Formula for maintaining the same Mv in different temperatures.

[geordiesniper]

Hi All, can anyone help me. Many years ago I got hold of some old copies of an American publication called Precision Shooter (I think it was called that). It covered all sorts of shooting issues and in one copy now long gone, it had a formula which allowed you to achieve the same muzzle velocity over a wide range of temperatures.
Initially you had to get your most accurate Mv and load and take note of the temperature on that day and by working out the cartridge efficiency value, you can extrapolate the load if the temperature changed thereby achieving the same Mv in a different temperature.
If anyone can provide the formula or any info it would be appreciated.
Thanks and good shooting to All.
Les

 

[TheSnake]

The formula is going to change for each different powder you use. I do not see how there could be one formula that fits all.

 

[jlow]

It’s an interesting idea but as TheSnake says, it depends a lot on the temperature sensitivity of your powder.

The other thing which is not frequently talked about is how hot your chamber is and how long your round sits in the chamber before it is fired. This does not affect first fired rounds, but if you are firing a string, this to me is much more of a factor than ambient temperature since the chamber can get really much hotter and any appreciable loiter time in the chamber will cook the round big time.

 

[300 RUM]

Yea each powder in question will have whats called a "specific heat number" and a burn rate "exponent and coefficient" also to figure you will need to know the "Kn" of the cartage, this varies from caliber to caliber and cartage to cartage or any combination of the two, also the Kn is a product of the exponent and coefficient. There is not just one formula to figure this, it's a gama maybe 5-6 formulas it's not a simple doing.

 

[geordiesniper]

Thanks guys, I have just this minute remembered (after reading your posts) that the article was specifically aimed at Viht N133 for the 6ppc cartridge, so will be no good for my 284 Shehane with Hodgdon 4831SC.
Les

 

[Win94ae]

Load develop in the summer, then carry the cartridge in your pocket during winter. Load the cartridge in the chamber when you're ready to fire.

 

[amlevin]

Load develop in the summer, then carry the cartridge in your pocket during winter. Load the cartridge in the chamber when you're ready to fire.

You realize that nobody will believe that something this simple will actually work ???


If you want to get more complex try using an internal ballistic's program like Load from a Disk or QuickLoad. By changing the "cartridge temp" one can get a feel for the velocity changes of a given powder and charge. From this information one should be able to generate some graphs that will show the temp/velocity relationship and give someone a good "guessing tool".

I've seen some people use the flameless hand warmers taped in place across the chamber area of their rifles for Cold Weather hunting. More to make sure that the action doesn't freeze due to the temp and any moisture. Could also help keep the action from being "sub zero" when you chamber that "pocket warmed" cartridge.

 

[garyw]

Load develop in the summer, then carry the cartridge in your pocket during winter. Load the cartridge in the chamber when you're ready to fire.

OR- keep the ammo under your wig!you slap head!

 

[geordiesniper]

Load develop in the summer, then carry the cartridge in your pocket during winter. Load the cartridge in the chamber when you're ready to fire.

OR- keep the ammo under your wig!you slap head!

But Garyw, I do not have a wig. I think that I had better buy one and give it a go!!!!!

 

[Leaddog]

It’s an interesting idea but as TheSnake says, it depends a lot on the temperature sensitivity of your powder.

The other thing which is not frequently talked about is how hot your chamber is and how long your round sits in the chamber before it is fired. This does not affect first fired rounds, but if you are firing a string, this to me is much more of a factor than ambient temperature since the chamber can get really much hotter and any appreciable loiter time in the chamber will cook the round big time.

Good post. I shoot F Class and if I load a round as the target is going down and have to wait 15 seconds with the load chambered, I can count on a 1/2 minute vertical with Varget. I always wait until the target is almost up before chambering a new round and take the shot witin 5 seconds or so. It helps.

 

[jlow]

Thanks! This occurred to me one day out of the blue and I modified my next outing where I was shooting for a group with a 1 min delay between rounds and my grouping with TAC tighten right up.

 

[Leaddog]

Nothing takes the place of a chrono, a means to measure conditions like a Kestrel, and a logbook. Over time you will have all the necessary data.

If you have tweaked Quickload with your base load, you can play what-if's with the program. Nothing replaces real world data from your rifle, however. Quickload falls short in it's inability to enter all atmospheric conditions to compute a density altitude model.

 

[garyw]

i will bring one to diggle on saturday if your there-900yd fclass

 

[geordiesniper]

i will bring one to diggle on saturday if your there-900yd fclass

Hi matey, at work unfortunately on Saturday so will not make it. But coming down on the 28th / 29th July for the 1000 yd benchrest comp on Sunday.
Good shooting