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Can Powder Density Change Over Time?


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I would normally think this is a stupid question but I ran into it. I had loaded some .40 S&W with AA #7 about 9 months ago using a set volume powder measure. I recently reloaded some more .40 S&W using the same powder measure and the remainder of the AA #7 I had used previously (had been resealed in the original container-stored at room temp indoors).

The charge weight this time was about 0.5 grain lighter than the first time. I double, triple checked the weights and made sure I was using the same powder measure. I used the same scale as the 1st batch and I double checked on an electronic scale. Since I was loading about 7.5 grains to begin with, this weight difference is significant.

Can the density of powder over time. My initial reaction is that it should be stable.

I tried to run a search but did not find this topic in previous posts.

Any comments? :wacko:

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This is from Mattburkett.com.

Something matt mentioned in his forum.

"Just some general chrono advice:

Always check your ammo at several different temperature variations. Keep one box in a cooler, one in the shade, and one in the sun (unless its AZ then it will blow up - JK). You maybe surprised at the amount of variation you get. Also test with the gun hot and cold.

Something else to remember, if you live in a humid environment, get the powder out of the powder measure and back into the can! Changes in the humidity level of the powder itself can seriously mess with the burn rate. At a minimum don't leave the top of the powder measure open.

Take care,"

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I really hate to admit to this!!!

I recently started shooting again and needed some ammo out of the blue to shoot a match.

Totaly unprepaired, my press had been untouched for more than a decade.

The powder measure was still full .

Against my better judgment I loaded a few rounds and compaired them to rounds I had loaded 17 years prior.

Across the chrono they were just the same!!

We live in a very dry climate and I doubt the outcome would be the same in a wet one.

This was AA#7 powder.


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AA7 is about as stable as it gets with powders.

The issues powder can have are temperature/humidity related. If it is stored in an airtight container, or in a stable/dry climate it WILL NOT change weight versus volume because it does not pick up, or lose water comtent. If the temperature is constant, the volume will stay constant, if not, it won't. Heat expands objects and cold contracts them. Not by a lot, but definitely enough to measure if your tools can resolve the change (most good grain scales can)

As long as your environment is stable, the result Gentleman JJim is getting is typical. I too have left AA7 in a measure for years and found it to be spot on next time I used that toolhead.

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