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How important is bc?

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  #29  
Unread 02-01-2010, 08:53 PM
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Re: How important is bc?

Quote:
Originally Posted by RockyMtnMT View Post
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
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  #30  
Unread 02-01-2010, 09:03 PM
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Re: How important is bc?

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).
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  #31  
Unread 02-01-2010, 09:56 PM
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Re: How important is bc?

Quote:
Originally Posted by Jon A View Post
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
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  #32  
Unread 02-01-2010, 10:19 PM
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Re: How important is bc?

[QUOTE]
Quote:
Originally Posted by RockyMtnMT View Post
Quote:
Originally Posted by MontanaRifleman View Post

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....
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  #33  
Unread 02-01-2010, 10:50 PM
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Re: How important is bc?

BC is everything... Wait a sec are we talking the province live in or... nevermind they are both everything...
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  #34  
Unread 02-02-2010, 12:01 AM
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Re: How important is bc?

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.
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  #35  
Unread 02-02-2010, 12:55 AM
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Re: How important is bc?

Quote:
Originally Posted by Fitch View Post
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?
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Last edited by Michael Eichele; 02-02-2010 at 02:45 AM.
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