Originally Posted by martyj
They were new Norma cases and have never been sized at the time.
General consensus is that you should always size cases before loading the first time. Many sources recommend full length sizing at first. I suspect that if you check them in your rifle first for fit you "might" get away with just neck sizing them if they feed through - though I've not tried it. I have tried full length sizing and then neck sizing new-to-me brass so that they matched what I'd be doing with later loadings. You could even remove the neck expander from your full length die so that the neck isn't manipulated twice in your brass prep - I do know loaders who do this.
If you hadn't touched them at all then I'd really wonder where your neck tension was at. That alone could cause the errors you see.
You could always look at the bullet that HSM used, measure the length to the ogive of one of their rounds, and then load your own using 7828 with the bullet seated at the same length. That's as close as you will get to matching what they do, though you still need to check various powder levels to see what is most accurate in your rifle.