Bryan,
Below is my e-mail to Steve Milholland ...
Below is my e-mail to Steve Milholland ...
JLK VLDs?
- Thread starter MontanaRifleman
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MontanaRifleman
Well-Known Member
Thanks again for the response Bryan,
I had thought about the cautions you mentioned and I will definitely be sure I am shooting a tight group at 300 yds before I try shooting through a chrono at that range. I will also use the "old" chrono down range
But as of yet, I only have one chrono and a second one is on the wish list along with a lot of other things including picatiny rail and rings for my Sendero, a good range finder (currently have a Nikon that is good to about 700-800 yds) Harris bi pod, etc., etc. With business being slow it could take all summer to get all the pieces together and I was hoping to be there by now.... but I digress....So if anyone wants to meet me at the Logan range and bring their chrony along, we could do some BC testing.
Yup, I sure will share the results if and when I do it, and I do want to do it.
Couple of questions on the JBM calc...
1) What's the difference between absolute and corrected pressure? I think I know but would like to hear from you and what do I use?
2) What does drag profile have to do with calculating the BC? I noticed that ii I changed the drag profile form G1 to G7 it drastically changed the BC even if I left everything else the same such as distance and velocities, near and far. This doesn't seem right to me so I must be missing something.
Also, a good article, but the graphs block some of the text.
Thanks,
Mark
This is tricky.
In JBM, if you check "Std. Atmosphere at altitude", then you don't have to tell it what the measured air pressure is (if you do it will ignore the input). Instead, it will use the air pressure that corresponds with the altitude input you use. This option is not recommended (by me) because it's more accurate to measure the air pressure directly.
If "Std. Atmosphere at altitude" is unchecked, and so is "corrected pressure", then the program will use the pressure you input directly to calculate air density. This is recommended.
If "Std. Atmosphere at altitude" is unchecked, and "corrected pressure" is checked, then the program will use the pressure that you input as the sea level pressure, and use the altitude input and the standard lapse rate to estimate the pressure at your altitude at that time. This practice was developed for pilots who need to calibrate altimeters to know where 'zero altitude' is, but unfortunately gets used by shooters to confuse themselves.
In my opinion, the best and simplest way to handle air pressure is to measure it directly, at the shooting location, and un-check the options in JBM that want to adjust it.
The drag profile, aka the drag function, is the standard that the BC is defined in relation to. G1 has historically been used exclusively, but there are other standards better suited for long range bullets.
Defining BC according to the G1 standard for long range (boat-tailed, sharp nosed) bullets results in a BC that changes a lot with velocity. However, if you define a BC in relation to the G7 standard, it's much more constant over velocity. This results in more accurate trajectory calculations.
As you observed, BC's related to other standards (G7 for example) are much lower than BC's related to the G1 standard. This isn't a problem as long as you tell the trajectory calculator what kind of BC you're giving it; otherwise you're mixing apples and oranges.
For example, let's say you measure the velocity drop over 100 yards to be 3000 fps to 2800 fps and the calculator says the G1 BC is .500 and the G7 BC is .250. If you now measure the velocity drop from 200 to 300 yards, the G1 BC will be something like .43 or .44, where-as the G7 BC will still be .250 or something very close to .250. Over the course of a long range trajectory, the G1 BC will change by as much as 10% to 15%+ where as the G7 BC will usually stay within a couple %.
Here are a couple links to articles that talk about G1 vs G7 BC's:
http://www.appliedballisticsllc.com/index_files/Berger155grainVLD.pdf
http://www.appliedballisticsllc.com/index_files/7mmNumberOne.pdf
BH mentioned BIB's new 6mm bullet. I noticed that Randy is advertising a G7 BC as well as a G1 BC for this new option:
BIB Bullet Co. (515) 438-4010 » New Products !
In the past I've been providing G7 BC's on request for Berger bullets. In the coming months, we're going to be making these G7 BC's more easily accessible, probably posting them on our website and printing them on our labels. We'll keep the G1 BC's for a while, but the eventual goal is to migrate to the more suitable standard.
Remember, if you use a G7 BC, you have to use it with a program that can accept it (JBM can accept G7 BC's). If you give your program a G7 BC, and don't tell it, then your computer will short circuit and fall into a black hole
Take care,
-Bryan
sscoyote
Well-Known Member
If you give your program a G7 BC, and don't tell it, then your computer will short circuit and fall into a black hole
Take care,
-Bryan
Dang, that's funny!!
MontanaRifleman
Well-Known Member
Bryan,
Thanks again for your reply. If I use absolute altitude I wont check standard or corrected, nor input my elevation as I understand it.
I'm still trying to chew and digest this G7 drag profile and It might take a little while. In the interim I'll use the the G1 and try to guesstimate the averge as best as I can.
While I have your attention, I thought I would run something else by you. Last year Walt replied to an email I sent him saying the VLDs minimum expanding velocity was about 1825 fps. Nosler advertises Accubonds to ba about the same. Interestingly, whne I read some of the threads and posts here, it seems the AB's will open at velocoities 200-300 fps lower at LR. If this is true, I wonder if the jackets and maybe the lead core might soften a little due to heat friction from high velocites and longer TOF? I know your more into the external balistics rather than the terminal ballistics, but I wonder if you had any thoughts on that?
If the opportunity presents itself, I may try experimenting on antelope. From my own perspective, if the VLD open at lower velocities down range, it would extend it's hunting value to me.
Mark
Thanks again for your reply. If I use absolute altitude I wont check standard or corrected, nor input my elevation as I understand it.
I'm still trying to chew and digest this G7 drag profile and It might take a little while. In the interim I'll use the the G1 and try to guesstimate the averge as best as I can.
While I have your attention, I thought I would run something else by you. Last year Walt replied to an email I sent him saying the VLDs minimum expanding velocity was about 1825 fps. Nosler advertises Accubonds to ba about the same. Interestingly, whne I read some of the threads and posts here, it seems the AB's will open at velocoities 200-300 fps lower at LR. If this is true, I wonder if the jackets and maybe the lead core might soften a little due to heat friction from high velocites and longer TOF? I know your more into the external balistics rather than the terminal ballistics, but I wonder if you had any thoughts on that?
If the opportunity presents itself, I may try experimenting on antelope. From my own perspective, if the VLD open at lower velocities down range, it would extend it's hunting value to me.
Mark
locotrician
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i find the jkl bullets to be more consistent when weight sorting or ogive sorting, but my gun shoots both the bergers and the jkls the same to 750 havent tried the jlks at 1000 yet.
Mark,
You're right, I'm more in tune with exterior ballistics than terminal, but I'm doing my best to learn and I'll take a stab at your question regarding minimum expanding velocity.
First thing to note is that there's no specific velocity at which a bullet will achieve adequate expansion, it's very much a gray area; a matter of degrees. The faster the bullet impacts, the more it will deform/expand. The slower it impacts, the less it will deform/expand. Now the tricky part is that everyone has their own idea as to what is adequate or acceptable expansion. Some like the bullets to open up like grenades while others think the best terminal performance is a bullet that retains it's original shape and weight at any impact speed for best penetration. These are some of the reasons why it's difficult to nail down a specific minimal velocity for reliable expansion. Having said that, we have found that Berger VLD's expand enough to kill things reliably down to 1800 fps. Can they kill at lower impact speeds? I'm sure they can, but maybe not as efficiently or with as much expansion.
After all the researching and studying I've done in the academic literature as well as places like this, I've come to the conclusion that shot placement trumps every other measure of terminal bullet performance. No matter how much velocity/energy the bullet has or how much it expands/penetrates, etc; if the heart or a major artery is punctured, the animal will die (think about archery). On the other hand, you can hit an animal with the most impressive performing bullet with a Jigawatt of energy but if you hit it in a non-vital place, it gets away, probably to die someplace where you won't be able to find it.
Bullet lethality is one of those subjects that has a lot of gray area and many strong opinions. Some say it's all about energy, some say it's all about bullet expansion vs penetration, some say shot placement, etc. My advice is to do your best to maximize all of the factors that are related to lethality, concentrating most on shot placement.
Sorry I can't give you a more concise answer, but maybe death just can't be calculated
-Bryan
J
JEREMY logan
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+1 on ''shot placemaent trumps"
