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anyone have problems with jbm?

 
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  #1  
Old 09-26-2013, 10:13 AM
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Join Date: Jun 2012
Posts: 24
anyone have problems with jbm?

hello

i ran some numbers over the jbm calculator and i dont trust the results i got.

i wanted to see a comparison of shooting a 208 amax in a .308 and a 140 vld in a 260ai and several unusual things caught my attention.

first thing that caught my attention was when i was entering the info for the temp i would be shooting in i always type in either 100 or 0 degrees for shooting year round, however, when i hit the calculate button i looked at all the info and the temp was always 59.2 degrees no matter what i did.

second thing, i used the g7 bc for both bullets[.324 for 208 amax and .313 for 140 vld] and when i calculated it showed up as .648 g1 bc and .612 g1 bc. and yes, i checked the g7 box and it still showed up as g1 both times.

third thing, and this is the most confusing for me, the .308 shows a whopping 212 ft-lb difference at 1200 yds [867.7 for the .308 and 655.5 for the 260 ai].
heres the confusing part, how can the 260 ai have that much less energy when the bullet is starting out 200 fps faster and the bc's of both bullets are nearly identical.
heres the numbers i inputted into jbm
velocity-2750 for the .308 2950 for the 260 ai
altitude 1312' asl
std. atmosphere at altitude box checked


i appreciate any and all input you guys and gals can give me
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  #2  
Old 09-26-2013, 12:07 PM
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Join Date: Dec 2010
Posts: 235
Re: anyone have problems with jbm?

As far as the lb-ft differences go I usually like to explain it in a way that most guys can relate to. Pick your favorite major league pitcher and think of him throwing a baseball at 100 mph and hitting you in the shoulder. Gonna hurt like hell, right? Now think of him throwing a bowling ball at 85 mph, if he could, and hitting you in the shoulder. There's a chance thats going to kill you. So, even though the lighter projectile is going faster, the heavier, slower one is going to pack a much larger punch. As far as the intricacies of jbm I cant help there.
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Old 09-26-2013, 05:29 PM
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Join Date: Aug 2012
Posts: 48
Re: anyone have problems with jbm?

Its my understanding that if you select the box std. atmosphere at altitude the program will ignore your inputs for temperature and air pressure and use the standard values for the given altitude you entered. If you want to enter a different value deselect that box. Also, be sure to consider the the "pressure is corrected" box. It will throw your numbers off if you're using a absolute pressure with that box checked.

As for the Bcs, I'm not really sure. Anytime I've used the program I have just scrolled down and selected the bullet from the drop down list. Maybe thats what you did as well and thats why it ignored the values you manually entered again??

Far as your energy comparison goes. I'd expect a 260ai with 140s to have more energy at that range then just about anything a 308 could shoot. Think your velocity is a little high for a 208 gr. My 308 shoots 168s at 2750. Seems like 40 grains heavier bullet along with reduced case capacity for a given coal would make that velocity unattainable. I very well could be wrong. If I am then that bullet at that speed might produce more energy at that range. It does have a higher bc after all. I'd start by verifying all the values and seeing what you get for a better comparision.

Hope this helps you -Sam
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Old 09-26-2013, 08:30 PM
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Join Date: Jun 2012
Posts: 24
Re: anyone have problems with jbm?

Quote:
Originally Posted by Sam Aspacher View Post
Its my understanding that if you select the box std. atmosphere at altitude the program will ignore your inputs for temperature and air pressure and use the standard values for the given altitude you entered. If you want to enter a different value deselect that box. Also, be sure to consider the the "pressure is corrected" box. It will throw your numbers off if you're using a absolute pressure with that box checked.

As for the Bcs, I'm not really sure. Anytime I've used the program I have just scrolled down and selected the bullet from the drop down list. Maybe thats what you did as well and thats why it ignored the values you manually entered again??

Far as your energy comparison goes. I'd expect a 260ai with 140s to have more energy at that range then just about anything a 308 could shoot. Think your velocity is a little high for a 208 gr. My 308 shoots 168s at 2750. Seems like 40 grains heavier bullet along with reduced case capacity for a given coal would make that velocity unattainable. I very well could be wrong. If I am then that bullet at that speed might produce more energy at that range. It does have a higher bc after all. I'd start by verifying all the values and seeing what you get for a better comparision.

Hope this helps you -Sam
thanks, ill look into it here in a little bit.

as for the velocity, i was under the impression that with alliants power pro 2000 mr, i could get close to the same if not a little more velocity that i could get with rl 17 and with a 28"-30" tube i thought 2750 was doable. maybe im wrong. wouldnt be the first time and probably wouldnt be the last
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