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About ballistics and some 'constants"
Help / Support / How-to's / Discussion related to PointBlank Software and External Ballistics in general
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Ghost
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Joined: Nov 20, 2011
Posts: 25
Location: European Forrest

PostPosted: Thu Nov 24, 2011 1:01 pm    Post subject: About ballistics and some 'constants" Reply with quote

I have read that for the Power(energy) the constant to use is the number '450411' but i learned in the past when i was for Many years with the 1GVP(now SFG), that it was '450240' i know the result isn't a big difference. But if want to know the exact Muzzle powerenergy i think it's interesting to know wich one is best to use my friends.

For example for my favorite Windrunner .338cal model98(nice to shoot around 1500yds or even 1500m: muzzlevelocity 2950fps, 250gr, BC 0.597, ImpactVelocity@1000yds 1642fps, site height 2.12" =Formula 250 x 2950² / '450411' = 4830.311 Power or MuzzleEnergy, and as i learned 250 x 2950² / '450240' = 4832.145 Power or MuzzleEnergy !

Altough if i calculate the Muzzle energy with my downloaded PointBlanckBallistic-software, then i got a complete other result after i gave in the 250gr & 2950fps numbers = result in PointBlanck 6551.422(FPE). So what the heck is that REAL 'constant' then? ? wtf

I also use regularly the PointBlank-ballistic software,mostly to compare if my estimations still being good, this software is very good. But mostly i only calculate everything the way that i've learned it in the past with 1GVP, on my calculator. No wind or forecast software. Fieldcraft & sight estimations only. That's why i like to use MilDot-systems also.
So, as you can see the result with both 'constants' is not a big difference , but for some specific calculations it can be necessary to know the exact 'constant'-number. Certainly to concern is the big difference in the MuzzleEnergy-result in PointBlanck in relation to my normal(without software)calculations.

Thx in front to everyone and those who like to learn/read about external ballistics too, Cool

Kind Regards,



Ghost


Last edited by Ghost on Sat Nov 26, 2011 8:11 pm; edited 2 times in total
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Ominivision1
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PostPosted: Thu Nov 24, 2011 1:44 pm    Post subject: Re: About ballistics and some 'constants" Reply with quote

I don't have my laptop with me that runs PB, but running your numbers on the on-line PB program I come up with 4838me which is fairly close to divide by 450240 that I've been using for a long time.

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Ghost
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Location: European Forrest

PostPosted: Thu Nov 24, 2011 7:12 pm    Post subject: Re: About ballistics and some 'constants" Reply with quote

Yes Omnivision,
On the online version i also can see it is at Zero yds 4830, that can be compared with the first 'constant' of '450411' i made and the same result. So i think it's that one.
But tried it again on the downloaded version in Ballistic menu and then below can choose 'misc', then can choose on the right 'Calculate MuzzleEnergy', there i've to put in the weight 250gr and velocity 2950fps and the result stay the same = 6551.422 Very strange though. I think that have to be fixed cause it's To far off from the other results and from the online result that use the correct '450411 constant'.(This is solved)
Of course if i click the tab 'Table data' or the 'Energy' there it give the correct energy, so it's defenitely the 'Calculate MuzzleEnergy' function in the 'Misc' tab that give the wrong result i guess!

Ghost
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Pumpkinslinger
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PostPosted: Thu Nov 24, 2011 9:08 pm    Post subject: Re: About ballistics and some 'constants" Reply with quote

The constant used in the energy formula comes from multiplying the number of grains in a pound X 2 X acceleration due to gravity.

7000 X 2 X 32.175 = 450450

The difference between the constants mentioned isn't enough to worry about IMO. If I figure the muzzle energy for a 140 grain bullet at 3000 ft/sec (using a .280 Rem for example) the difference between using a constant of 450450 and a constant of 450240 is only 1.3 ftlbs, or less than 0.05%. A velocity difference of 1 ft/sec or a weight difference of 1 grain makes a lot more difference than that.

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Ghost
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Joined: Nov 20, 2011
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Location: European Forrest

PostPosted: Fri Nov 25, 2011 5:03 am    Post subject: Re: About ballistics and some 'constants" Reply with quote

Yes my friend and it's not much or very less difference, but those numbers are there for some reason to be correct.
For example the correct 'constant' that PointBlanck use '450411' / 32.175(9.81m/sec²) = 6999.3939......etc, gives evidence that it isn't much difference at all.
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Ominivision1
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PostPosted: Fri Nov 25, 2011 9:58 am    Post subject: Re: About ballistics and some 'constants" Reply with quote

I just tried PB on my laptop and entered all your figures and the results are right on at around 4838 me. Maybe their is a difference in the way it calculates between Europe and the US.

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DallanC
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Joined: Jan 18, 2005
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PostPosted: Fri Nov 25, 2011 10:55 am    Post subject: Re: About ballistics and some 'constants" Reply with quote

Ghost wrote:
Yes my friend and it's not much or very less difference, but those numbers are there for some reason to be correct.
For example the correct 'constant' that PointBlanck use '450411' / 32.175(9.81m/sec²) = 6999.3939......etc, gives evidence that it isn't much difference at all.

Such small differences in results can be attributed to internal precision errors. Long ago in a much earlier version of PointBlank I used floating point values as they were faster. In later versions when 64bit computers became the norm, I have switched to using doubles to store values. This increased minute accurate by alot... there are other values as well to take accuracy even further but it could slow down the computations. Its kindof at a point of diminishing returns. I doubt the average shooter needs NASA'ish calculation accuracy though Smile


-DallanC
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Elvis
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Joined: Jul 27, 2008
Posts: 6970
Location: south island New Zealand

PostPosted: Fri Nov 25, 2011 12:53 pm    Post subject: Re: About ballistics and some 'constants" Reply with quote

man that sort of a difference could be a field mouse farting half way to your target!!! great job DallanC Its great as is is thanks again.

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Ghost
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PostPosted: Mon Nov 28, 2011 4:55 am    Post subject: Re: About ballistics and some 'constants" Reply with quote

Surprised Where's that fieldmouse

Very Happy
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