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Lets Talk Stepped BC's

 
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  #8  
Old 08-08-2011, 11:28 PM
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Re: Lets Talk Stepped BC's

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Originally Posted by jmden View Post
Broz,

Would you mind sharing your 'real world' shooting bc that you've found to work for those longer distances? Are you talking stepping g7 or g1 or just adding a tested amount of moa?

Jon
Jon, I have full confidence in the G7 BC to 1400. But I found I needed to add elevation as I moved out and the amount of elevation ever so slighty increases with distance. So yes, a need for a tested amount of moa.

Now, I have considered the fact it becomes harder to detect as even with a 1/2 moa rifle the errors add up at these distances. But I shoot alot, and I feel I have enough logged data to support my statments.

All this said, I am on the fence as to whether G1 or G7 will work better when stepped BC's are used.

So far, as for BC's. I have found that I do not need to venture far from advertised BC's if any. I am refering to the 30 cal 210 Berger and the new 300 gr Gen 2 .338 Berger Hybrids.

Jeff
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  #9  
Old 08-08-2011, 11:38 PM
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Re: Lets Talk Stepped BC's

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Originally Posted by Rocky Mountain View Post
I asked Bryan Litz in another thread on this topic if he thought it was needed the step the G7 BC for the really long ranges but i got no reply I hope this new threat might shine some light on this by Broz and Bryan

I asked this as well. I am not sure Bryan seen the posts. Or he is prepairing a proper answer that takes time.

So this is why I started this thread. I was hoping we could compile actual field results and come up with some good ways to "tune" stepped BCs.

I feel Bryan agrees there could be a need as range is increased. He was involved with the making of the "Shooter" program I am using and it has the stepped BC option in either G1 or G7.

Also, I feel I have found some ELR consistancy and accuracy with the new Berger Gen 2's. This has opened the door to longer shots and a need for a shooting solution at these distances. So, this is all kinda new territory for me as I strive for cold bore first round hits inside 1 moa.

Jeff
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  #10  
Old 08-09-2011, 09:04 AM
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Re: Lets Talk Stepped BC's

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Originally Posted by Rocky Mountain View Post
I asked Bryan Litz in another thread on this topic if he thought it was needed the step the G7 BC for the really long ranges but i got no reply I hope this new threat might shine some light on this by Broz and Bryan
Yeah, it has always seemed to me that stepped G7 bc might be the way to go for longer ranges. Do any ballistic computers allow that at this time?

Afterall, g7 is just a model and the bullet we are talking about is much closer to g7 model than g1 model, but still not exact obviously, so it seems logical to expect there to be a need for 'stepped' g7 bc for nearly any bullet closer to/using the g7 model...just not as dramatic as if you used the g1 model.

In terms of velocity banding, Litz and others I've seen use 3K and above, 2500, 2000, and 1500 and below...for g1. Might be a good starting point for g7...?
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  #11  
Old 08-09-2011, 09:16 AM
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Re: Lets Talk Stepped BC's

We're into some interesting territory here.

"Is it better to step a G1 or G7 BC?" Truth is, if you're going to step the BC, it really doesn't matter which one you use, you will correct it with the steps. If you start with the BC that's a better match for the bullet you're using, then you should need fewer steps, and the required steps will be smaller, but neither is 'better' than the other if you're able to step any way you like.

Having said that, there are problems with stepping BC's, regardless if it's G1 or G7. All ballistics programs that have stepped BC's allow you to define the steps according to velocity. The problem is that a bullet's drag curve is a function of Mach number, not velocity. This becomes a problem when temperature changes (Mach number changes with temperature). In other words, if you go shooting in 59 degree weather and decide that you have to step down the BC at 1200 fps (for example), that step is applied at Mach 1.08. But if the temperature decreases to 20 degrees, the Mach step is applied at Mach 1.15 instead of 1.08 where you intended it to be (because the speed of sound changes from 1116 at 59 degrees to 1038 at 20 degrees).

This is a pretty minor problem, but it is a problem with the way steps are handled (even in Shooter).

Another issue is that in order to truly reflect the actual drag of a bullet thru transonic, it would take about 15 to 20 data points to really model the drag of a specific bullet, not the 3 to 5 data points that are typically used. This number of data points is nearly impossible to derive from shooting drops, which is the preferred method (and only method available) for most shooters.

There are other potential issues with using stepped BC's that I won't go into here.

So what to do?

I've long been an advocate of G7 BC's for modern long range bullets. I contend that for practical long range shooting (meaning ranges that precision is adequate to hit practical (game) sized targets) the single, averaged G7 BC is all anyone needs. Broz observation is that the single BC is good to ~1400 yards, and I think that's about as far as 99.9% of the population will ever attempt to shoot at anything they actually intend to hit.

Having said that, I recognize that there is room for improvement in trajectory predictions beyond 'practical' ranges, and for those cases, the single averaged G7 BC can be improved on. The new chapter in my second edition (chapter 11: Extended Long Range Shooting) eludes to this improvement, which is to model the drag of a bullet with a custom drag curve rather than reference it to a standard projectile. In other words, don't use BC's for the bullet at all, but rather each bullet has a unique drag profile based on it's shape which is represented with a table of cd vs Mach number points. This is the best way to eliminate error in trajectory predictions due to drag modeling error.

There is a new software app in development that will allow you to download custom drag curves for the bullets you're interested in. It will have a conventional library of BC's (just like shooter has), but for those ELR shooters who need to model drag near and beyond transonic, you'll be able to get the drag curves for those specific bullets. This app is intended for android, iPhone/iPod/iPad, blackberry, and windows mobile devices. I hesitate to talk about it now because the original 'due date' was June, and it's still not even in Beta testing yet. But it will be a reality some day.

My hope is that most 'normal' long range shooters continue to use the G7 BC's because they really are adequate for most users. But for that small fraction of people who shoot these extended ranges, there will be an option that is better for their purposes than a single BC.

BTW, when I shot 1 mile with the 300 grain Hybrids, my group was a little high using the single G7 BC, and so was the other guys group (different rifle, same bullet). When I compared my G7 based prediction to the custom drag, the custom drag would have had me even higher. This would seem to indicate the native G7 BC was closer than the custom drag, but I didn't get too worried about it because of other uncertainties in the test, most importantly range which was measured with GPS, not directly lazed. So there could have been some range error which is why I'm not concerned that the custom drag solution was further off.

Having read Broz accounts that his G7 based trajectories have him hitting a little LOW at ELR (and he's got a good laser rangefinder for those distances ) I'm encouraged that the custom drag solution actually does provide a better prediction when range is known with more certainty. As an example, at 1000 yards, the G7 and custom drag based predictions are within 0.1 MOA. At 1760 and 2300 yards, the custom drag predicted drop is 0.3 MOA more than the G7 based drop. This would bring Broz predictions into better alignment with his observations than the native G7 based drop.

There are other features of the new ballistics app which will make it more capable for ELR shooting, but I don't want to let the cat entirely out of the bag! Plus I have no idea when the programmers will finish so I don't want to stir up too much hype too early.

So to sum up; yes, ELR shooters do have a need for more sophisticated trajectory predictions than the customary BC's can provide. No, stepping BC's is not the answer, rather full custom drag curves for specific bullets is the right approach, and it's on it's way.

BTW, bullets and ballistics aren't the only links in the ELR chain. Don't forget about adjustable bases (which Shawn is working on) and reliable means for measuring range. I just sunk over $5,700 into a PLRF-15 OUCH! (thanks a lot BROZ! ). Effective ELR shooting is not cheap and there are few people with the know how to pull it off (but several of them are right here!)

Take care,
-Bryan
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  #12  
Old 08-09-2011, 10:10 AM
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Re: Lets Talk Stepped BC's

Great post Bryan, It helps me out alot.

Some other thoughts I need to try and confirm. Please feel free to debate these too.

My first thought was to go into Shooter where I pick my load for my rifle and simply add a second bullet curve for beyond, lets say 1400 yards. I could probably slightly adjust the velocity and bc to fit the extnded LR shots.

Example:

Berger 300 Gr Hybrid Gen 2

Berger 300 Gr Hybrid Gen 2 1400 to 2000 yrds.



Also, I should add I have been using both spin drift and the effect of coriolis for a large percentage of these logged shots. I have over 250 rounds of the .338 Gen 2's alone down range and have logged much data. Although my success rate on first round hits have made me smile, I see room for improvement. I say this because I seen a pattern and started adding a small correction of elevation from past experiences. Example: I logged these shots as "called for" (CF 45 moa) (actual 46 moa) and marked the correct elevation with a star.

So being a doubting thomas I am. I want to reshoot a majority of these shots with out coriolis added and also on the flat as some were shot at as much as 16 degrees up hill. I just want to make sure that the small error I am dealing with is truely a function of a curve problem and not an error added by the programs correction for coriols or look angle.

I agree that this topic is for only a few shooters. I also agree that 1400 yards is a long way to engage game and my personal limit is not far beyond that. Conditions would have to be steller, and field of view around the animal would need to be open. My main mission here is not to extend my hunting perimeter. It is to challenge my skill level and this will greatly improve my closer shots as well.


Bout time you spent some of that moldy money and bought a good RF. I will take on part of that blame for a few members here buying PLRF's. But you will thank me for it later.

But, my friend, you and the folks at Berger have given us a bullet that is much more accurate and predictable at these extended distances. Now I am spending money too as a new door has opened.

So , Thanks! to you Bryan, Erric , Walt and the others at Berger too.

Jeff ---------------------------------------------------------------------------------------------------

Jeff
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  #13  
Old 08-09-2011, 12:34 PM
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Re: Lets Talk Stepped BC's

Quote:
Originally Posted by Broz View Post
Great post Bryan, It helps me out alot.

Some other thoughts I need to try and confirm. Please feel free to debate these too.

My first thought was to go into Shooter where I pick my load for my rifle and simply add a second bullet curve for beyond, lets say 1400 yards. I could probably slightly adjust the velocity and bc to fit the extnded LR shots.

Example:

Berger 300 Gr Hybrid Gen 2

Berger 300 Gr Hybrid Gen 2 1400 to 2000 yrds.

Jeff
BROZ,

Interesting ideas. How do you determine the correction for your second curve?
I used a similar two curved process with the .338 cal, 300gn SMK from 1200 to 1500 yards when I was using Exbal but since switching to "Shooter" and G7 B.C.'s I have not bothered.
However, I have recently been field shooting with them out at 1600 yards and have been toying with the idea of doing the same with the "Shooter" program and G7 B.C.'s due to my observed POI difference at the longer distance.

It gets you thinking when your trajectory is good to 1200 but then is slightly out at +1500.
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  #14  
Old 08-09-2011, 02:50 PM
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Re: Lets Talk Stepped BC's

Quote:
Originally Posted by Topshot View Post
BROZ,

Interesting ideas. How do you determine the correction for your second curve?
I used a similar two curved process with the .338 cal, 300gn SMK from 1200 to 1500 yards when I was using Exbal but since switching to "Shooter" and G7 B.C.'s I have not bothered.
However, I have recently been field shooting with them out at 1600 yards and have been toying with the idea of doing the same with the "Shooter" program and G7 B.C.'s due to my observed POI difference at the longer distance.

It gets you thinking when your trajectory is good to 1200 but then is slightly out at +1500.
Thats thinking outside the box Broz I can see two different curved inputs for different ranges working, what does Bryan think of this forward idea
This new app coming out sometime in the future sounds very interesting
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