How important is bc?

[RockyMtnMT]

+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

 

[BryanLitz]

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

 

[RockyMtnMT]

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

 

[BryanLitz]

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

 

[elkaholic]

+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

 

[Michael Eichele]

+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.

 

[BryanLitz]

Michael,

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

-Bryan

 

[Jon A]

I'd like to answer the question from a slightly different perspective.

Good stuff. I'll also add another perspective into the mix people don't think about much (but should). Higher BC bullets are less sensitive to changes in weather.

Much like wind drift, for any given temp or pressure change, the error will be less with the higher BC bullet. Or looked at another way, the higher BC the bullet you use and the faster you shoot it the farther you can go with simple drop charts before needing to worry about weather changes.

If using a PDA for every shot it's not that big a deal, but understanding when you actually need to use a PDA and when you don't is something people should know. That distance being farther increases the odds when speed is needed in the field.

 

[MontanaRifleman]

You are correct. Can you shoot a one shot kill at 1100 yards in a 10 mph variable wind with any bullet? Be honest.

Yup, I sure can - do it all the time OK, serious.... no, I would not take a 1000 shot on a game animal in a 10 mph wind (except maybe a speed goat). If I knew for sure it was a 10 mph wind, then yes I would because it could be accurately corrected for. It's not the amount of wind, it's the ability to accurately dope the wind. Which, as already mentioned, is where a higher BC helps to enlarge the degree of forgiveness. If I was antelope hunting @ 4000' elevation, with previously mentioned .5 vs .7 BC loads and had an 800 yd shot in an estimated 10 mph wind, a 3 mph estimation error would result in a 7" POI error for the .7 BC vs a 10 1/2" POI error for the .5 BC load. And yes, there is a big diff between .5 and .7 BC, bit that's what LR is all about. That's why all the interest in the high BC wildcat and custom bullets. That's why LR shooters are willing to pay up to 4x as much for them as standard bullets. I better stop there

Another big factor is that each increase of .5 BC gains about 100 yds effective range. Going from .5 BC to .7 BC in the above example takes you from 1000 yds to 1400 yds. I think that's very significant. And here again is where individual goals and priorites come in. Some guys just dont care to shoot past 600 or 800 or whatever. Other guys want to shoot as far as they can. To the later, BC is extremely important.

 

[elkaholic]

Yup, I sure can - do it all the time OK, serious.... no, I would not take a 1000 shot on a game animal in a 10 mph wind (except maybe a speed goat). If I knew for sure it was a 10 mph wind, then yes I would because it could be accurately corrected for. It's not the amount of wind, it's the ability to accurately dope the wind. Which, as already mentioned, is where a higher BC helps to enlarge the degree of forgiveness. If I was antelope hunting @ 4000' elevation, with previously mentioned .5 vs .7 BC loads and had an 800 yd shot in an estimated 10 mph wind, a 3 mph estimation error would result in a 7" POI error for the .7 BC vs a 10 1/2" POI error for the .5 BC load. And yes, there is a big diff between .5 and .7 BC, bit that's what LR is all about. That's why all the interest in the high BC wildcat and custom bullets. That's why LR shooters are willing to pay up to 4x as much for them as standard bullets. I better stop there

Another big factor is that each increase of .5 BC gains about 100 yds effective range. Going from .5 BC to .7 BC in the above example takes you from 1000 yds to 1400 yds. I think that's very significant. And here again is where individual goals and priorites come in. Some guys just dont care to shoot past 600 or 800 or whatever. Other guys want to shoot as far as they can. To the later, BC is extremely important.

AMEN!!

 

[RockyMtnMT]

Yup, I sure can - do it all the time OK, serious.... no, I would not take a 1000 shot on a game animal in a 10 mph wind (except maybe a speed goat). If I knew for sure it was a 10 mph wind, then yes I would because it could be accurately corrected for. It's not the amount of wind, it's the ability to accurately dope the wind. Which, as already mentioned, is where a higher BC helps to enlarge the degree of forgiveness. If I was antelope hunting @ 4000' elevation, with previously mentioned .5 vs .7 BC loads and had an 800 yd shot in an estimated 10 mph wind, a 3 mph estimation error would result in a 7" POI error for the .7 BC vs a 10 1/2" POI error for the .5 BC load. And yes, there is a big diff between .5 and .7 BC, bit that's what LR is all about. That's why all the interest in the high BC wildcat and custom bullets. That's why LR shooters are willing to pay up to 4x as much for them as standard bullets. I better stop there

Another big factor is that each increase of .5 BC gains about 100 yds effective range. Going from .5 BC to .7 BC in the above example takes you from 1000 yds to 1400 yds. I think that's very significant. And here again is where individual goals and priorites come in. Some guys just dont care to shoot past 600 or 800 or whatever. Other guys want to shoot as far as they can. To the later, BC is extremely important.

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

 

[RockyMtnMT]

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

So, does the boat tail help with wind deflection by reducing the time of flight? I apologize for the simpleton questions, I'm a little thick in the head. I'm having trouble visualizing how a boat tail reduces wind drift.

Thanks for your patience,
Steve

 

[Fitch]

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

Yes.

Fitch

 

[RockyMtnMT]

Yes.

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