Where to get variation of muzzle velocity data?

[jcampbellsmith]

A question re - Powder Temperature

You can specify your load's powder temperature at time you chronographed along with the variation of muzzle velocity (fps/mps) per degree (F/C) and the app will automatically adjust the muzzle velocity based on current powder temperature.

Where can one find data on muzzle variation? I'm shooting a 6XC with Reloader 17 and N160. Is it significant or not?

Thanks

JCS

 

[memilanuk]

I'm guessing you find it by experimenting, and recording the data. If there is some other source, I'm curious as well.

 

[JPeelen]

Barbara Wagoner did analyze data available. You can download her report from www.dtic.mil when you search for its AD-number: ada221155

Going from 70 to 5 degrees Fahrenheit you loose less that 3 percent of muzzle velocity according to her. German ballistician Carl Cranz got very similar results.

Unless you operate in very extreme conditions, temperature effect is very small compared to other disturbances. If temperature drop is very large, you also have to take into account change in air density. A very rough estimate is 50% of additional drop due to slower velocity and the other 50% due to increased air density.

Edit: I should add that published velocity data for different temperatures shows an extraordinary large variation from source to source, much larger than anything else I have seen in the ballistics field.

 

[Bryan Litz Ballistics]

The rule of thumb I use is:
0.5 fps/degree F for single based powders (like Hodgdons extreme and most stick powders).
1.5 fps/degree F for double based powders (like MAGPRO and most ball powders).

VERY generalized, but it's something.

-Bryan

 

[taliv]

some anecdotes

when i tested two years ago, i got .58 fps / * for H4350 and 1.7 fps / * for R17 in a 260AI.

may not be linear but that's the average over a roughly 70* swing from 30ish to 100 F

 

[taliv]

a 30 degree temperature swing on a target rifle during a 20 round course of fire that may total up to 25-30 shots in a short time. In the second case, increasing barrel temperature would make a large contribution to increased velocity, well above and beyond just the increase in ambient temperature.

if barrel temp over short time 30 shot CoF affected POI, then those shooters would already be tracking their temp and making adjustments, right?

 

[Canadian bushman]

Whenever i test this i tend to see a trend that suggest that the change in mv/F* is not linear.

Am i alone in this thinking?

 

[JPeelen]

No, you are not alone. But the problem is to find a curve that is actually better than the linear rule of thumb. We have very little reliable data. In such a case a simple rule of thumb is better than a more complicated estimate, which may be not really more accurate. (Application of Occam's Razor)

Carl Cranz, mentioned in my earlier post, found that at colder temperatures velocity dropped more than a linear assumption suggests. On the other hand, hot cartridges showed less rise in velocity than a linear assumption. This was with the German military rifle cartridge (S Patrone), which used single-base flake propellant.

These deviations from a linear rule of thumb only have a noticable effect at very large temperature changes. In any case a linear rule will bring you close.

 

[memilanuk]

In such a case a simple rule of thumb is better than a more complicated estimate, which may be not really more accurate.

Another way of phrasing it:

All models are wrong; some are useful. (Box, Hunter and Hunter)