supplemental background information. Rex is a bit of a mathematician I suspect. He enjoys showing the equations
and discussing the various drag functions, and reference bullets, and the history of the reference bullets.

Anybody ever seen a stuffed Teddy Bear shot out of a cannon? Its drag function is infinite.......:) Everything has a high BC in
comparison. I have heard some bullet makers have used the Teddy Bear as their reference bullet in the Form Factor.:)


 
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