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Calculation for different bullets without modifying optics
Help / Support / How-to's / Discussion related to PointBlank Software and External Ballistics in general
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lince
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Joined: Jul 19, 2006
Posts: 32
Location: Madrid, Spain

PostPosted: Fri Aug 18, 2006 1:31 am    Post subject: Calculation for different bullets without modifying optics Reply with quote

Howdy everybody!.
This is a generic ballistic problem, but maybe some PB expert can help me.
I'm a little bit lost with a problem: I'm able to calculate trajectories for different bullets and adjust the sight according to them
But I'd like to calculate the effects of leaving the sight adjusted for one of those bullets and them calculate the sighting distance for the other bullet. Is it possible?.

In practical terms:
Bullet 1: Remington Express Core Lokt, 7mmRM, ref. R7MM3, Pointed Soft Point Core-Lokt, 175 grains, BC=0,427, muzzle velocity 2860 ft/sec

Bullet 2: Remington Premier Ballistic Tip, 7mmRM, ref. PRT7MMC, Nosler Ballistic Tip, 150 grains, BC=0,493, muzzle velocity 3110 ft/sec

Depending on the game conditions I'd like to use one or another bullet, because of the different bullet behaviour (mainly relative to hard-point or soft-point), so for me it's interesting to know the effects of adjusting the optic sight for one of the bullets and know the ballistic behaviour of the other bullet without modifying the sight. In this way, I can choose the best adjust taking into account that the deviation with the other bullet will be minimum.
Any expert can help me?.
Thanks in advance.
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DallanC
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Joined: Jan 18, 2005
Posts: 3230
Location: Utah

PostPosted: Fri Aug 18, 2006 7:52 am    Post subject: Re: Calculation for different bullets without modifying optics Reply with quote

This is a new feature others have requested. I'm looking into it.


-DallanC
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lince
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Joined: Jul 19, 2006
Posts: 32
Location: Madrid, Spain

PostPosted: Mon Aug 21, 2006 2:38 am    Post subject: Re: Calculation for different bullets without modifying opti Reply with quote

This is good news!. Again, thanks a lot. I'm waiting desesperately for new version Cool
Anyway, do you have some hints to "play" to try to look for some results about it making some home-made calculations?.
Again, thanks in advance.
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DallanC
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PostPosted: Mon Aug 21, 2006 9:02 am    Post subject: Re: Calculation for different bullets without modifying optics Reply with quote

You could look at the difference in Total Drop field between the two and make a rough guess according to that.

Example:

Load 1 has a TOTAL drop of 10" at 200 yards.
Load 2 has a TOTAL drop of 12" at 200 yards.
Load 2 is sighted in dead on at 200 yards.

Load 1 drops 2" less at 200 than load 2 so it should be hitting 2" high @ 200 yards.
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st_albert
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Joined: May 31, 2006
Posts: 38

PostPosted: Wed Aug 23, 2006 5:42 pm    Post subject: Re: Calculation for different bullets without modifying opti Reply with quote

If you have Microsoft Access 97 or greater installed on your computer, there is an Access application called "baltraj" that can be used in the way you described. To get it, go to Fr. Frog's website here:

www.frfrogspad.com/ballisti.htm

and scroll down to the "freebie ballistics program" section near the bottom. But I warn you, it isn't nearly as user-friendly as Pointblank, and you WILL have to read the documentation in order to make any sense out of it. (Well, at least I did Very Happy )
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lince
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Joined: Jul 19, 2006
Posts: 32
Location: Madrid, Spain

PostPosted: Mon Aug 28, 2006 4:32 am    Post subject: Re: Calculation for different bullets without modifying optics Reply with quote

Got it!.

Just in case someone else wants to get some values I'll explain my steps:
1) Get trajectory values for two different bullets (let say bullet1 and bullet2)
2) For bullet1, calculate the straigth line values, given the elevation angle, i.e. the sum of "total drop" + "elevation" values
3) For bullet2, calculate the elevation values from this straight line, i.e. "bullet2 elevation with sight adjusted at bullet1" = "straigh line values" - "bullet2 total drop"

Notes:
1) I performed all these calculations importing all data in MS-Excel, so it's very easy to get the values and also some nice graphs
2) There is one suposition that it's not fully true: the "total drop" values are changing depending on the elevation angle of the trajectory, but assuming little variations in elevation angle, we can also assume that total drop values will remain the same.

I don't know if DallanC is using something closed to this method, but it can be easily implemented for quick comparisons.
Anyway, HTH.
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st_albert
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Joined: May 31, 2006
Posts: 38

PostPosted: Mon Aug 28, 2006 5:56 pm    Post subject: Re: Calculation for different bullets without modifying opti Reply with quote

And then there might be another source of variation in point of impact, if the velocities of the two loads differed in such a way that the bullet was leaving the muzzle at a different point in the barrel's vibrational cycle. This concept is discussed in detail elsewhere on this site:

www.huntingnut.com/ind...cle&sid=17

I did an experiment where I started with a relatively mild load and worked up to near maximum in stages. (The procedure is described in the article mentioned above.) It seemed to me that the differences in point of impact were in some cases greater than could be explained by the different velocities. In other cases, near the "sweet spot" of velocity, the POI did not vary much at all with variation in muzzle velocity.

So I guess ballistic calculations will give you a good first approximation, but it might be wise to test it out in the field as well.

That's what keeps it interesting!
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