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Up/Downhill corrections

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Unread 05-14-2008, 07:44 PM
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Ok, I sent an Email to Gerald Perry of Perry Systems and we chatted on the phone as well.
This is the email that I sent.

From: JWP13051@aol.com [mailto:JWP13051@aol.com]
Sent: Wednesday, May 14, 2008 9:06 AM
To: gperry@perry-systems.com
Subject: Uphill, Downhill

The Exbal program as far as I can tell doe not have a feature to indicate if the angle is uphill or downhill. The reason that I ask is that I was involved in a discussion and the affects of gravity on shooting uphill or downhill at 45 degree of angle. I took the position that it did not matter and a couple of guys claimed that gravity would effect velocity at Longer distances is this fact or not


This is the reply that I recieved from Mr. Perry

Gravity is not the issue for any minute difference between up hill and down hill shots. It is the air density differences which are pretty small unless you are shooting from an airplane. J

Gerald L. Perry
Perry-Systems - Ballistic Calculator Software

__________________________________________________ _____________________

When speaking with Mr. Perry he went into a little greater detail as to the many variables of shooting uphill vs downhill as to why that thier is no difference in the trajectory. He stated that without being adapted at advanc ed calculous one would not understand all of the math equations and many variables that goe into predicting trajectory. I asked him about programs that allow imput for positve and negative angles and Mr. Perry stated that if the program showed a difference in trajectory then it was incorrect.

Anyone that wants to ask Mr. Perry these question just go to his web site and get the contact info and ask away... It boils down to whom do you believe is correct and I believ that Exbal is IMHO.

Yes we should all keep this civil, and I applogize for my part..
range it,check the wind, dial in correction, aim and only one shot
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Unread 05-14-2008, 08:08 PM
Join Date: Jun 2007
Location: North Idaho
Posts: 1,891

Thanks for the invite. If we have two programs that are not the same then one is wrong and one is right or they are both wrong. The only way to prove out is to shoot them. This is the kind of work I do for Nightforce scopes on occasion, proofing ballistic models. The only way to prove it out would be to find a location that allows long range over 1000 and a fairly steep angle like 30 degrees or better and access to both the shooting postion and target easily so you can shoot it each way in the same conditions and compare results. I just have to know. I tend to lean toward the theory that it won't make as much difference as the one program shows. The question has been raised and now we need to find the answer.
Shawn Carlock

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Unread 05-15-2008, 10:54 AM
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Originally Posted by pstimac View Post
Let's see... I just checked Exbal's site and see that they don't ask for a latitude or compass direction of your shot... so they are not taking the coriolis effect into account.......hey... maybe you can use this as proof that the earth is not round at your next "Flat Earth Society" meeting.

Exbal does indeed have the ability to take into account Coriolis effect, but this is not necessary with rifle rounds. In talking to Mr. Perry he pointed out that the effect is less than the ability to correct the scope and that it is of course necessary when firing Mortars and artillery field pieces that shoot at high angles and are in the air for sometime.

Perry's Ballistic Calculator

Scroll down and it shows which programs have the feature. I will hitch my wagon to Exbal
range it,check the wind, dial in correction, aim and only one shot
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Unread 05-15-2008, 07:01 PM
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My Exbals must be older versions than described because mine do not calc spindrift or coriolis. There are no inputs such as lattitude or twist direction in mine.

A Varmint Hunters mag has a write-up from Pejsa defining coriolis as around 1moa at 1kyd depending on various things. It can be more or less. He sums errors like this up as insignificant for similar reasons as Perry. But all the insignicants certainly add up, and I worry about ballistic software providers who generalize 1moa errors..
Seems like they're in the wrong business.
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Unread 05-15-2008, 07:06 PM
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Perry told me that the max coriolis effect at 1K was less than 2" (1/4 MOA is 2 1/2") so it appears that we have not found any agreement here with the various program providers..
range it,check the wind, dial in correction, aim and only one shot
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Unread 09-13-2008, 09:50 AM
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Re: Up/Downhill corrections

Originally Posted by Buffalobob View Post
I am not going to get into an argument about it since I need Shawn to send me another cheekpiece being as I already used the one I ordered on a different rifle and now my new rifle is nearly done and it still needs a cheekpiece.

The acceleration of gravity is 32.2 ft per second squared.

The equations for calculating the velocity effect is 1/2 GxTxT. So for a time of travel of one second which would be about a 1,000 yard shot, the acceleration would generate a velocity of = 1/2x32.2x1x1 = 16.1 fps. If an angle is involved then multiply it by the sine. Thus, at 1000 yards we would see a small effect.

Now then lets us shoot at something 2000 yards away and things will get significant because "time of travel" is a squared function and it will take about 3.5 seconds to get there.

So the added or subtracted velocity = 1/2x 32.2x3.5x3.5 = 197 fps. Correct that for the angle of say 45 degrees and you get 197 fps x Sine 45 (0.707) = 139fps. (we use the sine function rather than the cosine function)

Being as one shot is uphill, its velocity is reduced by 137 fps and the other one is going downhill, its velocity is increased by 137 fps. So total difference is 274 fps by the time it reaches the target.

This is a simplified calculation that assumes the bullet path is a straight line and that is not true. The bullet travels along a curve and depending upon the angle a bullet fired uphill may actually start out going "up" but then change to "level" flight at some point and then tip over into "down" flight as it nears its target.

Think about things like this. If you go out in a boat and fire a 22 rifle straight up in the air gravity finally stops it and then begins to accelerate it back down and you will see and hear it "plunk" into the water. This is the simplest of all situations. If you have never done this then you simply don't meet minimum qualifications to be a redneck.

This is what I think. Perhaps I am wrong. It would not be the first time in my life I was wrong.

Anyway, I have no intentions of arguing about it. I use Exbal on my PDA and JBM on my PC.
One of the problems with this argument is that it assumes we are shooting in a vacuum with no atmosphere - I think. If you took this to an extreme, a bullet shot straight up and then falling back to earth would be going at the same speed when it got back to earth and hit you in the head. They don't. The friction of travelling through the atmosphere is a big factor and ensures that the bullet is slowed as it returns to earth to a terminal velocity much slower than the velocity that it left the muzzle at. If it has an effect, it's so small as to not be relevant.
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Unread 09-13-2008, 02:47 PM
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Re: Up/Downhill corrections

I think. If you took this to an extreme, a bullet shot straight up and then falling back to earth would be going at the same speed when it got back to earth and hit you in the head. They don't.
But we agree gravity is a downward force and it only affects things in one direction - downward.

So to reword the example I use to support my hypothesis ( I might not actually be correct).

If you go 2000 yards up in the sky and shoot a bullet straight down then the acceleration from gravity will add about 197 fps to the velocity you observe as it strikes the ground.

If you fire a bullet straight up then deceleration by gravity will reduce the velocity observed at 2000 feet by about 197 fps.

There will be a total difference in velocity of 394 fps approximately (the effects of air drag will cause the actually added or subtracted velocity to be somewhat less than calculated).

This is true irregardless of whether you shoot in a vacuum or not.
The Smokin Fur Rifle Club
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