Temperature and humidity factor - how do they affect my loads?

[elkfirst]

I've seen various posts where long range shooters record temperature and humidity conditions when making what I consider extraordinary shots. I am currently working up some loads for a couple of my rifles, namely a 270WSM and a 7-08. These days, when I go to the range to test my loads, it is usually upwards of 20C (68F). Will the external ballistics change when I go hunting in the fall or winter and the temperature changes drops to 10C (50F) or even down to -10C (14F)? Are the ballistic changes that I hear of affecting bullet path or affecting the way powder behaves. I've even considered putting my loads in a thermos lunch bucket with some isolated ice pack to cool them down and simulate a temperature closer to hunting season. Am I way out in left field here?

 

[Jim Hundley]

In my humble opinion,that is exactly what I do to try to simulate "normal" hunting conditions here in Texas.I keep the ammo cool at home,then put it in a container on top of the ice in my drink chest to keep the components very cool and dry.I also space the timing between shots to keep barrel heating to a minimum and then put my barrel under the trucks air conditioner vents to throughly cool before the next group.I do this when the temperature is above 70F.
My partner has a CO2 bottle rigged with plastic tubing hooked up to a bore guide that is inserted into the chamber and lets the gas do the cooling,while keeping his ammo very cool.Others have suggested wet towels,etc.Hope this helps.

 

[Mikecr]

Air density rises with lower temps.
Your velocities drop with cooler powder.
Your barrel and bedding might change.

Ballistics programs account for air density. You could probably figure velocity change @ 1.8fps x degrees under 70.
Range time at temps helps with equipment prediction.

 

[Jim Hundley]

+1

 

[redbone]

The Temp in Arkansas is 103 an the Heat index is 112.
It is just to hot to shoot For me
Most of the time i do not work up loads in Late June all of July an August.
Redbone

 

[winmagman]

I used to do the ammo in the cooler thing and shoot during the summer, but since mirage is usually worse in the summer, and your heart is beating harder from any exertion in the heat, and I can't stand it when the sweat is running in my eyes, I've given up doing that.

Now I do like Redbone, shoot in the spring, fall, and winter. Besides It just doesn't make good sense to me to develope a load when temps are 65 - 75 degrees warmer than what I hunt in.

Chris

 

[Bart B]

Temperature and humidity do effect ballistic coefficient. BC goes higher as they get higher. Sierra's reloading manual has excellent technical explanations. A good ballistics software program could show the effects on a given cartridge's trajectory both will have when they change.

Shooting cold ammo from a cooler will result in a lower muzzle velocity providing you shoot each round as soon as it's chambered. Leave that cold round in a hot barrel more than 30 seconds and the powder will heat up then produce a higher muzzle velocity to require about a 1/4th MOA elevation change up for each minute it's in the barrel.

Hot ammo or cold, its bullets will still go through the ambient temperature, humidity and atmospheric pressure conditions the round's fired in. To see changes in trajectory for different atmospheres, you just about gotta shoot in them then record your sight settings for each one when that particular load's used.

A good barrel properly fitted won't change point of impact as it heats up and rounds aren't left in the chamber more than 30 seconds before they're fired. I've never seen any accuracy change from 20 to 105 degrees F, 5 to 90 percent humidity and 200 to 8300 feet altitude with the same rifle and ammo so I don't think bedding changes any significant amount. But sight elevation zero settings will change.

 

[elkfirst]

So, what I now deduce is that temperature affects both external ballistics and powder behaviour. Just a few days ago, it was 25C (77F) here in the evening. I shot some loads of 270WSm using Hornady 140gr, behind 59gr of IMR 4831. Grouping was 1.25" and velocity averaged 3175 fps. Yet the IMR manual states a velocity of 3081 fps for a 60.5 gr load of this same powder. My loads clocked nearly 100 fps faster with 1.5 grain less powder. This must be the obvious results of this temperature differential that we speak of. One would seemingly be quite impressed about the gain in velocity, however there must also be a gain in pressure which could be a concern, although my spent casings showed no such signs. I think that from what I'm reading is that likely the idea to keep the loads and barrels as close to hunting season conditions as possible is likely the way to go. Keep in mind, I frequent this site for the great deal of reloading information to be learned, but I am by no means a "real" long range hunter. I shoot big game to 400-500 yds, that's it. I'll likely be huntig anteloppe in September when it's still pretty warm and need to work up my loads now, not a week before season opens.

 

[Bart B]

[ QUOTE ]
Grouping was 1.25" and velocity averaged 3175 fps. Yet the IMR manual states a velocity of 3081 fps for a 60.5 gr load of this same powder. My loads clocked nearly 100 fps faster with 1.5 grain less powder. This must be the obvious results of this temperature differential that we speak of.

[/ QUOTE ]I don't think it's local atmospheric conditions. You probably used a different barrel as well as powder lot and maybe a different bullet, primer and case. All of which can easily result in a 100 fps difference from published load data even in identical atmospheric conditions. Plus two people can easily get more than a 50 fps difference shooting the same ammo and rifle from bags atop a bench just because they hold the rifle differently.

 

[elkfirst]

[ QUOTE ]
Plus two people can easily get more than a 50 fps difference shooting the same ammo and rifle from bags atop a bench just because they hold the rifle differently.


[/ QUOTE ]

I must admit that I didn't think of that, but how do the mechanics or physics of holding a rifle differently than another actually work? - interesting though.

 

[Bart B]

[ QUOTE ]
...how do the mechanics or physics of holding a rifle differently than another actually work?

[/ QUOTE ]It's caused by how hard the rifle's held against the shoulder. A hard hold reduces the amount of rearward recoil which lets more energy push the bullet out the barrel resulting in higher muzzle velocity. You'll get higher velocity with the rifle's solidly fixed in place (butt against a tree?) than fired unheld in totally free recoil.

 

[elkfirst]

Again, I hadn't thaught of that, but it makes perfect sense. Thanks Bart_B and all others for your valuable input.

 

[goodgrouper]

[ QUOTE ]
I've never seen any accuracy change from 20 to 105 degrees F, 5 to 90 percent humidity and 200 to 8300 feet altitude with the same rifle and ammo

[/ QUOTE ]

So let me get this straight. You have never witnessed a sweet spot or "node" get overpressurized from 90 degree weather and shoot worse groups or open up standard deviations?! Boy, somebody needs to find a sweet spot over a chrono on a cold day and then fire that ammo over a chrono on a July afternoon! Can you say "wow"?

Where do I purchase one of these properly fitted barrel lazer guns? I need one badly. I'm gonna have to call Kirby and complain that my gun barrel won't shoot a good group when the crown is so hot it's setting off my neighbors ammo on the bench next to me! /ubbthreads/images/graemlins/wink.gif

 

[goodgrouper]

[ QUOTE ]
It's caused by how hard the rifle's held against the shoulder. A hard hold reduces the amount of rearward recoil which lets more energy push the bullet out the barrel resulting in higher muzzle velocity. You'll get higher velocity with the rifle's solidly fixed in place (butt against a tree?) than fired unheld in totally free recoil.

[/ QUOTE ]

Oh, and by the way, PS did an article on that very topic awhile back and found it to make a difference in the order of something like 10 fps or less. /ubbthreads/images/graemlins/shocked.gif