Hi, and welcome to the forum! To be honest I don't think you will find much, if any difference between Lapua and Norma brass in the .300 Win. Mag. Both are a quality manufacturer. While there is a chance you might find a bit more case to case variation in the Norma, it's going to be so slight, especially in the large .300 Win. Mag. case, that it's not going to be enough to cause any discrepancies.I’ve been using Norma brass for my 300WM. Let’s say I’m using 77.2 of H1000 in Norma brass, how much different will Lapua brass be? Just bought Lapua for the first time.
I would weigh several of each, and see what you come up with in case to case variation. My guess is that you're not going to find much, if any difference. Both cases are going to be very close in case to case weight. Especially when compared to each other.