Ok when your manually inputting your BC, what does the pull down tap G1, G2, G5, G6, G7, G8, GI, GL, RA4 mean. It makes a HUGE difference in your exterior ballistics.
Range Report JBM Calculation HELP
- Thread starter jrbet83
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Re: JBM Calculation HELP
There are two ways to specify bullet data in JBM.
One way is to manually enter the BC which you have obtained from an outside source. You will enter the BC, and specify the drag model.
The other way is to pick a bullet from the bullet library. Anything in the bullet library with the notation (Litz) behind it is a G7 value. Note that when you do that, the program does not indicate the value of the BC it is using.
Most questions about JBM entries can be answered by clicking on the ? link next to the entry point.
There are two ways to specify bullet data in JBM.
One way is to manually enter the BC which you have obtained from an outside source. You will enter the BC, and specify the drag model.
The other way is to pick a bullet from the bullet library. Anything in the bullet library with the notation (Litz) behind it is a G7 value. Note that when you do that, the program does not indicate the value of the BC it is using.
Most questions about JBM entries can be answered by clicking on the ? link next to the entry point.
Re: JBM Calculation HELP
What Lindy said is right on. There is more here:
http://www.jbmballistics.com/ballistics/topics/dragfunctions.shtml
The bottom line is that manufacturers pick a drag function and BC, they go together -- you can't change the drag function without changing the BC and expect to have decent results. Almost all manufacturers use the G1 BC. The ones that done (e.g. Berger) will tell you it's a G7.
Thanks!
Brad
What Lindy said is right on. There is more here:
http://www.jbmballistics.com/ballistics/topics/dragfunctions.shtml
The bottom line is that manufacturers pick a drag function and BC, they go together -- you can't change the drag function without changing the BC and expect to have decent results. Almost all manufacturers use the G1 BC. The ones that done (e.g. Berger) will tell you it's a G7.
Thanks!
Brad
Re: JBM Calculation HELP
I've found that, regardless of the BC model used (G1 or G7), the BCs published by Bryan Litz in his book (Applied Ballistics for Long Range Shooting) are more accurate than those supplied by most manufacturers (other than Berger). As Lindy said, even if you don't spring for the book, you can still select bullets in JBM that use Litz' BCs. I highly recommend the book, however. It is a trove of useful information.
In practice, there are few bullets that most of us use that don't fit either the G1 or G7 models. So I'd focus on those. Depending on the bullet, you may not see much difference between the two out to 500-600 yards. After that point, since you'd likely use only boat tails for long range shooting, you'd want to stick with the G7 model and get as accurate BC as possible for input.
Although most of the bullets covered in Applied Ballistics are boat tails and better fit the G7 model, there are a few exceptions. For example, one of those is the .308 200 grain Nosler Partition, which fits better with the G1 model. How can you tell? Litz measured BCs at four different velocities, calculated an average and provides a variation on that average. Whichever model has the lesser variation is a better fit.
I've found that, regardless of the BC model used (G1 or G7), the BCs published by Bryan Litz in his book (Applied Ballistics for Long Range Shooting) are more accurate than those supplied by most manufacturers (other than Berger). As Lindy said, even if you don't spring for the book, you can still select bullets in JBM that use Litz' BCs. I highly recommend the book, however. It is a trove of useful information.
In practice, there are few bullets that most of us use that don't fit either the G1 or G7 models. So I'd focus on those. Depending on the bullet, you may not see much difference between the two out to 500-600 yards. After that point, since you'd likely use only boat tails for long range shooting, you'd want to stick with the G7 model and get as accurate BC as possible for input.
Although most of the bullets covered in Applied Ballistics are boat tails and better fit the G7 model, there are a few exceptions. For example, one of those is the .308 200 grain Nosler Partition, which fits better with the G1 model. How can you tell? Litz measured BCs at four different velocities, calculated an average and provides a variation on that average. Whichever model has the lesser variation is a better fit.
Re: JBM Calculation HELP
I thought other than the really high VLD bullets like the Bergers and Scenars, the normal boattails like SMKs AND Amax's actually fit the G5 model the closest?
I thought other than the really high VLD bullets like the Bergers and Scenars, the normal boattails like SMKs AND Amax's actually fit the G5 model the closest?
Re: JBM Calculation HELP
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: ReaperDriver</div><div class="ubbcode-body">I thought other than the really high VLD bullets like the Bergers and Scenars, the normal boattails like SMKs AND Amax's actually fit the G5 model the closest? </div></div>
Well, they would, but who's got G5 BCs for anything? Also, consider that most boat tails are closer to G7 than G1, yet G1 still provides pretty close results out quite a ways. And it is less like the G5 model than G7 is. Take a look at the various models here:
http://www.frfrogspad.com/drgshape.htm
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: ReaperDriver</div><div class="ubbcode-body">I thought other than the really high VLD bullets like the Bergers and Scenars, the normal boattails like SMKs AND Amax's actually fit the G5 model the closest? </div></div>
Well, they would, but who's got G5 BCs for anything? Also, consider that most boat tails are closer to G7 than G1, yet G1 still provides pretty close results out quite a ways. And it is less like the G5 model than G7 is. Take a look at the various models here:
http://www.frfrogspad.com/drgshape.htm
Re: JBM Calculation HELP
If you were confident your chronograph was reasonably accurate, you could shoot a load at long distance, but short enough that you were sure it was still supersonic, and then back out a G5 BC using a ballistic program.
That is, put the conditions in which you shot and the velocity, specify a G5 drag model, and change the BC until the program output agreed with your data.
Or you could use the testing method Bryan described in his book.
The primary advantage of the G7 vs. G1 drag model is that the G7 is more accurate in terms of terminal velocity. That's usually not much of a factor for military/L.E. snipers, because they're probably going to take a given shot even if the bullet is going to go subsonic. Look at the current hot story about a new sniper distance record. If the distance was accurate, those bullets were going subsonic.
If you were confident your chronograph was reasonably accurate, you could shoot a load at long distance, but short enough that you were sure it was still supersonic, and then back out a G5 BC using a ballistic program.
That is, put the conditions in which you shot and the velocity, specify a G5 drag model, and change the BC until the program output agreed with your data.
Or you could use the testing method Bryan described in his book.
The primary advantage of the G7 vs. G1 drag model is that the G7 is more accurate in terms of terminal velocity. That's usually not much of a factor for military/L.E. snipers, because they're probably going to take a given shot even if the bullet is going to go subsonic. Look at the current hot story about a new sniper distance record. If the distance was accurate, those bullets were going subsonic.
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