Reloading a temperature sensitive powder

[clowdis]

Was discussing with a friend my issue with IMR4320 in a Palma load last week. With an ambient temperature of 75* I could load a round take about 15-20 seconds to get position, aim and fire a shot with no effect on elevation setting, ever. With an ambient temperature of 85* the elevation setting would move over 2 minutes. During a 20-25 shot string the barrel temperature varies from maybe 100* to 200* at the end of the string. The question is "why does 10* of ambient temperature effect the shot elevation so much while the barrel temperature variation of around 100* has little to no effect on elevation"?

 

[rardoin]

Perhaps absolute barometric pressure or density altitude change in addition to powder temp change?

 

[clowdis]

Perhaps absolute barometric pressure or density altitude change in addition to powder temp change?

Same day.

 

[normmatzen]

The powder doesn't care a hoot what the barrel temp is, only the powder temp!
I carry my ammunition to a match in a thermal bag so they will all be near my home internal temp right up to touching the trigger.

 

[K22]

Never used IMR 4320 but used a lot of IMR4064, IMR 4198, and IMR 4350 which I assume are similar since they're "stick" powders also.

The only powders I have significant problems with regarding temperature shifts were ball powders, namely H380, BLC2 and to a lesser extent H335. I experienced pressure surges but in all cases this occurred in 90 degree+ weather.

I do notice some point of impact changes during large temperature variations (i.e. 40* to 80*) from winter predator to summer varmint seasons even with the so called temperature stable powders but this may be due to changes in air density rather than strictly temperature change affects on the powder, the colder air being more dense.

 

[Toejoe12]

Mirage, light condition change, and wind all can have an impact. Don't be too quick to fall back on temp sensitivity on the powder. A 10 degree change in temp shouldn't give a 2 MOA change on its own.

 

[Ned Ludd]

Was discussing with a friend my issue with IMR4320 in a Palma load last week. With an ambient temperature of 75* I could load a round take about 15-20 seconds to get position, aim and fire a shot with no effect on elevation setting, ever. With an ambient temperature of 85* the elevation setting would move over 2 minutes. During a 20-25 shot string the barrel temperature varies from maybe 100* to 200* at the end of the string. The question is "why does 10* of ambient temperature effect the shot elevation so much while the barrel temperature variation of around 100* has little to no effect on elevation"?

Did you measure muzzle velocity of these shots? That might provide some insight.

Heating of the powder inside the case of a loaded round doesn't happen instantaneously. There probably isn't that much of a temperature swing within the powder inside the case in a 15 to 20 seconds time interval. I have seen people test this using a temperature probe and my recollection is that it took as long as 2 to 3 minutes for the powder inside a loaded round to fully equilibrate to a hot chamber temperature. At 75 degrees ambient temperature, your loaded rounds may have still been very close to ambient temperature when you pulled the trigger. In fact, they were probably very close to that when you left your house. However, they were probably not at ambient temperature to start with when it got to 85 degrees, unless you specifically let them equilibrate to ambient temperature prior to firing, something I usually try to do. Finally, a significant portion of the powder burns outside the case within the first 6 to 8 inches or so of the barrel. A screaming hot barrel can therefore contribute to velocity excursions even if the temperature of the powder inside the case hasn't had enough time in the chamber to change markedly when it is fired.

The bottom line is that there can be a number of different factors that contribute to velocity excursions and/or to where the groups print on a target at some specified distance. Ambient temperature, the temperature of the rounds themselves, the temperature of the barrel, the atmospheric changes that occur as the temperature goes up; all these things could be at play in the situation you described. In addition, the small samples sizes we typically shoot can often look like a big discrepancy, but just as often the differences aren't reproducible from day to day, or with larger sample sizes.

 

[clowdis]

Ned,
I was shooting into Silver Mountain targets and the velocity was available over the 7 day period but I didn't have any way to save plots since I wasn't using my own computer. I remember the 1,000 yd. velocity was in the 1350 area on the lower temperature days but I don't remember a reading for the day I had the elevation deviation. I did load some other ammo "just in case" with IMR 4166 which is a temp. stable powder that I used on Sunday and elevation went back to normal. Ammo was not left in the sun but the shooting equipment box I had it in was. Still, the brass was not hot to the touch. It seems to me that with the thermal conductivity of brass being kind of high the chamber would heat the powder charge faster than the ambient temperature, but apparently like you said, that isn't the case. Even with the elevation change the 4320 shot good groups and showed no sign of excess pressure, but I'm sure that if I had remembered to look at the 1,000 yd. velocities that would have given me a clue. The Hodgdon max. charge is 48 grains and I was using 46.5 so I wasn't pushing the charge envelope with 155 grain bullets.

 

[Texas10]

The brass case in not in close contact with the chamber, hot or not. Rat turd in the violin case if sized properly, therefore heat conduction and charge heating is not likely. Atmospheric changes are very likely, what's called density altitude due to temp and humidity changes as the day warms. Much more pronounced as altitude increases or days that start out very humid in the morning.

Possible your load is on the edge of a node, and small swings in air density pushes it past the optimal bullet exit time. Find the middle of the node and load to that velocity and perhaps you'll have less POI swings.