Hopefully to never use that bullet OR that powder again for that caliber - Unless it becomes perfectly clear the issue was something else entirely.
On more than one occasion I've found a wrong weight bullet mixed in with a box. Usually seen when buying "seconds". Not very likely to be this, however.
Another time long ago when using a balance beam I mis-read where I meant to set it. Caught the problem before I went too far, fortunately.
Another extremely unlikely scenario is a wrong diameter bullet being loaded. (Again, not likely, but it never hurts to check the diameter of one or two from the first box of a new lot.)
These days I use two electronic scales for most rifle loads - in part, to make me look twice.
The first being the scale in the RCBS Chargemaster.
That charge is then dumped into another powder pan sitting on a different good quality electronic scale, and trickle adjusted or dumped if needed.
(The first pan going back to the Chargemaster so it can immediately start the process again.)
Not saying this is "the" way to do things, btw.
That bullet or that powder. Yes, that's the direction I'm leading because that bullet seems to not be working correctly. Too bad, because they are really good bullets.
The problem is, the tech guys at VV recommended that powder, and the guys at Woodleigh also said it should work. Going back to my notes, here's the email I received from Woodleigh:
Hi Don,
It should be safe to substitute the Hornady load for our 160 gr. I have a
listing of 37.5 gr H4350 for 2100 fps, this powder is faster burning than
N160, so similar charges of N160 would be quite a safe starting load.
Signature line of that email redacted because I didn't ask him if I could share.
Email from VV:
I'm afraid we have no data for the 6.5x54MS that we've worked up in our lab. I did find one source that listed a charge of N160 with the Hornady 160 grain round nose, starting at 37.0 grains and reaching maximum at 40.0 grains. According to the chart, it should give a starting velocity of about 2,000 fps, working up to about 2,200 fps at the top load. As I said, this isn't data that we (Lapua or Vihtavuori) worked up ourselves, but it may be a good place to start. As for the selection of N160, it's ideally suited to this combination, and would have been the choice I made as well.
In both cases I went below the minimums mentioned above. My reason for this was trying to match the pressure curve from QL. I was prepared to deal with a squib. Overpressure was a concern, I think for good reason.
I absolutely agree that misreading a balance beam could be an issue. I'll go back to my procedures, but I don't think I was too far off on this one. When i pull the powder from the remaining loads, I'm going to measure that, and that should tell me if I bumped it a bit, without realizing it. The problem with the electric scales is they more easily lose calibration/zero. If I had a chargemaster, etc. I'd double check on the analog anyway.
My "procedure" for scale:
I actually mark each piece of brass with the load, using a fine point Sharpie. This is especially done for "progressive" loads (i.e. working up to a safe max) or for finding Optimal Charge Weight. My experience with OCW brought my .30-06 from a 1.25 MOA to 0.75 MOA, while staying well below max load.
Check my "paper" (book, listing, reloading app provided by the manufacturer, whichever).
Match paper to brass (Sharpie Load).
Match load to scale. Actually say weight out loud, because saying it is better than thinking it.
Check scale again. Add powder.
Once appropriate powder is trickled in, add powder to case. Read load off case.
Compare that number to scale to ensure it is correct.
If making a progressive load, step scale to next load.
Once the bullet is seated, I load them in my bullet case in order, so at the range I can go one at a time, in order. I also had my spreadsheet to record velocities for each load, and compare the sharpie marks on the case to the next load on the spreadsheet.
I did a "sample check" of the bullets, both weighing them and checking diameter. I'll admit I should have checked more than 2 or 3 of the 50, but these weren't "seconds", and Woodleigh has a reputation for quality.
Those are the things I did to get to this point. I contacted the manufacturers of both the bullets and the powders for their recommendations. I compared both to a software program, just to be sure. I weighed everything, checked for jam, checked for COAL, checked for case length. If I left anything out, I want to be enlightened.