I know this is an "old" thread, but it popped up bold in my feed at the weekend. I have read all the posts included the work to achieve a satisfactory solution.
OK: If I had $20 to place in a pool on this, it's the Norma cases. I have had exactly the same experience in another belted cartridge w/Norma cases, .300 WM.
Now, let it be known I have a very *positive* bias to Norma brass. It was the "name" brass when I started loading in the UK back in the late '70s. And in my .223 Rem F/TR rifle, I actually rate it better than Lapua for accuracy and brass life. But, I think Norma is having a problem with belted magnum cases.
I have an unfinished and unpublished article on Norma versus Peterson Brass -- pretty much the same price point, not quite Lapua price, but definitely top shelf brass.
Norma is the lightest .300 WM brass I've tested by quite a bit, very close to Nosler which makes one think they may be the manufacturer, but concomitant with the low weight, it also has the highest capacity. Which would make you think it would tolerate heavier charges, but it doesn't. It is *very* sticky in the chamber with charges that other cases shrug off -- Peterson, FC, PPU and so on.
Internally the Norma cases are really clean, no need to debur flash holes, but you can see the lighter construction around the case head versus the other cases, especially around and above the belt.
And so, I think, this is their undoing. My sense is, the Norma cases expand excessively just above the belt, and, well, stick, when no other case does.
FWIW, I too am shooting these cases in a rem 700 but I'm not sure it's a rifle-related issue, I (sadly) think it's more a Norma problem w/belted cases. Wish it weren't since I have 200 of them..... which basically I don't use.
And just to confound everyone, the best performing cases in my .300WM have been FC; plated and unplated.
My view from the East........