I came to the end of a 1lb imr4350.
I have been throwing 55gns.

New jug same setting on the thrower weighs heavy. Why such a difference?
I know powder lots vary but I have never seen a weight difference like this.

About a 1/2 grain off...

Probably a little denser.

It would be a good time to check out the "load by volume" theory.

Moisture content.

I'm a proponent of loading by volume.

Jordan, PLEASE EXPLAIN THIS LOAD BY VOLUME. powdr

Don't know much except when I got my Hawkins long ago, that is what the instructions said to do with black powder. Never go by weight, but by volume. miles

I did a loading by volume experiment a while back. I had two lots of IMR3031, purchase dates separated by fifty years. The newer, denser lot was the reference since the powder measure was already set up for it. It shot extremely well as always. The older, less dense lot shot all right, but the velocity was down a good bit. So for the next range trip I adjusted the measure to throw charges with the older, less dense powder to match the weight of the standard load with the newer denser powder. Groups tightened right up and velocity increased to the reference load level within a few feet per second. These were long shot strings, at least ten and maybe twenty shots. So in this case loading by volume did not work out.

I'd provide numbers and specifics but all my stuff is packed away and stored as I'm still recovering from major flood damage.

There have already been mentions of moisture content, but many handloaders don 't realize how often it happens--or how much weight powder can lose simply by drying out. I'm acquainted with the folks at a piezo pressure lab here in Montana, and during their start-up they had problems with consistent results. It turned out this was partly due to powder drying out so quickly in the dry Montana climate, because most powder's produced in much more humid parts of the world. In fact it dried out so quickly, they could weigh a typical charge in a scale's pan and with half an hour it would lose several tenths of a grain. They eventually built an air conditioned room for powder storage and handloading (previously only the shooting room itself had been climate controlled) and left jugs open as little as possible, which solved the problem.

Before hearing that I frequently left the canister open on my bench while loading, so it would handy if I needed more, but since then have started pouring out the minimum amount needed and putting the lid back on ASAP--which according to my loading seems to have helped consistency after opening a new container.

If I wasn't on top of closing lids on jars, jugs and the powder measure I'd have to worry about powder getting heavier most of the time in my locale.

I have been doing that for years, but not because I was worried about the powder changing density. Being fairly klutzy, it keeps me from spilling some of the remaining powder when I accidentally knock the canister over.

Glad to know I'm not the only one.