My usual load is:
Vihtavuori N560: 67,5 grains
Barnes TTSX 140 grains
CCI Large Rifle Magnum primer
OAL 3,33” (84,6 mm)
Speed at about 3 m (10ft) 960 m/s (3159 fps)

Recently, while checking the zero before hunting, I measured the speed of four rounds averaging 914 m/s (2999 fps).
Has anybody an idea of such a large difference with the previous speed?
The rounds were loaded in April (no, not on the 1st), using the last powder in the container. Is it possible that the solvent evaporated, giving place to a different propellant?

Never had this happen. Unlikely the powder deteriorated. Check your chrono. Same rifle I assume.

That is very likely what happened. Apparently at least some VV deteriorate in relatively short order. I had a similar experience using the 9.3x62 and VV150 (see previous thread about deteriorating powder) and using the same charge from a fresh can added about 130 fps. I just dumped the dregs from the first can rather than waste more time on the subject. So yes, you are most likely correct. Slamfire has a great deal to say on this subject and it is most useful information. Despite what we might like to think, powder doesn't keep forever.

"Deterioration" of powder is the least likely cause of the apparent variance you've found. In over 50 years of reloading (45 of that owning a chronograph) I've never experienced that kind of rapid change in the energy content of powder. I'd look several other places first.

I agree Stonecreek.
I'd run a bunch more of them over a chrony and see what it says again. I'd also shoot it at distances you are prepared to fire at game at (for several reasons) to know what your drop is not calculated off of velocity. The deterioration of your powder is the least likely problem you are faced with.
My guess is inaccurate high initial readings of your chrony on the original shooting is more likely. Run them over the chrony again in different lighting and see.

Ok, simple test: open a new, sealed can of the same powder. Use the same load. If you get your former higher velocity then it just might be the powder in the old can had something wrong with it.

Sometimes the simplest answer actually is the correct one.

It has happened and I don't mean to be cute; Is it possible that instead of 67.5 it was 65.7?
roger

Was there a wide variance in temperatures from when you got the 3150fps and then the 2999fps? I'd guess a chrony problem first though. Powder will last many, many years unscathed if stored halfway properly.

Could it be a change in air temperature? I know some powders are temp sensitive. I recently re-chronographed my 7mm load with RL22. I got in the high 2900's when it was 80 degrees F out and it dropped down around 100 fps less when I retested it at 50 degrees F.

I'm curious, you should measure the length of the cartidges now that they have dropped in velocity and see if it has lengthened any more than when you first seated them. I have an experiment I want to try, but haven't gotten around to it yet. I know when you seat a bullet, it compresses the air quite a bit inside the cartridge. I've seated bullets and then put them under water and noticed bubbles spewing out of the case neck from the compression of air leaking out.
What I'd like to do it seat to the correct length, then wait a few weeks and remeasure the length and see if the air pressure has pushed the bullet out at all. If this does indeed de-seat the bullet slightly, it would be worth bumping them back down to the original length after some of the air has had time to escape.

I've heard of powder moister contamination, but most powder is coated so well, you usually don't need to worry about it unless you are storing your powder in a powder hopper in moist environments for long periods of time. Primers are more likely to get contamination for surrounding oils or sweat and oil from your fingers.

I've also had a fold out chronogragh that has given me very different readings depending on the day. I switched a ProChrono solid piece and all my problems went away.