How important is bc?

[Fitch]

So, two bullets, all things being equal, other than the boat tail. Time of flight to a thousand yards is .2 seconds different. How much wind drift difference does that make?

Steve

Not enough information - you need to specify the cross wind speed. For expediency I'll assume 10 mph at 90 degrees.

delta Wd = 12 * Ws * delta Tlag

delta Wd = difference in wind drift in inches
Ws = cross wind speed component in ft/sec
delta Tlag = time of flight difference

Given:

delta Tlag = 0.2 seconds
Ws = 14.667 ft/sec or 10 mph

Calculate drift:

delta Wd = 12 in/ft * 14.667 ft/sec * 0.2 sec = 35.2 in

You "really" need to get a copy of Brian's book and read it from cover to cover. Trust me on this, it isn't rocket science. The book makes it very clear.

Fitch

 

[Jon A]

Wind drift is less related to time of flight than it is to BC. If you try and make a direct correlation between TOF and wind drift you're just going to confuse yourself—because a big, slow, high BC bullet can have much less wind drift than a faster lower BC bullet—even if it has a longer time of flight.

It's not a matter of how long the bullet is in the air, it's a matter of how much effect the air has on the bullet while it is—which is in a sense exactly what BC measures for you. The higher the BC, the less affect the atmosphere has on the bullet; from slowing its velocity to pushing it sideways in the wind (or up or down for up/down drafts).

 

[Fitch]

Wind drift is less related to time of flight than it is to BC. If you try and make a direct correlation between TOF and wind drift you're just going to confuse yourself—because a big, slow, high BC bullet can have much less wind drift than a faster lower BC bullet—even if it has a longer time of flight.

It's not a matter of how long the bullet is in the air, it's a matter of how much effect the air has on the bullet while it is—which is in a sense exactly what BC measures for you. The higher the BC, the less affect the atmosphere has on the bullet; from slowing its velocity to pushing it sideways in the wind (or up or down for up/down drafts).

Wind drift is directly proportional to lag time. Lag time for a given bullet is defined as the difference between the flight time in a vacuum and the flight time in air to reach the distance of interest.

Very simply:

Wd = Ws * Tlag

Wd = wind drift in feet
Ws = cross wind component speed in ft/sec
Tlag = lag time in seconds as defined above.

This isn't complicated. Most ballistics programs will give time of flight in air. The time of flight in a vacuum is the distance in feet divided by the MV in ft/sec. For example, 300 feet /3,000 ft/sec = 0.1 second flight time to 100 yards in a vacuum for a bullet with 3,000 ft/sec muzzle velocity.

BC gets into the act because it represents the physical properties (weight, cross sectional area, and form factor) that cause the lag time. The way BC is defined a higher BC bullet will have shorter lag time and thus less wind drift. BC is a convenient one number index of how well the bullet will penetrate the air with out losing velocity.

Works for me anyway.

Fitch

 

[MontanaRifleman]

So a .5 to .7 bc change makes a 3.5 inch difference in POI at 1000yrds when there is an error of 3mph in the wind judgment. Not much for such a large difference in bc. And again we are talking about a shot that most of us long range hunters are not willing to take on a game animal. If I knew the wind was steady, no sweat. But that just isn't how it works.

It goes back to my original post. I'm not sure that it makes a big enough difference to worry 'much' about. More realistic difference, in bc, to look at is .1. Then the difference becomes much less at 1000 yrds.

Steve

Actually that 3.5" diff is at 800 yds. At 1000 yds it would probably be about 5" or more, and in any case it might mean the diff between a good hit or a bad hit or a miss. Your definition of a realistic diff might be .1, but when we're comparing comercially availble bullets to wildcats and customs it could easily be .2 or more. You just need to lean on GS to get some of those high BC HV's out

Repeat after me.....

BC IS EVERYTHING.... BC IS EVERYRTHING.... BC IS EVERYRTHING....

 

[LongBomber]

BC is everything... Wait a sec are we talking the province live in or... nevermind they are both everything...

 

[Sticker]

It can make a difference in you hold on range. I like to set my rifles to be at zero at 300 yards. High BC bullets may be 3 inchs high at 175 yards, lower BC bullets may be 5 inches high at 175 yards. It makes a difference on being right on the range if you are more than 350 yards. On the setting of 3 inches high , I missed an elk in Canada because I set my sight the at 2 inches high at a 100 yards, on my .280 Ackly, because I was planning on moose at close range. A guide offered me an elk opportunity and told me he was 300 yards. He was probably 400 yards. I was conditioned to the 300 yard setting so I held dead on. I hit him in the front leg running on the 3rd shot. If I had set at the accustomed 3 inch setting, I would have been 5 inches low instead of 16 inches low and I would not have lost the elk. With a lower BC bullet , I woud have missed either way because of an error in range estimation.

 

[Michael Eichele]

Wind drift is directly proportional to lag time. Lag time for a given bullet is defined as the difference between the flight time in a vacuum and the flight time in air to reach the distance of interest.

Very simply:

Wd = Ws * Tlag

Wd = wind drift in feet
Ws = cross wind component speed in ft/sec
Tlag = lag time in seconds as defined above.

This isn't complicated. Most ballistics programs will give time of flight in air. The time of flight in a vacuum is the distance in feet divided by the MV in ft/sec. For example, 300 feet /3,000 ft/sec = 0.1 second flight time to 100 yards in a vacuum for a bullet with 3,000 ft/sec muzzle velocity.

BC gets into the act because it represents the physical properties (weight, cross sectional area, and form factor) that cause the lag time. The way BC is defined a higher BC bullet will have shorter lag time and thus less wind drift. BC is a convenient one number index of how well the bullet will penetrate the air with out losing velocity.

Works for me anyway.

Fitch

You could get that info in the 4th edition of the Sierra reloading manual years ago. What is your point? We all know that windage is partly based on TOF however, TOF in and of itself is not what makes for wind drift. In other word, the shorter the TOF doesnt always equate to less drift.

I may be way off base here but it seems as if you dont agree with Jon A. Jon A is sopt on here. You are too.

Comparing time in a vacume versus time in the apmosphere is how you find wind drift in inches yes but does little if any to show you the effects of a high BC bullet at low velocity versus a low BC bullet at high velcoity. The example below shows that the lag time for both is nearly identical. What the lag time DOES NOT show you is how fast a bullet started and what it's BC was. It is simply 2 numbers one of which is subtracted from the other.

A bullet with a very short TOF at 1K may have a TOF of 1.2300 seconds where another may have a TOF of 1.5500 seconds. If the bullet that has 1.2300 second TOF is a 168 SMK at 3600 FPS at the muzzle and the bullet that has a 1.5500 second TOF is a 208 AMAX at 2600 FPS at the muzzle, guess which one will drift less. Well.......................... What is your guess?


Scroll down for the answer












Did you guess the 168 SMK?? If you did then you are wrong.



Even though the 168 got there 0.32 seconds faster at 1K, the 208 actually arrives with a couple less inches of drift despite exiting the muzzle at 1000 FPS less than the 168.

How can that be? Because the 'lag time' of each was near identical even though one bullet was very fast and one was very slow. That is the point. This is how a high BC bullet can have the same drift at slower speeds than a very fast low BC bullet. It is NOT the TOF that determines the wind drift rather how slowly or fast the bullet slowed down over the course of it's time in the air.

The reason low BC bullets at super high velocities drop less yet drift more than slow high BC bullets is because drop is a function of 2 principals. 1: Air drag and 2: gravity. The shorter the TOF, the further it will travel before hitting the ground which is VERY importand for a flat trajectory whereas with windage, sure TOF is a factor for windage but not near as much a factor as is for drop. Gravity is not a major component in windage like it is drop. The idea isnt to get a super short TOF for windage, the idea is to use a heavy bullet with a good form factor even if the TOF is not impresive. The high BC bullet WILL drift less than a low BC bullet even if they are 1000 FPS different on the muzzle end. Have you noticed that when you run the numbers using TOF and TOF in a vacume for your 'lag' time that niether factor in gravity?

 

[Fitch]

You could get that info in the 4th edition of the Sierra reloading manual years ago. What is your point? QUOTE]

I was answering a direct question.

Steve asked the question "So, does the boat tail help with wind deflection by reducing the time of flight? "

I answered "yes".

Then Steve asked

"So, two bullets, all things being equal, other than the boat tail. Time of flight to a thousand yards is .2 seconds different. How much wind drift difference does that make?

I estimated an answer to his direct question about time difference. He was looking for understanding. When people are looking for understanding it is usually best to answer their direct question. So I did that.

I understand BC. When I'm doing ballistics for my self I don't use time lag. I use QuickTarget Unlimited because it accepts data from QuickLoad and I can enter the G7 BC data from Litz's book in it if that is appropriate for the bullet.

BTW: If it matters, the information I gave on time lag happens to be correct. If you look in Litz's book, Chapter 5, you will find wind drift explained in terms of lag time. The delta Tlag equation I used in the difference reply was derrived from his equations on page 68.

Fitch

 

[Mikecr]

Michael, I believe you're muddling up TOF -vs- LAG within it.
The 168smk may have arrived earlier, but it lagged more. That is, there was a larger differential between it's potential vac arrival(w/resp to MV), and it's actual arrival.
There would be less lag with the big AMAX regardless of MV, because it's velocity drops off slower than the 168.

And with drop, it's just the rate of gravity over a bullet's TOF. Air density plays no role in it, other than to change that TOF, by influencing BC, which causes LAG, and allows increasing drift at the same time.

BC works equally well for both drift and drop, all by itself.
Otherwise, we would have to invoke two BCs. One for drop, and one for drift.

 

[RockyMtnMT]

Back to the boat tails. Does the long hunter that works out to 1000 yrds really benefit from the boat tail? I know that it is a very general question, but I am not convinced that it is not more of a sales gimmick than a benefit. It's not like we have a lot of choice, they pretty much all have the boat tail, unless you look at short range bullets.

I think, but I'm not sure, that we all agree that a flat base bullet is inherently more accurate. Do we gain enough from the boat tail to off set the accuracy?

The nose design is the most responsible for how the bullet performs in flight. Am I wrong? As the bullet slows the boat tail becomes more important?

Steve

PS I have very much enjoyed the discussion.

 

[Michael Eichele]

Michael, I believe you're muddling up TOF -vs- LAG within it.
The 168smk may have arrived earlier, but it lagged more. That is, there was a larger differential between it's potential vac arrival(w/resp to MV), and it's actual arrival.
There would be less lag with the big AMAX regardless of MV, because it's velocity drops off slower than the 168.

And with drop, it's just the rate of gravity over a bullet's TOF. Air density plays no role in it, other than to change that TOF, by influencing BC, which causes LAG, and allows increasing drift at the same time.

BC works equally well for both drift and drop, all by itself.
Otherwise, we would have to invoke two BCs. One for drop, and one for drift.

Sorry to confuse you Mike. I didnt get TOF confused with the lag within it. My words may not have sounded clear but I get it. I was just pointing out that it is not TOF that is responsible for windage rather the lag within it. That is, how fast or slow the bullet looses its velocity.

Sorry, I dont agree with you that gravity is the sole function of drop. Drop is directly related to TOF and TOF is directly related to air density. You dont have to invoke 2 BC's for wind and drop. That is why the calculations for drop include gravity as a factor. If there were no air density, the trajectory would be purely parobolic. In other words, the highest point would always be at the exact center of the arc. In the world we live in the highest point in the trajectory is closer to the target than we are. Also, that is why we cannot simply take the angle of a shot's cosine and multiply it by the yardage and aim for the corrected yardage. In a vacume this would work. In our air it does not work. Why? Because gravity is NOT the only influence on the bullet. If gravity was the only influence that simple system would work just fine yet we multiply the cosine by the drop from borline. Why? Because drop from boreline reflects gravity and air density. So yes, air density absolutely plays a role in bullet drop as well as wind.

 

[Michael Eichele]

You could get that info in the 4th edition of the Sierra reloading manual years ago. What is your point? QUOTE]

I was answering a direct question.

Steve asked the question "So, does the boat tail help with wind deflection by reducing the time of flight? "

I answered "yes".

Then Steve asked

"So, two bullets, all things being equal, other than the boat tail. Time of flight to a thousand yards is .2 seconds different. How much wind drift difference does that make?

I estimated an answer to his direct question about time difference. He was looking for understanding. When people are looking for understanding it is usually best to answer their direct question. So I did that.

I understand BC. When I'm doing ballistics for my self I don't use time lag. I use QuickTarget Unlimited because it accepts data from QuickLoad and I can enter the G7 BC data from Litz's book in it if that is appropriate for the bullet.

BTW: If it matters, the information I gave on time lag happens to be correct. If you look in Litz's book, Chapter 5, you will find wind drift explained in terms of lag time. The delta Tlag equation I used in the difference reply was derrived from his equations on page 68.

Fitch

Below is the quote you replied to in which invoked my reply. FWIW, I realize your numbers are correct. I dont need to look at Litz's book to know you were correct. That was never the point. I was not calling your numbers into question. I have written a couple of different ballistic calculators. Trust me, I understand lag time as ALL of my windage factors are calculated based soley on them. If you read my first post again, you can deduce that my comments were meant to agree with you even if it was from a different angle.

Wind drift is less related to time of flight than it is to BC. If you try and make a direct correlation between TOF and wind drift you're just going to confuse yourself—because a big, slow, high BC bullet can have much less wind drift than a faster lower BC bullet—even if it has a longer time of flight.

It's not a matter of how long the bullet is in the air, it's a matter of how much effect the air has on the bullet while it is—which is in a sense exactly what BC measures for you. The higher the BC, the less affect the atmosphere has on the bullet; from slowing its velocity to pushing it sideways in the wind (or up or down for up/down drafts).

Sorry if I mussunderstood you. I was refering to your response to Jon A and not Steve. It appeared that you were dissagreeing with him and felt that he was spot on. I cant say you were wrong as you were not. I just thought you missunderstood what he stated.

M

 

[Mikecr]

Boat tails are not a hoax, and base drag is a significant part of total drag.

Flat base bullets ARE more accurate.
But at some point/range drift becomes far more difficult to manage than accuracy. This is currently the condition 1Kyd competitors are in, even if they completely deny it. They are all, every single one, shooting heavy per cal bullets(most are boat tails), even though these bullets will not shoot as accurate(as demonstrated closer) as lighter flat base bullets could..
Yet they continually claim 'accuracy' is their top priority... No,,, their setups demonstrate BC first, -followed by accuracy. They get the highest BC bullet shooting as accurately as they can.
Otherwise, they'd be shooting 6PPCs/30BRs at 1kyd.
Oh but those deluding themselves using the recent mid-range mighty mouse cartridges for 1kyd? Well that wheel just keeps going roundy roundy. Their results will be as inconsistant as all who have tried the same in the past.
This is because the devil's in the wind.

You just cannot deny the importance of BC at long distances.

 

[MontanaRifleman]

Steve,

I cant argue whether or not flat base bullets are more accurate than boattails. I dont have enough experience to make a conclusion but I have read a number of opinions that they are so I'll accept it. Question is, how much more accurate are they? Boattailis are apparently accurate enough for 1K plus hunting so I think accuracy is a moot point. And boatail bullets, having a better BC, are less affected by enviromental conditions making them potentially more accurate in a sense at longer ranges.