Powder temperature and MV SD

[Williwaw]

Full disclosure here ... I'm a rimfire guy who knows nothing about reloading but I reckon you all can answer my question if anybody can. I know MV increases as powder temperature increases ... but what happens to MV SD?

I am being specific here, I'm not talking ambient temperature but powder temperature. If there is something to be gained I'll use a controlled temperature container to keep my ammo at the optimal temp. I'll keep it one ice if that decreases SD.

Thanks for your time.

 

[David Christian]

Here is a quick stab. Depending on where the load fits within the node window, I have seen the SD both increase and decrease. Go figure…

 

[RegionRat]

If we look at good examples of single base extruded powders that are known to be insensitive, say H4350, Varget, etc., they do pretty well if the starting point was a good stable load window to begin with, however...

As an example, if we were to visit Camp Perry on a hot summer, we will see several examples of Varget go wild in summer heat.

Those are typically folks who loaded too hot based on cooler weather in spring and the summer temp increase takes the load over the edge. Even Varget and H4350 taken too high and hot will go wonky.

By definition, what goes awry is the ES/SD along with the average. That same powder at a lower charge might have been fine on the hot day.

So two part advice. Pay attention to the pressure range of the powder. Don't look for trouble and you won't find any. Then, pay attention to the temp and DA of where/when you shoot. Make sure you anticipate the DOPE and pressure of that ammo under those conditions.

Learning to keep stable DOPE for pre-loaded ammo taken to varied climates, requires experience and skill.
Good Luck and YMMV.

 

[Williwaw]

That pretty well answers my question, thanks guys.

 

[Straightshooter1]

Full disclosure here ... I'm a rimfire guy who knows nothing about reloading but I reckon you all can answer my question if anybody can. I know MV increases as powder temperature increases ... but what happens to MV SD?

I am being specific here, I'm not talking ambient temperature but powder temperature. If there is something to be gained I'll use a controlled temperature container to keep my ammo at the optimal temp. I'll keep it one ice if that decreases SD.

Thanks for your time.

When MV changes, SD will change. SD will get larger as powder heats up and visa versa when powder cools down. How much the SD's will change due to temperature changes will depend on the powder's particular sensitivity to temperature changes.

Some years ago I ran a test comparing my Lapua Center-X, Federal UltraMatch and CCI Standard Velocity on how they respond to hot vs cold temperatures. I was curious which did better in either in hot condition vs cold conditions. I put 20 rounds of each in a container with dry ice where the interior temperature was below 40°F. Another 20 rounds of each was kept out in the open where the ambient temper was 90°+F. Interesting, the CCI SV's did better being cold with an SD of 7.6 vs 13.3. My Center-X did best with the warm ones with and SD of 4.7 vs 6.6 and likewise the UnltraMatch did better being warm with an SD of 6.0 vs 12.6. The only problem with these SD numbers is there more to it than the powder as I'm not sure how much the bullet lube contributed to these results. But I thought maybe you'd find this info interesting.

 

[Fast14riot]

For my N555 loads, 5fps per 10°F.

This is if I leave my ammo box in the sun or even just ambient temp. I shoot in NorCal, my loads are tested generally above 90°F ambient and when testing I do not intentionally cool my ammo. I want to know how it will react on that 105°+ day in August and if my load will hold up. During the 4 "winter" months, I'll add a couple tenths to bring velocity back up, but generally I see my best SD numbers in early season with ambient temps around 65°F.

I do tune to find the best load for my atmospheric conditions, up to 500'asl, and it has worked for me when I do travel to other ranges. I have yet to see a solid high temp, sea level load completely fall apart at altitude. I have seen the reverse fall apart, though.