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How important is bc?

 
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  #8  
Old 02-01-2010, 04:23 AM
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Re: How important is bc?

I don't know, I think if you asked 10 people here a good 9 out of the 10 would agree. Or at least they would have in years past. It seems there are many new members here intent on learning all the same old lessons the hard way all over again for themselves.

The single biggest obstacle to hitting at long range is the wind. The simple comparisons above show one load drifting 50%-60% more than another...and the answer isn't obvious? The most likely reason for a miss is you misjudged the wind. For any particular misjudgment, this means you miss your point of aim by 50%-60% more. Turning a 6" miss into a 10" miss. Turning a 9" miss into a 14" miss. That's enough to put the bullet out of the chest and into the guts. That's enough to turn a double shoulder shot into a miss.

Maybe some are so good at judging the wind they can purposely put themselves at such a disadantage because their excess talent will make up for it. I know I'm not one of them. I also try not to shoot blindfolded or with one arm tied behind my back and I don't shoot myself in the foot before taking a shot.....

For those who say they can't shoot in the wind anyway so they don't even try...I guess they can convince themselves of anything. I don't think I've ever shot really long range where there was no wind at all. If you look closely enough, there's always a couple mph this way or that. Saying shooting in the wind is hard so it doesn't matter if I don't try is a copout that will limit success.

Whatever the conditions, with a load that drifts a whole bunch less in the wind, you'll be accurate with it to a longer range. Whatever the range, with a load that drifts a whole bunch less in the wind, you'll be able to be accurate with it in much worse wind conditions.
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  #9  
Old 02-01-2010, 07:09 AM
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Re: How important is bc?

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Originally Posted by elkaholic View Post
Actually, a boattail doesn't make that much difference until you reach subsonic which is WAY out there, depending on B.C. of course.....
Rich
I'm new at this so I'm trying to learn. Reading everything I can find. Trying to follow along here and understand the discussion. Litz's book is the best reference I've found so far. Looking in the book, I'm not so sure that's true. I'm sitting here looking at the graph, figure 2.7 on page 23, in Litz's book. It compares the drag for the G1, G7, and a modern long range bullet. G1 has higher drag everyplace.

Near as I can tell, a boat tail won't matter much at less than 300 yards for sure. But as the range gets longer it makes more and more difference. Especially with bullets on the lighter end of the spectrum where a small gain in BC is a big percentage.

Surprisingly, there is also data in the book that shows some very low drag bullets don't do well subsonic - they get rather unstable. I think one could say flat base bullets actually do better subsonic than some low drag bullets because they are more stable (center of pressure is less aft of center of mass compared to long nosed VLD type bullets).

That said, 1800 fps, which is the stated minimum expansion velocity for quite a few modern bullets, is supersonic so subsonic performance is academic since they shouldn't be used at ranges where they will be subsonic for terminal performance reasons. That makes the trade on the side of higher BC vs subsonic instability a good one since subsonic really doesn't matter if the bullet won't do the job after impact.

I think three things (not in any particular order - I see them as a chain, not a pyramic) matter quite a bit when it comes to hitting game at long range:

One is at what range does the bullet drop below it's effective expansion velocity? More BC extends that range.

The most important factor relative to making a good hit, when hunting is wind drift. Range can be measured, wind is considerably more random and measuring it accurately over the distance of a long range to target across verying terrain is a practical impossibility. More BC reduces wind drift which increases the hit probability - which tells me that all other things being equal, picking the bullet with higher BC is better for hit probability.

Then there is what the bullet does after it gets there. I don't really understand the trade off between terminal performance and BC very well. It seems to me this is an important thing to understand. I'm liking how low drag bullets shoot, I need to understand more about their terminal performance. At the moment my practice is to respect the stated minimum velocity but I'm not sure that is enough.

Sitting here thinking about it in real time my tentative conclusion is that the two most important things in bullet choice, in order of importance, are suitability of terminal performance for the game to be hunted at the intended max range and ability to shoot them accurately out of the rifle in question. Ballistic coefficient is more or less important depending on the range, and that depends on the game.

Which brings me back to the original question, where does BC get important given game and range. My answer to that is when the wind drift "uncertainty" is over half the the size of the kill zone on the animal. Not the expected wind drift which may be several kill zones in magnitude but which is compensated for by estimating wind and making sight corrections, but the uncertainty in wind drift. i.e. one measures the wind and concludes the wind drift will be 40 inches but the uncertainty is a foot or more given terrain variations. Measuring the wind will reduce that uncertainty. Higher BC will reduce that uncertainty. But I think the range at which it becomes important varies with the size of the kill zone on game in question.

Fitch
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Old 02-01-2010, 07:23 AM
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Re: How important is bc?

.........................

Last edited by Mikecr; 02-01-2010 at 07:25 AM. Reason: delete
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  #11  
Old 02-01-2010, 07:24 AM
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Re: How important is bc?

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Originally Posted by Fitch View Post
where does BC get important given game and range. My answer to that is when the wind drift "uncertainty" is over half the the size of the kill zone
This is perfect Fitch.
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Old 02-01-2010, 08:25 AM
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Re: How important is bc?

Quote:
That said, 1800 fps, which is the stated minimum expansion velocity for quite a few modern bullets, is supersonic so subsonic performance is academic since they shouldn't be used at ranges where they will be subsonic for terminal performance reasons. That makes the trade on the side of higher BC vs subsonic instability a good one since subsonic really doesn't matter if the bullet won't do the job after impact.
Well if guns and bullets that don't develop 1800 fps shouldn't be used to hunt things, that certainly eliminates a lot of them.



One of the interesting aspects of BC is its reliance upon SD. This is a double edged sword and one should be careful with it. The 200 grain Wildcat 7mm bullets and similar have extremely high SDs but they become sensitive to launch conditions. However, high SD is a wonderful thing when it impacts an animal.
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  #13  
Old 02-01-2010, 08:41 AM
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Re: How important is bc?

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Originally Posted by Mikecr View Post
This is perfect Fitch.
+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
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  #14  
Old 02-01-2010, 09:28 AM
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Re: How important is bc?

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Originally Posted by MontanaRifleman View Post
Steve,

Good question and it's all relative to what's important to the shooter and what he is willing to pay. If 600 yds is all the farther you're going to shoot, then it just doesn't matter a whole lot.

For me, since becomimg interested in LRH, I've come across some good ambush spots that are in excess of 1K. Last year I found a good spring bear ambush spot in the range of 1000-1300 yds across a canyon. I'm hoping to get a shot there this year. In this case BC is very critical and every littel bit helps. It extends my max effective range and It helps buck wind.

The difference out of my RUM for a .5, .55 and .6 BC 180 gr @ 3400 fps bullet means ranges of 1100, 1225, and 1325. A BC of .7 gets my bullet on target with a good bit more thump and a significant diff in wind drift. The diff in wind drift between .5 and .7 @ 1200 yds is about 55%, 78" vs 50" per 10 mph.

Bottom line is it's all relative based on a person's goals and priorites. Ask 10 different people and you'll get 10 different answers.

-Mark
You are correct. Can you shoot a one shot kill at 1100 yards in a 10 mph variable wind with any bullet? Be honest. I don't think I have that confidence. I will not take that shot with the wind. Calm, that's another story.

We are talking huge jumps in bc in this theoretical scenario. But usually we would be talking about a bullets of different shapes and weights. So then the mv will change. In that scenario, a lower bc lighter bullet can out run the higher bc bullet out to beyond practical distances for wind and drop.

The boat tail is another subject. I think they are kinda like fishing lures. First thing they gotta catch is the fisherman. The boat tail extends flight distance by reducing drag at the very end of the maximum range of the bullet. Unless I am missing something here, the boat tail is useless to the hunter. An fact it a detriment to the hunter. It is inherently less accurate than the flat base due to turbulence upon the exit of the barrel. The boat tail is the fishing lure part of the bullet. In today's market everybody thinks they need a boat tail. I shoot them because they are on the bullets that have the terminal performance that I want. If I had the option I would go with out the boat tail.

So now we have looked at theoretical bc's. Let's look at the bc's of real bullets and real world effects on wind drift and drop. I don't want to start bc ******* matches, I just want to see real world comparisons of the bullets that we all are using.

Steve
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