Thank you very much indeed for your observations and advice - very kind of you.
Yes, my barrel is 20" (rather than the 25" used for the Viht data) and the velocity seems OK. Interestingly, and on your suggestion, I have just looked at one of the flattened primers and it has "top-hatted" slightly (but no cratering around the firing pin depression). I use a variety of brass, and have never had any difficulty in extraction, but all primers with the N133 have exhibited flattening - which is why I was pleasantly surprised to see that the N140 showed no signs. Perhaps it is a slower round; I'll chrono some at the weekend and see what I get, but there was no sooting of the cases.
I'm now less concerned about the full case/slight compression; thanks for the link to the 6mmbr QL review. Very interesting.
Once again, thanks very much for taking the time to provide such a helpful response to my query.