Velocity changes with temperature

[tacotime]

Tuned up a nice mild low-end load in the 45-70 with 4198 and it chrono'd somewhere in the 1300's when I set it up while temperatures were in the high 50's. 
 
Then later when the temp was in the mid 70's it was running in the upper 1200's.  Same bullets, cases, powder, primers and digital powder scale. 
 
Or there could be another variable in the loading process.  But...  are we expecting velocity to rise or fall with maybe 20 degrees higher temperature? 
 
Is 20 degrees enough to cause a noticeable velocity change in the same load?

[MZ5]

You may expect velocity (and pressure) to increase with temperature.  This is particularly true of old standby powders such as the IMRs, and there's a very nice correlation for them, actually (excluding extreme desert or arctic conditions).
HOWEVER, these days IMR 4198 is no longer produced in Canada at the old DuPont facility.  It's now produced in Australia on the Thales line right alongside H4198.  I don't say the burn rates are identical, but the IMR-labeled stuff is claimed to be 'extremely' temp-resistant now, just like the rest of the Thales-produced Hodgdon stick powders.
I don't know for sure why you saw what you saw, but I'll tell you this:  Both ammunition _and_ rifle temperature drive pressure/velocity, particularly rifle temperature.  Also, you can watch some of the 'extreme' powders actually slow down some as temps increase.  I don't say that's what happened here, I just share it for information.

[jhalcott]

 Yes gun/ ammo temps will affect velocity. Did you take an average of the cooler temperature shots and compare it to an average of the warmer temp. shots? A SINGLE shot can vary quite a bit with out any component changes. Moving the chrono a foot or so either way can also give different results!

[tacotime]

If memory serves, these were 4 shot averages each time. 
 
How does a warm (not hot) rifle barrel change the pressure in a fired round?

[PowPow]

How does a warm (not hot) rifle barrel change the pressure in a fired round?

Can't say what's happening behind the bullet, but if the air in the barrel ahead of the bullet heats up 20 degrees because the barrel  is warmer, it about 4% less dense. (40 degrees warmer is about 8% less dense).  Seems like that would make the bullet go faster through a warm barrel.

[MZ5]

How does a warm (not hot) rifle barrel change the pressure in a fired round?

Burning nitrocellulose releases lots of heat (ever burned a pile of smokeless on the ground?  It's HOT!).  That heat is a large part of what drives the gas expansion that accelerates the bullet down the barrel.  So, how much heat is produced by the burning of the propellant charge is important.  The colder the steel of the barrel is (and to a lesser but still important degree, the brass, lead, copper, & powder of the cartridge components), the more heat that steel pulls out of the reaction, thus leading to less pressure built up.  Less pressure results in less acceleration, hence lower velocity.

Therefore, perhaps obviously, the warmer/hotter the steel of the barrel, the less heat it pulls out of the reaction, thus leading to more heat energy available to create pressure & velocity.

See?

[MZ5]

Sorry; hit the wrong button.

[tacotime]

So if loads were identical then each shot should be a little faster as the barrel warms.
 
But more to the point, a load worked up in August should shoot faster than the same load fired in January...
 
Still curious why my loads shot slower in warmer temps.  Possible loading issue then...

[MZ5]

So if loads were identical then each shot should be a little faster as the barrel warms.
 
But more to the point, a load worked up in August should shoot faster than the same load fired in January...

In the general sense (and absent geometry or chemistry efforts to make the powder temperature-resistant), that is correct.

[tacklebury]

Some powders are more sensitive to it than others.  I've found, with a bit of research, a few to avoid.  I know personally, even though I like and use it, that H110 can be hard to ignite in cold temps.  That's why I always load it with magnum primers to get the extra heat.    Even in the general descriptions on the powder makers sites, there are some comments that state this powder is temperature stable or vice versa.
 
Example:

With Reloder 17 reloaders have a powder that meters easy and consistently while providing maximum velocity even in extreme weather. This consistent performance is what reloaders have come to expect from Alliant Powder - and Reloder 17 proves worthy.

• Designed for short magnum case capacity
• Similar burn speed to IMR 4350
• Meters easily and consistently
• Consistent maximum velocity in extreme weather conditions

I have had good luck with Reloader7 in most of my rifle applications across a range of temperatures.