In a couple weeks I'm going to be hunting in West Texas and the temperatures will be colder than I am able to test velocities at home. I live in Houston and dont think I'll be able to test velocities below 50 before I go. Temperatures have been as low as the high teens this past week in west texas. I'm shooting a .308 Win. with 208gr Hornady AMax with 43gr of RL15 (1gr short of max load)and at 70degrees I'm shooting 2550fps with 9SD. If I can test velocities at 50degrees will the velocity change from 70-50 continue to drop at the same rate to 15-20 degrees?
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Range Report Temp. and Velocity Change
- Thread starter bjax919
- Start date
Re: Temp. and Velocity Change
The change of muzzle velocity with temperature might not be exactly linear, but, if I were hunting, I'd just assume that it was.
The change of muzzle velocity with temperature might not be exactly linear, but, if I were hunting, I'd just assume that it was.
Re: Temp. and Velocity Change
There would be two parts to this.
First, the exterior ballistics. For any velocity measured at about 20 feet out, there are ballistics programs for the exact bullet shape you're using that will tell you exactly what the trajectory of the shot will be.
For example:
http://www.jbmballistics.com/calculations/calculations.shtml
So, once you have a measured muzzle velocity (regardless of temperature) and a manufacturer's BC for your bullet, you then measure the environment conditions and then plug all those in to the correct program. The mathematical model it gives back will then be really close or right on with what you would shoot in those conditions.
The second part is 'internal ballistics'. That means powder ignition and the pressure curve for your gear, dependent upon temperature, which would result in some particular muzzle velocity. (That velocity is what you'd plug into the first part, above). This part's tougher to figure out, but is specific to any particular set of gear and loading. Who knows if the relationship is linear or not - it's best to just measure it, then you'll know. If you have to SWAG it, then it becomes like shooting across a valley and using an educated guess to say what the winds are going.
The net effect of temperature on cartridges, and therefore muzzle velocity, and therefore ballistics, however, can be generalized in some gear as 1000 feet density altitude effect per x degrees C. (You'll have to do some web searching for 'x').
It's probably negligible however, for .308 loads at hunting distances over 30 Fahrenheit degrees. Wouldn't worry at all about cartridge temperature effects on muzzle velocity over that difference.
What's maybe not negligible (but still not critical for you) is the effect of substantially different density altitudes on your 308 ballistics at hunting distances.
However, on top of that, maybe there's just not much difference between density altitude in Houston and west TX anyway. DA is a combo of station pressure, temperature, and humidity. A change in one can be completely offset by a change in another. When you drive up from Katy through Austin, you're higher and the air a little less dense, but it's also less humid (making it better to live in July then Houston, right?
) So maybe the two things cancel each other out and you don't have to worry about DA change.
Anyway, measure your muzzle velocity at 50 deg F in Houston and use that as velocity input to the program, but use the real expected air temp, station pressure, and rel humidity of your hunting site to prepare your ballistics reference card for the hunt.
There would be two parts to this.
First, the exterior ballistics. For any velocity measured at about 20 feet out, there are ballistics programs for the exact bullet shape you're using that will tell you exactly what the trajectory of the shot will be.
For example:
http://www.jbmballistics.com/calculations/calculations.shtml
So, once you have a measured muzzle velocity (regardless of temperature) and a manufacturer's BC for your bullet, you then measure the environment conditions and then plug all those in to the correct program. The mathematical model it gives back will then be really close or right on with what you would shoot in those conditions.
The second part is 'internal ballistics'. That means powder ignition and the pressure curve for your gear, dependent upon temperature, which would result in some particular muzzle velocity. (That velocity is what you'd plug into the first part, above). This part's tougher to figure out, but is specific to any particular set of gear and loading. Who knows if the relationship is linear or not - it's best to just measure it, then you'll know. If you have to SWAG it, then it becomes like shooting across a valley and using an educated guess to say what the winds are going.
The net effect of temperature on cartridges, and therefore muzzle velocity, and therefore ballistics, however, can be generalized in some gear as 1000 feet density altitude effect per x degrees C. (You'll have to do some web searching for 'x').
It's probably negligible however, for .308 loads at hunting distances over 30 Fahrenheit degrees. Wouldn't worry at all about cartridge temperature effects on muzzle velocity over that difference.
What's maybe not negligible (but still not critical for you) is the effect of substantially different density altitudes on your 308 ballistics at hunting distances.
However, on top of that, maybe there's just not much difference between density altitude in Houston and west TX anyway. DA is a combo of station pressure, temperature, and humidity. A change in one can be completely offset by a change in another. When you drive up from Katy through Austin, you're higher and the air a little less dense, but it's also less humid (making it better to live in July then Houston, right?
Anyway, measure your muzzle velocity at 50 deg F in Houston and use that as velocity input to the program, but use the real expected air temp, station pressure, and rel humidity of your hunting site to prepare your ballistics reference card for the hunt.
Re: Temp. and Velocity Change
I've got the trajectory down, I did load development earlier in the year for this and it was consitantly 70 degrees. I've always used Varget but it didn't work with this bullet in my rifle so this is the first time I've used RL15 and not aware of its temperature sensitivity. My issue is the last time I was at this ranch the closest I got to a Mule Deer Buck was just under 600yds and the does closer to me smelled me so I'm anticipating a long shot which I'm capable of I just need to know the temperature sensitivity of RL15. Hopefully someone has experience with with bullet/powder combo or powder.
The Alititude is definetly different! American Shooting Center in Houston where I typically shoot has an altitude of 50 or 60ft so going to 4200-4600 is very different.
Thanks for the assistance.
I've got the trajectory down, I did load development earlier in the year for this and it was consitantly 70 degrees. I've always used Varget but it didn't work with this bullet in my rifle so this is the first time I've used RL15 and not aware of its temperature sensitivity. My issue is the last time I was at this ranch the closest I got to a Mule Deer Buck was just under 600yds and the does closer to me smelled me so I'm anticipating a long shot which I'm capable of I just need to know the temperature sensitivity of RL15. Hopefully someone has experience with with bullet/powder combo or powder.
The Alititude is definetly different! American Shooting Center in Houston where I typically shoot has an altitude of 50 or 60ft so going to 4200-4600 is very different.
Thanks for the assistance.
Re: Temp. and Velocity Change
Really wish there was a ballistics program I could use on my Blackberry but have only found them for the IPhone.
Really wish there was a ballistics program I could use on my Blackberry but have only found them for the IPhone.
Re: Temp. and Velocity Change
I have not used the programs below for BlackBerry but have used the JBM its a good program. What bullet are you using at what FPS, RL 15 from the temp you zero at is about .7ft per degree velocity loss if that. Its not going to have much effect at 600 yds. BallCoef.606 post above is good info. other factors are going to be at work besides a little velocity loss. Plug accurate data into JBM hight of scope, temp. should be close. http://www.jbmballistics.com/calculations/calculations.shtml
http://sites.google.com/site/freeballisticcalculator/
I have not used the programs below for BlackBerry but have used the JBM its a good program. What bullet are you using at what FPS, RL 15 from the temp you zero at is about .7ft per degree velocity loss if that. Its not going to have much effect at 600 yds. BallCoef.606 post above is good info. other factors are going to be at work besides a little velocity loss. Plug accurate data into JBM hight of scope, temp. should be close. http://www.jbmballistics.com/calculations/calculations.shtml
http://sites.google.com/site/freeballisticcalculator/
Re: Temp. and Velocity Change
Using Hornady 208gr Amax at 2550. I got my trajectory card from jbm and I also just found this program below but dont know how accurate it is and I cant take it to the feild with me. I guess I can always make a chart of possibilities below I go.
www.jacksonrifles.com/files/pejsa%20ballistics.xls
Using Hornady 208gr Amax at 2550. I got my trajectory card from jbm and I also just found this program below but dont know how accurate it is and I cant take it to the feild with me. I guess I can always make a chart of possibilities below I go.
www.jacksonrifles.com/files/pejsa%20ballistics.xls
Re: Temp. and Velocity Change
Here's an idea - why not pack your ammo in salt ice until just before you shoot it over the chrony in Houston? Then you can be confident what muzzle velocity a cartridge will produce after sitting in a magazine for hours at the freezing point, regardless what happens to the bullet after that.
Here's an idea - why not pack your ammo in salt ice until just before you shoot it over the chrony in Houston? Then you can be confident what muzzle velocity a cartridge will produce after sitting in a magazine for hours at the freezing point, regardless what happens to the bullet after that.
