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Temperature and Humidity

4K views 13 replies 4 participants last post by  docv_73 
#1 ·
How much does temperature and humidity effect trajectory?

The reason I ask is yesterday I shot a 3 shot group at 300 yds all within about an inch and a half. I then dialed in the dope for a 600 yard 24"x36" steel target and shot 3 more. They didn't group great but I could consider that shooter error as I am fairly new at shooting distances over 200 yards. They were all within about 8 inches of each other and roughly the center of the target. I went back out today and shot with the same dope and was shooting about 2.5 MOA high on my first 3 shot group. The temperature I would say was about 10-15 degrees warmer the second day. I'm not sure about humidity but it didn't feel that much different. I guess I'm looking for an explanation for the variation between the two days. Could it have been the atmospheric differences or could have been the ammo was from two different boxes?
 
#2 ·
Temperature and Humidity will not make that much difference at that range. From range to range, all through the year, and all over the midwest, I do not see that much variance in my zeroes.

Odd. The change was strictly elevation, correct?

-Nate
 
#3 ·
Yes. Strictly Elevation. At first I thought maybe when I dialed back to zero I went the wrong way but I set up a 100 Yard target and shot and hit dead on. I am shooting Federal American Eagle 150 Gr. FMJ Boat Tail and the last shot on the first day finished off one box. So on the second day I opened a brand new box. I bought both boxes at the same time but am not entirely certain if they were the same lot. Regardless if it was the same lot or not, I don't think the ammo would shoot THAT different....just throwing out some more info.
 
#4 ·
Was there anything different between day 1 and day 2--such as:
a)how the gun was held to the shooter,
b) how the gun was positioned on a bag/bipod/other,
c) any difference in the time of day that could give different illumination of the target (or range ndicator) ??

Did you clean the gun between day 1 and day 2? oil the trigger? alter the bag? tune the bipod?

Was day 2 at 300 yards or 600 yards?

Temperature can't be blamed for more than 35% of the change in dials.

Did you take careful notes?
 
#5 ·
Day 2 was 600 yds. No difference in position or range. Literally left the target in the same place overnight and went and shot the next day. The only thing I did was took down my trigger weight by about a pound, maybe a little more. Took some notes, didn't really go crazy, just temperature, elevation and windage dialed onto the scope for the 300 yd shots and the 600 yard shots.
 
#6 ·
Different bullets with different BC will vary SLIGHTLY from this, but as a general rule you will see about 1 MOA for every 20 degrees. If you sight in at 70 degrees, the next trip out is 50 degrees, ALL OTHER VARIABLES UNCHANGED) you should see about a 1 MOA drop. The bullet will rise with temp increase. A raise in humidity will cuase a bullet drop, and a drop in humidity will cause a bullet rise. The temp of the ammo will make a differnce too. If it is an overcast day on day one, but on day 2 it is sunny and your bullets are exposed to the sunlight, and are heated up, this will caus esome rise also.

A difference of 2.5 inches seams like a lot, BUT...

If day one was overcast with 65% humidity, and day 2 was sunny - warming your rounds, 15 degrees warmer, with 20% hunidity you would see a significant bullet rise. 2.5 inches still seems like a lot, but it is possible if the conditions all conspired against you. That's the number one reason for a shooter's log book. To record the conditions each time you shoot, so you can see the difference between each outing based on the given conditions, and life of your platform.

I will have to look it up to remember what it is called, but there is a number used to adjust bullet dope, that is one number based on a combination of elevation, temp, and humidity - as an atmospheric condition, istead of having to adjust individually for each condition seperately. The more expensive "wind guages" have that built in. They will give wind reading, elevation, temp, and humidiyt, and then that atmospheric condition number, I just can't recall what it is called.

If I find it, I will post the info. I think there is even an online article somewhere I saw once about it, If I find that I'll post the link too.

Edited to add the following after rereading your post

It looks like the 2.5 inch difference is at 600 yards? If that is correct, a difference of 10 - 15 degrees warmer, a rise of under 1/2 MOA would seem about right. If that is 100 yards, my previous posting would stand at the suprise of that much difference.

But, how good is your scope, and have you box tested it? It might be an issue of repeatability of your scope's elevation dial.
 
#7 ·
Another possiblity... did you shoot at the same range, from the same place, and facing the same direction?

A lot of guys will roll their eyes at the mention of this word, but Coriolis is a real thing that can cause havock.


There are 2 types of coriolis (one causes verticle POI change, the other causes horizontal POI change), one is based on where you are on the planet (lattitude from equator, and north or south of the equator) and the second is based on what direction you are shooting.

The numbers are constant, and do not change, so if you shoot from the same place, and facing the same direction, once you adjust the dope into your scope, it will always be the same. But if you zero on the range facing 80 degrees to the left (west) of true north, and the next time out you shoot from the same place, but 80 degrees to the right (east) of true north, IF TRUE NORTH IS YOUR CENTER, this can have a difference in impact anywhere from 6 - 14 inches at 1,000 yards, depending on BC, twist and twist rate. That's the big advantage of custom barrels being left twist instead of right twist. In the northern hemisphere, that allows the rifle twist's "spin drift" (gyroscopic drift) to counteract the coriolis effect, virtually cutting the effect in half.
 
#8 ·
Doc,

I'm useing a Leupold Mark 2 6-18x40mm scope and no I have not tested the repeatability of it. Could you post some tips on how to do so?

Also I was shooting 2.5 MOA high at 600 yards the second day, not 2.5 inches. The first day I had 14.5 MOA dialed and the second day I was shooting way high with the same elevation dialed. I droped my elevation to 12 MOA and was getting hits on the target. I appreciate your help and input. You seem very knowledgeable.

Murph
 
#9 ·
Murph McMannus said:
I'm useing a Leupold Mark 2 6-18x40mm scope and no I have not tested the repeatability of it. Could you post some tips on how to do so?
Alright. First: chekc your mounts, rings, bases, and what'avyous for proper torque. #6 screw, bases to rifle should be 15 in/lbs; #8 screw, bases to rifle should be 25 in/lbs; mounts to base should be whatever the manufacturer recommends, if they are heavy steel like badger, then go with 65 in/lbs, if not, start with 50 in/lbs; #6 screw, ring halves, should be set to 15 in/lbs with a STRICT MAXIMUM.

Now then, set up at one hundred yards. This is called a "Box Drill:"
1) Fire a group of three shots, note impact location of all three for later ref.
2) Adjust scope elevation "UP" 4 minutes of angle.
3) Maintaining ORIGINAL point of aim, fire a second group of three shots.
4) Adjust scope windage "RIGHT" 4 minutes of angle.
5) Maintaining ORIGINAL point of aim, fire a third group of three shots.
6) Adjust scope elevation "DOWN" 4 minutes of angle.
7) Maintaining ORIGINAL point of aim, fire a fourth group of three shots.
8) Adjust scope windage "LEFT" 4 minutes of angle.
9) Maintaining ORIGINAL point of aim, fire a final group.

The final group should land RIGHT on top of the first group, and the other three should form a "perfect" square (within the accuracy limitations of the shooter and rifle system). If it isn't or doesn't, let us know before proceeding further.


Now that you are done with that, it's time to check your retical perpendicularity.

1)From your original zero from the last drill, move elevation "UP" 10 minutes of angle, and fire a three shot group, again, maintaining original point-of-aim.
2) Again, move the scope elevation "UP" 10 minutes of angle, and fire a three shot group, maintaining original point-of-aim.
3) Safe the rifle and range, and go down to the target with a plumb-bob of some sort, and means to mark the center of the groups.
4) Hang the plumb line such that you may judge the tracking of the three groups. The centers of those three groups should all fall along the vertical plumb line (do NOT substitute the target, target frame, posts, etc for your datum...as they are almost never square). If the groups do not fall on the line, then either your scope is not mounted correctly, the reticle was installed at a cant, (an ongoing QA/QC problem with Leupold) or both.


Well, let us know.
-Nate
 
#10 ·
That's what I was going to say...

The plumb-bob is a trick I need to remember. I like that. I usually go into making a level target, then leveling the gun and firing at the cross section of an upside down capital "T." My way you shoot at where the lines meet, add 8 MOA up, and aim at the same spot. The round should touch the line about 8 inches high. The plumb-bob seems a lot faster, and you don't have to bubble level the target when you set it up.

The box test, I do the same, except I shoot 1 round then go 4 MOA left, than do the box in 8 MOA increments, three times so there is eventually a 3 shot group at each corner, fired one at a time, then I come back to zero and fire 2 more to make a 3 shot group there, to make sure it will "return to zero."

His way is cheaper. :D

2.5 MOA is A LOT. Possible, but as I said, would have required the level of conspiracy in nature that God usually reserves for screwing with me. I doubt He dislikes you that much. :wink:

You should probably take the above advice and make sure nothing has come loose, is torqued properly, and loc-tite it all.

Good luck.
 
#11 ·
Murph McMannus said:
Doc, ...

You seem very knowledgeable.

Murph
My wife thought I was sexy once too. :p She also thought I liked safe jobs that would have me home nights and weekends. 8) Keep that in mind.
 
#12 ·
natdscott said:
...a plumb-bob of some sort...
hehehe...I should have mentioned this: by "some sort" I mean a 6-foot piece of mason's line tied to a 1/2" drive socket or other relatively heavy object. Hang it from a nail, heavy tack, staple, whatever really...just as long as it is free to align itself vertically along the paper.

-Nate
 
#13 ·
Thanks Guys for the advice.

Doc,
Yes 2.5 MOA is ALOT! Thats why I am hopeing that it was some sort of mental mistake. That way I could give myself a kick in the rear end and take it as a lesson learned.

Nate,
I'm a surveyor, I have a few actual plumb bobs and gammon reels laying around that I can use. Question though... Is there a certain way I'm supposed to mount my scope so my crosshairs are perfectly (or as close as possible to) square? The scope was mounted when I purchased the rifle, but I'd prefer to know for sure rather than assume they are square.
 
#14 ·
Look into a "level-level-level" scope mounting kit. There are lots of scope leveling devices out there, but most make the cross hairs level with the stock or with the acope base. The scope needs to be level with the centerline of the bore, which is closest to achieveable by leveling with the action/receiver.

If it is properly leveled, it may not look level to the shooter, and just because it looks level to you, doesn't mean it is.

A way to do it, without the kit, is done the same way as checking it afterwords, which you already have the instructions for, above.

The plumb-bob test is the scope level test. The second shot?group of shots after adding the MOA elevation should touch the line 8-10 MOA above the point of aim. If it does not, the scope is canted. Loosen the rings and make TNIY adjustment to cant it in the direction away from the group towads the line. Carefully retighten to the proper torque, and shoot again. When it falls where it is supposed to, loosen and loc-tite, then properly torque down each scre ONE AT A TIME.

Wheeler makes a kit you can buy for between $80 and $110 (depending on where you get it) in either a 1 inch scope body or 30 mm scope body kit. You can buy the extra kit parts (add 30 mm to a 1 inch kit) for about $50. The kit has the level-level-level kit, alignment tools, lapping rod, lapping compound, adjustable torque handle, with interchangeable heads, printed instructions and a DVD for how to mount a scope. All you need is the scope, rings base, and a bunch of annoying know-it-alls like us to tell you how to check your work, and you'll be all professional-like.
 
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