Tempeture Sensitivity of Powder

[scsims]

Is the sensitivity a fuction of the tempeture of the powder in the cartridge itself or just the air tempeture?

I was wondering this because I shoot from a bench just out of my back door and my guns and cartridges are room tempeture when I begin shooting.

[lisagrantb]

Powder temp. makes the biggest difference especially when shooting long range. Some of the IMR powders can vary as much as 2-3 fps per 1 degree change in powder temp. I started using my ballistic computer to input the actual powder temperature and it has improved my accuracy from one day to another when the temperature is different.

[goodgrouper]

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"Is the sensitivity a fuction of the tempeture of the powder in the cartridge itself or just the air tempeture?

I was wondering this because I shoot from a bench just out of my back door and my guns and cartridges are room tempeture when I begin shooting. "




Both. Air temp changes the temp of the brass which conducts to the powder which increases combustion. If you put your ammo in a cooler and pull it out and shoot it in 90 degree weather, it will shoot like it was in a colder climate (because it was!) and if you have your ammo on the dashboard in the sun with the windows rolled up on a 40 degree day, it will shoot like it is 70 or 80 degrees outside. This of course starts to equalize in both scenarios as the brass and powder equalize temps to the ambient air temp.

And contrary to popular belief, IMR powders are not always as temp sensitive as they are sometimes portrayed. Several years ago, there was an article in the VHA where temp gauges were used to determine velocity changes in powder lines in severe temp swings. The findings were that IMR powders tested were actually one of the more stable powder lines and faired well against the so called "extreme" powders from Hodgdon. If memory serves, IMR 4064 actually changed speeds less than Varget which was surprising to me.
The worst powders for temp swings were all the ball powders regardless of manufacturer.

In my own testing, IMR powders are much more consistent from lot to lot than Alliant powders, and just about as dependable in cold/hot situations as Hodgdon. I did have one lot of IMR4198 that was horrible though. It had peak pressure curves that would change dramatically from 20 degree swings in temp but I have had that in a lot of H4831 too so it must just be a "lot" thing.

BTW, Hodgdon now owns IMR and for years they gave out free information showing how much less H4831 changed in temp than IMR4831. Well, they bought the company, and it's strange I haven't seen those comparison charts ever since! Interesting....

[johngfoster]

I've found leaving a round chambered longer between shots even when waiting for the barrel to cool will mess with the velocity.

[Mikecr]

I just hope that Hodgdon doesn't ruin IMR powders.
I wish right now they would change their designations so that people would stop assuming they are the same(like H4350/IMR4350). I'm afraid that they will likely meld all into one lineup though. THe death of IMR..

[Boss Hoss]

Originally Posted by johngfoster

I've found leaving a round chambered longer between shots even when waiting for the barrel to cool will mess with the velocity.

Bingo-----that is a very common mistake people make when shooting off the bench for groups or while working up a load. Letting the round sit and cook does nothing for consistency..