OzFclass

FTR, 185s - rifle hammered cleans at 300, 600 and 1000 (less a few x's)
at the end of the queens a bit of vertical had crept in. ~1200 rounds down it by this time.
A couple of years later I get around to re-tuning it. ~1300 rounds down it now.
I'm using the same lot of powder, cases, primers bullets ...but I have moved to bore tech carbon and copper cleaner from butches bore shine.
first thing I do before doing too much is shoot the original load over a magneto....wow it is ~70-80 fps slower than the readings I had when I first tuned the load and again after 500 rounds.
I'm using the magneto on another rifle and barrel and all is reading as expected there.
I run a series of .2 grain increments to see where the rifle wants to shoot now and what the numbers look like. I've selected a powder charge (1/10th more than I had and ~50 fps slower) and have loaded up for a seating depth test...then I started thinking a bit more...

Originally I thought changing the cleaning product has changed the barrel and accounts for the speed decrease...then a thought...could the remaining powder in the opened but tightly resealed powder (4kg) container have absorbed moisture over the last 2 years and that accounts for the speed decrease?

Have I just wasted my time re-tuning against this powder?

I have another 4Kg unopened container of the same lot number I could open to prove it but I thought I would throw this out there for thoughts.

Thanks in advance

It is likely that your batch of powder has absorbed some water which has increased its weight, so you will need to load a little more powder by weight (but similar volume or thrower setting).
It is also likely that your barrel has slowed down slightly with a few more rounds down its throat, so likely you will need to bump the load slightly even with the unopened tub.
Another consideration is that I have found velocity readings can vary 20 fps or more when you move the magnetospeed around, either back and forth or closer/further away from bullet travel.

If your batch of powder has absorbed a slight amount of water, it will likely still work fine, you will just need to adjust the weight up slightly to account for it. (actually, nah its no good, I'll give you $10 for it)

Thanks Rod - I appreciate the reply.
I'll push on as all the signs are there that I can tune it in as good as it has been.
One of the flags for me that I might have something unusual going on is the range of FPS from lowest to highest change (.7 grain) was significantly reduced as well in that it only produced a FPS increase of 28. I would have expected something more around 50-60 fps.