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Zero shift on travel to new location.

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Unread 01-02-2005, 10:45 AM
Join Date: Jun 2007
Location: North Idaho
Posts: 1,891
Re: Zero shift on travel to new location.

No doubt that gravitational variances due to latitude and density will effect bullet flight. I believe that in most cases this effect to be very small,(much like the rotation of the earth causing windage issues when firing north or south). The situation that I have run into has occured at the same location on different trips to the range. With the gravity issue constant, I just can not see any other place to turn other than conditions not read correctly or not read at all. If we can come up with conclusive evidence that a particular effect is causing this zero change in the degree we are seeing, I am all ears. Since I have no way to measure or guage thermal updrafts down range, my ideas will probably remain theory even though I believe them to be correct. This is an interesting topic, I had not thought of gravitational variances until it was mentioned. I look foreward to more info on this one. [img]/ubbthreads/images/graemlins/smile.gif[/img]
Shawn Carlock

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Unread 01-02-2005, 11:05 AM
Platinum Member
Join Date: Sep 2004
Location: on the rifle range in Utah
Posts: 2,704
Re: Zero shift on travel to new location.

Brown dog,
I like that phrase-ballistic nerdity! True, true.
Yes, as the link says, it is a very small amount that this effect has, but you're right. There are other factors involved that seem to make up the rest of the voodoo. I wasn't even considering all the other obvious stuff like thermals, wind, and humidity. These have a much bigger effect than this gravity thing, but I would bet that gravity anomalies have more to them than what meets the eye! Murphy's law seems to have it out against shooters more often than not. On top of that, it is awfully hard to make a scientific observation of this phenomenons actual impact because you can't eliminate the other variables in the equation. One little puff of wind at the apex of your trajectory, or one little raindrop falling on your bullet would render the experiment void. If only we had access to indoor 1000 yard ranges at home and at some place 6 degrees north of home!
I will see if I can get some of Pejsa's article on a link or something for you. It is very interesting. He also says there is difference in impact if your shooting west and east! Because the earth is spinning at something like 1520 fps toward the east, a bullet fired from the west to the east at 1000 yards is going towards a LOWERING target that is lowering at 2.62 inches per second! He also talks about shooting at the poles, and shooting north and south. And he throws into the mix firing in different air temps and barometric pressures. I'll tell you more about it later when I have time to type.
thanks, --goodgrouper
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Unread 01-02-2005, 11:16 AM
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Join Date: Oct 2001
Location: Altus, OK
Posts: 180
Re: Zero shift on travel to new location.


Great post on the gravity. I too am a bit a ballistic geek. Thanks for the informative post. The first thing I thought of when I read his post was thermals, but it seemed a little extreme in the variation. I didn't consider humidity as I'm sure he's practiced in different hunidities and ambiant temps.

Excellent post! Two thumbs up on the info!

Learn from others mistakes, you won't live long enough to make them all yourself
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Unread 01-02-2005, 04:32 PM
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Join Date: Aug 2003
Location: NC, oceanfront
Posts: 4,222
Re: Zero shift on travel to new location.

I sounds like a vertical effect of earth's rotation(also cause of coriolis). Great article by Ken Howell on this in latest VH mag. Were you shooting 270deg bearing or 90deg. Well maybe not, seems only significant at long distances.
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Unread 01-02-2005, 05:52 PM
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Join Date: Sep 2002
Location: Nebraska
Posts: 313
Re: Zero shift on travel to new location.

What about the location of the sun? Meaning was it at your back at your home range and then in front or to the side of you at the new location? I am no ballistic guru but have read in a few different places that David Tubb says the light location plays a role on your zero. To what extent i don't remember. I will look for some of the links where he posted it. I am sure some of these problems are addressed in his new book.
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Unread 05-11-2011, 09:18 PM
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Join Date: May 2011
Posts: 68
Re: Zero shift on travel to new location.

Gravity changes, yes. We have all heard of the Coriolis Effect. The earth is spinning at about 1,000 miles per hour at the equator. That is fast. If you shoot north or south at the equator, you should have almost no Coriolis Effect as the target and you are moving at or near the same speed and the target is not rising or dropping in relation to you. Hold on I am getting there....

If you shoot east, your bullet should hit high since the earth rotates to the east and the target is dropping in relation to you. If you shoot west, your bullet should hit low. Get it?

You are standing on a ball that is spinning. When the bullet leaves the gun, it will continue to travel toward the original target location. The earth does not stop spinning when we shoot, so the angular relationship between you and the target is ALWAYS changing. Lets say, just for an example, that the earth rotated 1 while your shot was in the air. Your shot would be off by 1 because both you and the target are revolving around one another. This is the coriolis effect.

I guess a classic example would be shooting at the moon. You would have to aim where the moon WILL be in the amount of time it takes the bullet to get there.

If you change latitude and/or shooting direction, your zero can change.

I hope that is clear enough.
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Unread 05-11-2011, 09:25 PM
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Join Date: May 2011
Posts: 68
Re: Zero shift on travel to new location.

EDIT: since 1= 60 minutes, the shot would be off by 60 MOA.

There, that should be better.
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