Sight variation factors

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semtav
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Sight variation factors

Post by semtav » Fri Jan 29, 2010 1:05 pm

Trying to figure out the basics for sight variation.

Last few times I've been out to the gong range practicing, my sight elevation was pretty close to the other times. Yesterday, they all needed an extra 5 moa except one target. The second to the farthest (approx 500 yds) was the same as before.

Temp was at least 10 deg colder, so I figure that is one factor.

Wind? It was less than 5 from the left where the other times its been either less than 5 from the right or direct headwind.

Humidity ? More humidity = less air density,

Air stability?

these are the things that affect airplanes, although temp affects it oppositely( Colder air= better performance), wondering if humidity and others are same or opposite.


And why would 4 targets need a 5 moa change and one no change?

this has all been with 40-65 and smokeless (7544 powder)
thanks
Brian

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montana_charlie
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Re: Sight variation factors

Post by montana_charlie » Fri Jan 29, 2010 2:30 pm

Colder air is denser, so the bullet flies slower.
A wind from the right will cause a right-spinning bullet to climb up on top of it while it will dig under a left wind.
These are a matter of physics, so they are to be considered.

But answering your question is beyond my experience and knowledge level.

CM
Retired...twice. Now, just raisin' cows and livin' on borrowed time.

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Ranch 13
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Re: Sight variation factors

Post by Ranch 13 » Fri Jan 29, 2010 2:56 pm

Light makes an awful lot of difference.
I've often time thought a person really should carry one of the old light meters we used to use with 35 mm camera's, and mark the light meter reading in our log books.

41Wyo
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Re: Sight variation factors

Post by 41Wyo » Fri Jan 29, 2010 3:36 pm

Here is something that be of interest, if it's accurate...

Humidity: At the same temperature, air with a higher dew point will be less dense. Density will increase if either mass increases while the volume remains constant or if volume decreases while mass remains constant.

Density of air will vary as the temperature and moisture content in the air varies. When the temperature increases, the higher molecular motion results in an expansion of volume and thus a decrease in density. The amount of water vapor in the air also affects the density. Water vapor is a relatively light gas when compared to diatomic Oxygen and diatomic Nitrogen. Thus, when water vapor increases, the amount of Oxygen and Nitrogen decrease per unit volume and thus density decreases because mass is decreasing.

Water vapor (H2O) is composed of one Oxygen atom and two Hydrogen atoms. Hydrogen is the lightest element at 1 atomic unit while Oxygen is 16 atomic units. Thus the water vapor atom has an atomic mass of 1 + 1 + 16 = 18 atomic units. At 18 atomic units, water vapor is lighter than diatomic Oxygen (32 units) and diatomic Nitrogen (28 units). Thus at a constant temperature, the more water vapor that displaces the other gases, the less dense that air will become.

You may be familiar with the concept that moist air is less dense than dry air. This is true when both have the same temperature or when the moist air is warmer. Said in another way, air with a greater percentage of water vapor will be less dense than air with a lesser percentage of water vapor at the same temperature. Often people erroneously believe that moist air is denser than dry air because very moist air is more difficult to breathe than dry air.

Or…. Fill a 12”x12”x12” box with ½” dia. lead balls (dry or low humidity level air). Weigh the contents. This is expressed as weighing X pounds per cubic foot or the material’s dry bulk density. Remove half of the lead balls and replace them with the same number of ½” dia. wooden balls (moisture in the air) to re-fill the box. Weigh the contents. Which is lighter?....the mixture with the wooden balls. So our medium of “air to be shot through” is lighter with moisture present, hence less resistance.

John Boy

Re: Sight variation factors

Post by John Boy » Fri Jan 29, 2010 11:52 pm

Brian, I too done have an honest answer for the reason or reasons why. But I'll say this ... No 2 days are constant in terms of conditions and there is a possibility shooting the XMP5744 that not doing the 'muzzle up' routine for each shot, the velocity changed. Earlier you indicated you'd like to switch to black powder. With proper reloads using BP, betcha that this mystery won't happen! :D

semtav
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Re: Sight variation factors

Post by semtav » Sat Jan 30, 2010 4:55 pm

this is like Deja Vu, every time I ask something I wind up finding it answered elsewhere:

http://www.shilohrifle.com/forums/viewtopic.php?t=14930

I have a hard time believing humidity has no effect, but they try to discount it in aviation too.

I bought a portable wind and temperature guage, now I gotta get a barometer too :roll:


One of these days Shilo is going to get around to approving my registration. :!:

Or maybe they found out I have a bunch of Brownings and aren't :|

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montana_charlie
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Re: Sight variation factors

Post by montana_charlie » Sun Jan 31, 2010 11:43 am

semtav wroteCOLONOne of these days Shilo is going to get around to approving my registration.
If you've been waiting a week, it probably got sidetracked, somewhere.
Start over and see if that works...
CM
Retired...twice. Now, just raisin' cows and livin' on borrowed time.

semtav
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Re: Sight variation factors

Post by semtav » Tue Feb 02, 2010 12:55 am

montana_charlie wroteCOLON If you've been waiting a week, it probably got sidetracked, somewhere.
Start over and see if that works...
CM

I just looked at their member list in descending Joined date. Looks like they are in the process of being hacked. way too many foreign and strange members just joined last couple days. Think I'll stay away from joining and just lurk for a while.

CrossXstix
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Re: Sight variation factors

Post by CrossXstix » Tue Feb 02, 2010 6:32 am

Try giving them a call and see if they can pull yours up and approve it.


X
David

semtav
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Re: Sight variation factors

Post by semtav » Tue Feb 02, 2010 8:02 am

I sent them an email, I'll let it go at that for a while.

kokomokid
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Re: Sight variation factors

Post by kokomokid » Fri Sep 23, 2011 1:33 pm

I never got on shiloh forum either but I will just lurk awhile also.

checker
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Re: Sight variation factors

Post by checker » Fri Sep 23, 2011 4:22 pm

Aside from getting off the topic of sight variation, I apologize, but in order for me to be able to get on the Shiloh forum, since I do have a website, I had to call them, and tell them I'm real, not a spammer, and while I had them on the phone, I bought something from them. Within 30 minutes I was approved. I don't own a Shiloh Sharps, and I really don't think it matters to them, however I'm sure they would like to sell one to me! :) It's a great forum. So is this one. The ASSRA forum, well, "yawn". :? :? :? Tim :)
"There is nothing simple about a simple point pattern"

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