Powder and Temperature

[Palladan44]

Say you dump an entire 30 round magazine from an AR-15. Then you quickly reload, and chamber another round. Its reasonable to suspect that the inner breech is still several hundred degrees? Hell yes it is......Brass being a pretty good thermoconductor will heat up pretty quick, and heat up the powder or primer.
Could it possibly heat the powder/primer enough to chemically alter it? (I know it wont be hot enough to cook the round off, but will it damage the powder or primer?) I had an FTF, ammos fault, a few months later, and I think it may have been the round that I chambered after the Magazine dump, but then never fired it. Ill never really know.

Second scenario is firing 10mm using a maximum 11.0 Charge of blue dot, firing from stock Glock 20. Worked up the load during the summer, and was able to get to the book maximum of 11.0 grains. No pressure signs such as flattened primers, or excessively bulged cases, and accuracy was great.... Fired the same gun with the same rounds outdoors when it was approximately 15 degrees out. The ammunition sat overnight in the truck, and was probably 5 degrees above that night. I could feel how cold the rounds were when i loaded them up.... they shot very snappy, seemingly more so than usual, but those variables from session to session vary based on perception and are usually false.... accuracy was now pretty poor, with my groupings quite a bit larger than normal....now for the smoking gun... primers were visibly flattened more than usual, the case bulge was now highly visible. Of the 15 rounds fired, there were 3 cracked cases!
Now, i know that as temperature decreases, the flexibility of metals or any material becomes less. Its possible that the brass cracked due to colder temps, but its more likely that the temperature altered the burn characteristics of the powder. Upon more research, this is a definite "thing" Blue Dot tends to be one of the more temperature sensitive powders, according to other accounts ive read. Im just here to testify to this and it is a real thing.
Any accounts experiencing real changes in your loads based on temperature?

 

[Blue68f100]

Some powders are more sensitive to temp than others, while some are reverse temp sensitive. Yes letting a round heat up in the chamber is going to impact the performance, velocity. Most powders start breaking down around 140F. It's a time vs temp curve that determines the time to where it will decompose. Normally in a rifle it's not going to be that hot after multiple firings to cause the powder to degrade. You will get a higher velocity though. But as things heat up so goes your accuracy. The reason most rifle shooters only shoot so many rounds before they allow the barrel to cool.

I know if your using WST as the temp would get colder your velocities will go up. Most powders gain velocity as the temp rises. The reason we always tell users to test again during the winter if shooting during the summer.

I had some loads for my 6.5 CM starting blowing primers with a 35 F temp increase, 55 to 90. Here in Texas its common to get 40 F swing in temp. Yesterday we were 70F this morning 31F.

 

[Otto]

Did you run any of the loads over a chrono?
If not, then everything is just speculation.

 

[Palladan44]

Did you run any of the loads over a chrono?
If not, then everything is just speculation.

Velocity and pressure can be related, up to a point. But not always. Especially near top end, the point of "diminishing returns" happens where velocities flat line, but pressures continue to increase... So even if the loads chronod' the same that doesnt mean pressures are the same. However, yes you are correct, I am still speculating. That part is true.