Well a very enjoyable thread. I have 2 300 Win Mags and a 03-A3 in 308 Norma Mag and submit the following observations. The 1st 300 Win Mag is a pushfeed M70 with a max load of RL22 and 180 gr NP fired by a FED 215 it gives great accuracy and 3100 fps Chrony average. The magazine in this gun allows cartridges no longer than 3.33" for reliable feeding. The 2nd gun is a M700 ss with plastic stock,it has a tighter chamber and cases used in the M70 are not resizable to be used in the M700. so I keep them seperated ,it would not even come close to tolerating the loads I use in the M70 with no problems. The M700 has a 3.6"+ magazine and longer throat so I can seat the bullet further out for more case capacity, but it did not help. 70 grs of IMR 4350 is all it will shoot with accuracy using the 180 NP. Both guns will shoot with various loads, bullets in the 165-168 gr range at 3000-3100fps with great accuracy and have shot many deer with them.
The 308 Norma was built on a 03-A3, they rechambered it and opened the bolt face and of course still retains the 06 magazine length. This was a semi popular project when Norma brought out the 308 Norma in the early 60's.I have both 308 Norma brass and reformed 338 Win Mag brass for it. Of course the reformed 338 Win Mag brass is .048" short compared to new Norma stuff. I have allways had better accuracy with the correct length Norma brass.The factory 180 gr Dualcore Norma ammo is loaded to 3.25" and all factory and handloads feed well from the magazine . Can't tell you if the mag rails were altered, I never have looked, it just works and I don't fix what isn't broke.Most of my reloading for the Norma has been centered around 165 gr bullet weight as it is all you need for deer. This rifle is over 10 lbs loaded so it don't go to elk country.I have also found that I don't need to load it full throttle for the desired results.
So I think it boils down to this for me ,loaded up or loaded down they all do the same thing 100 fps one way or the other as long as you put the bullet where it belongs. I like them all and have shot plenty of deer and a couple of elk with the 2 300 Win Mags. You don't have too get the last fps out of them for their effectiveness. Magnum_Man