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? about ballistic programs

 
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  #8  
Old 04-27-2009, 07:16 PM
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Re: ? about ballistic programs

Quote:
Originally Posted by LouBoyd View Post
It depends on why your trajectory doesn't match the calculations.

Is the manufactures published BC wrong? Some are most are close.

Is your chronograph wrong? Maybe if you chronograph is triggering on the muzzle blast shockwave instead of the bullet.

Did you enter the scope/sight centerline over the bore correctly in the ballistics program? That's very important if you calulated clicks/moa to match. To nit pick that should be the distance the line of sight passes over the centerline of the bore >AT THE MUZZLE<.

It's not likely the calculations are significantly in error. Just about all software for PCs uses the same ballistic tables and basic equations
published by Robert F McCoy of the US Army Aberdeen Balistics Research Labs. There a notable exceptions like Art Pejsa's program.

So what's left? The atmosphere of course.

I mostly shoot at 5600' elevation. If I don't set the air density correctly the calculated trajectories are significantly different from actual trajectories. Air density can be calculated from temperature, standard barometic pressure, elevation, and humidity or it can be measured directly with an air density gauge which actually weighs a volume of air.

I agree 100%. There's alot of variables that have a big influence on trajectory that alot of people don't account for, on the given day that they're shooting. That's why it's important to do alot of shooting in differen't conditions to give an educated opinion.
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  #9  
Old 04-27-2009, 07:22 PM
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Re: ? about ballistic programs

Quote:
Originally Posted by 300saum View Post
There's alot of variables that have a big influence on trajectory that alot of people don't account for, on the given day that they're shooting. That's why it's important to do alot of shooting in differen't conditions to give an educated opinion.
That is about the best advice yet. It is time spent at the LR line that gets it done. Not how if my scope is really .26 MOA and my BC is really .523. Gather practical data, use practical data and be happy!
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  #10  
Old 04-27-2009, 08:31 PM
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Re: ? about ballistic programs

well once I run my tests and establish what I'm gonna use for my BC, during practice I enter these conditions
bar-pressure
temp
Range
and of course wind

also, I try to do most of my practicing in the same conditions I hunt in, during season the temps are 30-50 degreesF, and normal Bar-Pressure is around 31.7, I don't enter angle of fire unless its more than 20 degrees.
I was under the impression that some makers established a BC of they're bullets and adjust it for sea level and some don't.

I also set my chrony up at 5 yards and back drag the ballistic program to get a true (or truer) MV.

Just trying to figure out why it works.
RR
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  #11  
Old 04-28-2009, 07:48 AM
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Re: ? about ballistic programs

Quote:
normal Bar-Pressure is around 31.7
RR,
Where do you live? You might want to check your barometer. Standard barometric pressure at sea level is 29.92.

From: Pressure on Earth, Highest

""The highest recorded atmospheric pressure, 108.57 kPa (1085.7 mbar or 32.06 inches of mercury), occurred at Tonsontsengel, Mongolia, 19 December 2001.""

If your routinely measuring barometric pressures of 31.7, I suspect your gauge may be off. Just something to check.

Quote:
I was under the impression that some makers established a BC of they're bullets and adjust it for sea level and some don't.
All of the bullet makers advertised BC's are corrected for standard sea level conditions. Not doing so would result in useless BC's. The important question is: which standard atmosphere are the BC's corrected for? (that's right, there's more than one standard definition).
The Army Standard Metro (ASM) 'standard' sea level conditions are: 29.53"Hg, 59 deg F, 78% hum.
The International Civil Aviation Organization (ICAO) 'standard' sea level conditions are: 29.92"Hg, 59 deg F, 0% hum.

The ASM atmosphere is the older of the two, and was updated to the ICAO somewhere around the '60s (I think). Some bullet makers still use ASM. Berger uses ICAO. There's about 1.7% difference between the air density defined by the two standards, which correlates directly to a 1.7% difference in BC. As long as the ballistic software uses the same atmosphere as the bullet's BC is corrected for, there's no problem. There's only a problem if there's a mismatch (bullet and program assume different atmospheres). Worst case scenario, it's only a 1.7% error, so not that big of a deal, but a potential problem anyway.

Take care,
-Bryan
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Last edited by BryanLitz; 04-28-2009 at 07:57 AM.
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  #12  
Old 04-28-2009, 07:52 AM
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Re: ? about ballistic programs

A 3% error in the chronograph requires a 13% adjustment in BC to compensate.
A 6% error in the chronograph requires a 32% adjustment in the BC to compensate.


That is why I do not own a chronograph. I have no data to dispute my word when I decide to exaggerate my muzzle velocities.


In my testing of the Nosler 160 AB in a 7mm Wby out to a 1000 yards, I found the BC and the advertised top MV to be exaggerated by Nosler. Unfortunately (or otherwise) when I converted that rifle to a different caliber I erased all the computer files on the 7mm Wby so I do not have my exact numbers anymore except to remember them as being slightly low.

I find that the 115 Berger in 6mm is almost exactly the old published value out to 1K but with my methods I would not really be able to discern the small adjustments BSL made to the BC to say whether he is right or I am right. Perhaps if I let my daughter shot the groups so they were tighter I would be able to reduce the shooter error inherent in shooting drops.
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  #13  
Old 04-28-2009, 08:25 AM
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Re: ? about ballistic programs

Quote:
Originally Posted by Buffalobob View Post
A 3% error in the chronograph requires a 13% adjustment in BC to compensate.
A 6% error in the chronograph requires a 32% adjustment in the BC to compensate.


That is why I do not own a chronograph.
That's why I own 2!!!

Shooting thru the Oehler (8 foot rail) and CED on several occasions has yielded agreement within +/- 10 fps (usually w/in +/-3 fps). Now that doesn't guarantee that they're both right, but it gives me more confidence than if they disagreed.

-Bryan
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  #14  
Old 04-28-2009, 09:30 AM
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Re: ? about ballistic programs

Quote:
Originally Posted by bsl135 View Post
That's why I own 2!!!

Shooting thru the Oehler (8 foot rail) and CED on several occasions has yielded agreement within +/- 10 fps (usually w/in +/-3 fps). Now that doesn't guarantee that they're both right, but it gives me more confidence than if they disagreed.

-Bryan

Bryan,

I use a CED M2 with the IR screens. Do you use the IR screens, or the regular ones?

Not shooting straight through a chrono can also impart errors, also in my experience, the closer to the sensor the bullet passes, the better.

The farther away a sensor is from a bullet, the wider area the sensor sees (like the beam of a flashlight, it gets larger the farther is shines). I'm sure these things can add up to some error, but have never heard how much it can be.

AJ

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