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How important is bc?

 
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  #15  
Old 02-01-2010, 09:48 AM
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Re: How important is bc?

Quote:
Originally Posted by bsl135 View Post
+1, well said.

So far most answers have dealt with BC as it relates to wind deflection, and in particular, the benefit of having a high BC. I'd like to answer the question from a slightly different perspective. Consider the importance of knowing an accurate value for the BC. Everyone's assuming that they can range the target and get their vertical POI on the mark, but to do that you need to know an accurate BC. In this context, the lower the BC is and the slower the MV, the more important it is that you know the BC with great accuracy. High BC bullets with high MV are less sensitive to computational errors.

As an example, consider a .308 Winchester shooting a 155 grain bullet, G7 BC of .230 at 2900 fps. If you have +10% of error in the BC (.253 vs .230) when you run your calculations, at 600 yards that results in 3.4" error in drop. At 1000 yards, the error is 25.8" difference in drop.
For a 300 grain .338 bullet with a G7 BC of .375 at 3000 fps, the same +10% error in BC (.413 vs .375) results in only 1.3" error in drop at 600 and 9.1" error in drop at 1000 yards.

My point is that it's more important to have an accurate BC for the lower performing bullets than the higher performing bullets. In the case of the 300 grain .338 at 3000 fps, it would take about 38% of error in BC to cause the same error in predicted drop at 1000 yards as the 10% error for the 155 grain .308 at 3000 fps MV.

On a different note...
1) Boat tails are effective at reducing base drag at all speeds (supersonic and subsonic).
2) They are effective inside 300 yards, but the ballistic performance advantage is quite small compared to a flat base that many consider the advantage to be negligible, especially if the flat base bullet can be made to shoot more precisely which is often the case.
3) They can be bad for stability in the transonic zone.

-Bryan
Bryan,

Thanks for joining in. Can you talk more about the boat tail. It was explained to me years ago that the boat tail has more to do with the shock wave of the bullet in flight that the air friction. As the bullets slows down the shock wave moves back toward the tail of the bullet extending the maximum range of the projectile. So in a war situation the side with boat tails doesn't have to get as close to the enemy to be effective.

Thanks,

Steve
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  #16  
Old 02-01-2010, 10:00 AM
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Re: How important is bc?

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Originally Posted by RockyMtnMT View Post
So in a war situation the side with boat tails doesn't have to get as close to the enemy to be effective.
This is true, but not for the reason you described.

The boat tail isn't there to affect the shock wave, or skin friction. It's there to reduce the third component of projectile drag: base drag.

If the airflow separates off the bearing surface as in the case of a flat base, then the low pressure area (suction) behind the bullet applies to the full cross-sectional area of the bullet. However, with a boat tail, the airflow separates off a reduced diameter, so the low pressure applies to a smaller area, thus reducing base drag.

The mechanism described above applies pretty much the same at supersonic, transonic and subsonic speeds. The effect is reduced drag, which extends range. Boat tails aren't only effective at the end of the trajectory, they're effective for the entire trajectory, which extends the range.

-Bryan
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  #17  
Old 02-01-2010, 10:25 AM
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Re: How important is bc?

Thanks Bryan,

I knew I was missing something. Is there any correlation between the boat tail and wind drift? If I am correct in my thinking, the boat tail only affects the drop. Which is very predictable. How much gain in drop, or range, is there by simply adding the boat tail?

Steve
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  #18  
Old 02-01-2010, 10:29 AM
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Re: How important is bc?

The boat tail increases BC, which reduces both drop and wind deflection.

How much of an effect the boat tail has depends on it's length and angle. Also remember that the boat tail reduces the bearing surface length compared to a similar weight flat base bullet. The reduction in in-bore friction can mean higher MV.

-Bryan
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  #19  
Old 02-01-2010, 10:50 AM
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Re: How important is bc?

Quote:
Originally Posted by bsl135 View Post
+1, well said.

So far most answers have dealt with BC as it relates to wind deflection, and in particular, the benefit of having a high BC. I'd like to answer the question from a slightly different perspective. Consider the importance of knowing an accurate value for the BC. Everyone's assuming that they can range the target and get their vertical POI on the mark, but to do that you need to know an accurate BC. In this context, the lower the BC is and the slower the MV, the more important it is that you know the BC with great accuracy. High BC bullets with high MV are less sensitive to computational errors.

As an example, consider a .308 Winchester shooting a 155 grain bullet, G7 BC of .230 at 2900 fps. If you have +10% of error in the BC (.253 vs .230) when you run your calculations, at 600 yards that results in 3.4" error in drop. At 1000 yards, the error is 25.8" difference in drop.
For a 300 grain .338 bullet with a G7 BC of .375 at 3000 fps, the same +10% error in BC (.413 vs .375) results in only 1.3" error in drop at 600 and 9.1" error in drop at 1000 yards.

My point is that it's more important to have an accurate BC for the lower performing bullets than the higher performing bullets. In the case of the 300 grain .338 at 3000 fps, it would take about 38% of error in BC to cause the same error in predicted drop at 1000 yards as the 10% error for the 155 grain .308 at 3000 fps MV.

On a different note...
1) Boat tails are effective at reducing base drag at all speeds (supersonic and subsonic).
2) They are effective inside 300 yards, but the ballistic performance advantage is quite small compared to a flat base that many consider the advantage to be negligible, especially if the flat base bullet can be made to shoot more precisely which is often the case.
3) They can be bad for stability in the transonic zone.

-Bryan
Some very good posts and you can always learn something new. I knew that boattails had SOME affect at closer distances, but it seems that it is more than I thought. Thanks for the correction guys..........Rich
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  #20  
Old 02-01-2010, 11:50 AM
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Re: How important is bc?

Quote:
Originally Posted by bsl135 View Post
+1, well said.

So far most answers have dealt with BC as it relates to wind deflection, and in particular, the benefit of having a high BC. I'd like to answer the question from a slightly different perspective. Consider the importance of knowing an accurate value for the BC. Everyone's assuming that they can range the target and get their vertical POI on the mark, but to do that you need to know an accurate BC. In this context, the lower the BC is and the slower the MV, the more important it is that you know the BC with great accuracy. High BC bullets with high MV are less sensitive to computational errors.

As an example, consider a .308 Winchester shooting a 155 grain bullet, G7 BC of .230 at 2900 fps. If you have +10% of error in the BC (.253 vs .230) when you run your calculations, at 600 yards that results in 3.4" error in drop. At 1000 yards, the error is 25.8" difference in drop.
For a 300 grain .338 bullet with a G7 BC of .375 at 3000 fps, the same +10% error in BC (.413 vs .375) results in only 1.3" error in drop at 600 and 9.1" error in drop at 1000 yards.

My point is that it's more important to have an accurate BC for the lower performing bullets than the higher performing bullets. In the case of the 300 grain .338 at 3000 fps, it would take about 38% of error in BC to cause the same error in predicted drop at 1000 yards as the 10% error for the 155 grain .308 at 3000 fps MV.



-Bryan
This is exactly why I run my loads over two chronies and drop tests. I rarely find the real BC to be in line with the published BC but it does happen on rare occasions. However, I ussually find them to be somewhat close. I use .490 for the 168 AMAX, .514 for the 178 AMAX, .523 for the 180 ACCUBOND, .671 for the 208. At least out of my rifles.
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  #21  
Old 02-01-2010, 12:14 PM
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Re: How important is bc?

Michael,

What distance do you test your BC's over two chronies?

-Bryan
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