Just how does high humidity &high temps affect accuracy?

[jonbearman]

The title says it all.yesterday it was hot(90) and 92% humidity. My normal load acted goofy ,2 would go in 1 hole and the rest would start going a bit haywire. The groups were like a quarter than the usual 3/8th's size groups I am used too. It is a savage with a bull barrel- 1in10 mcgowen match barrel with a sss trigger and it's bedded,and it is in a bvss stock. I shoot off a hart rest and a bumble bee bag.

 

[jbpmidas]

In weather extremes, the fault may lie in the shooter more than we care to admit. I live in Mississippi, and I am very familiar with the type of weather you're referring to. I don't shoot my best in cold weather either.

I do believe, sometimes, high temperature can raise pressures with certain powders and it won't let barrels cool down, which may explain you having two in one hole and the rest spread out.

 

[Scotsman1]

John, I'm in South Louisiana, and Ive been playing hell keep mine in tune as well. The humidity in the swamp is madness. I too have been shooting a 1-10 for a week now, since I changed my load to shoot 87,88, & 90's. Two good ones and a flyer! Ran a ladder test yesterday and got crazy results with a super low load! Normally running 31-32.5 grains of Varget. And I dropped it all the way down to 28g. Low vel, but it finally shot low 3's! Terrible was the best I got. Went home disgusted and sweaty. Idaho is calling my name!

I'm glad you posted this, curious to learn some info from the fellas.
Scotsman

 

[wnroscoe]

2 in a hole and one out is classic double grouping. This is a sign that your load is out of tune, I know, obviously.

I've ran through this time and time again with various barrels/calibers. 99% of the time, for me, the culprit has been seating depth.

For 30 calibers I try to find a case neck that measures .306", 6mm's @ .241" and so on, you get the picture. Don’t get too caught up on the favorite powder flavor of the day, if your rifle doesn’t like it it'll let you know......listen to your rifle.

I usually start from a soft seat and let the chamber finish the seating job. If strong land marks are on the bullet I'll shorten the COAL by -.010" and re-check. This is usually where I start. With VLD's I'll start with what ever my soft seat method gives me.

Pick a starting point and run a ladder, chances are, you'll find a load pretty quick. If nothing shoots prior to hitting pressure, change powder.

 

[vmaxpro]

Out of all my guns and different loads the only one that will shoot in any conditions is my 223 with varget. I also noticed cci primers dont seem to like high humidity, but that was just my observation

 

[LHSmith]

I have never seen a definitive explanation as to why atmospheric conditions affect accuracy so much. My guess is it's such a complex combination of both interior and exterior ballistics, that no one has ever truly got a handle on it. Although Hodgdon markets some powders as "extreme", they still are affected by weather changes.
The only way to get a handle on it is to shoot in a variety of conditions.....and shoot a lot and learn that particular powders quirks.
This is why BR competitors are constantly adjusting their powder charge throughout the day ( tuning)......or depending on the weather forecast, or match locale, will refrain from using their favorite powder because they know it will act quirky and be impossible to find a tune that will be competitive. V133 goes quirky below 30 % RH, AA-2230 goes quirky in high temps, H-322 falls apart in cold temps,the infamous "T" powder fails when temps get real hot.....the list goes on and on........and then you eventually must contend with a new lot of powder which might act different than the last lot ( unless you were smart and bought several dozen kegs).

 

[boltfluter]

John, I'm in South Louisiana, and Ive been playing hell keep mine in tune as well. The humidity in the swamp is madness. I too have been shooting a 1-10 for a week now, since I changed my load to shoot 87,88, & 90's. Two good ones and a flyer! Ran a ladder test yesterday and got crazy results with a super low load! Normally running 31-32.5 grains of Varget. And I dropped it all the way down to 28g. Low vel, but it finally shot low 3's! Terrible was the best I got. Went home disgusted and sweaty. Idaho is calling my name!

I'm glad you posted this, curious to learn some info from the fellas.
Scotsman

I live in Meridian Idaho and it was only 98 deg. here yesterday. But as they say, it's a dry heat. Yea right. ;D ;D . 12% humidity if that helps. All in all, a great place to live.

Regards, Paul

www.boltfluting.com

 

[bheadboy]

i have some curious thoughts on this, loaded at home -
the primer is seated tightly at home temp/humidity, often air conditioned
the power is enclosed by the bullet case neck tension
the first bullet fired clears the bbl of any humidity - high or low with burned gases

So in flight humidity increases resistance, temp reduces resistance, mirage is the result of these two actions with wind drift causing shifts

Consider

Bob

 

[Win94ae]

Humidity doesn't seem to have an effect. Heat effects accuracy/precision.

I've shot in varying humidity and have found no ill effects.

Powder is heat sensitive, some on a lesser scale, (Hodgdon Extreme powders.) I develop loads for different times of year. Those of you that use factory ammo will have to find ammo that works in specific temperatures.

 

[bheadboy]

humidity combined with heat will cause a higher degree of mirage than low humidity.

Bob

 

[jonbearman]

With regards to mirage,does it give you a false sight picture?Also I am using xbr8202 for my powder with 168 gr hornady match bullets in federal premium brass. I am about .020 off the lands and it has always worked in most weather conditions.

 

[bheadboy]

Jon, mirage is a big factor, with a number of components which i will summarise, but the short answer to you is a great big YES

light moves through air,
the image you see is transmitted by light
if DENSITY of air is consisntant the light moves uninterupted
if the density of air varies the light movement is altered as density varies
the density of air is subject to two things, heat which causes expansion and decrease in density
humidty which increases or decreases density with %, high increases density and low decreases density

Finally the real bug on the windshield, WIND, as air moves due to wind, rises due to heat expansion (vertical heat waves) the IMAGe of the target goes through air that is of varing desnity and direction of movement thus the visibilityof the image will be distorted in shape and location.

There are a number of books and articles on how to "read" mirage, but this is the cause.


Bob

 

[Turbulent Turtle]

i have some curious thoughts on this,
(snip)

So in flight humidity increases resistance, temp reduces resistance, mirage is the result of these two actions with wind drift causing shifts

Consider

Bob

Curious thoughts indeed, considering the fact water vapor is less dense than air. So in high humidity the bullets actually encounter less resistance and will fly faster. This is also the reason you can sleep at night without hearing clouds crashing to the ground.

 

[bheadboy]

Bayou, you aare right i mis typed, humidity decreases resistance , what i ment to type was humidity increases image movement.

Bob

 

[jonbearman]

This is the stuff I need to sharpen up on. Thanks Bob.

 

[bheadboy]

Joh,

some thoughts i put together for my sons and grandchildren

a few facts
if the waves are moving right to left the wind is moving right to left and the target is to the right of where you see it

if the waves are boiling verticle the image is above where the target actually is

if the waves are moving right to left at a 4:00 to 10:00 angle the wind is moving the image up and to the left the actual target is to the 4o point

if the waves are moving left to right 7:00to 1:00 the target is actually at the 7:00 area

the same is true of a wave movement from 10-11:00 towards 4-5:00

The same is true of waves movement from 1-2:00 towards 7-8:00

The wave direction indicates the wind direction and the target is alway at the sorce of the wave direction.

the intensity of the wind determines the distance the image is actually moved

12 and 6:00 winds offer the least movement and usully the target is more clear
3-9:00 winds give full effect to movement and deformed image
1-2:00 and 4-5:00 and 7-8:00 and 10-11: give about 1/2 the target movement vs its actual position

Wind speed/vertical boil is the determining factor as to how far the image moves from true

Everything above is for a wind behind you -- if the wind is in your face, the movemet of the image you see is the reverse of that above, basicly a right to left mirage with wind in the face will move the image to 7-8:00 - low left vs the actual target position,
etc.


Bob

 

[kentuckytroutbum]

Listen to bheadboy, he is dead on about the effects of mirage. I've seen it so many times at the 300 & 600 yd line for NRA HP matches.

If you want to see the boil, focus your spotting scope at a point halfway to your target. Without changing focus, look at your target, you'll see the boil, and the direction that it is moving. Then compensate for it as bheadboy states.

Bill

 

[doc gordon]

my 1000 range,private and i'm an occasional guest,has the middle 500 over a lake.i notice a distinct vapor trail whe others are shooting but only over the water however on extremely humid days i may notice most of the course.it seems to me this would have to slow bullet!what are your thoughts.south al is humid most of the time !!

 

[bheadboy]

this is a vapor trail just like you see when a air line jet fly over at high altitued.

friction of bullet going through the moist air and the vacume as air is pushed aside caused condensation which quickly is reabsorbed

Bob