Something that I always found odd was how much weaker than the 9.3mm weapons the .338NM was, while the .338 Lapua Magnum is pretty much on par with them. However, I did a bit of search and I am pretty sure I found out what is wrong:
The .338 Norma Magnum was originally developed to basically allow people to load cartridges with projectiles that have significantly lower friction coeficients than those of the .388 Lapua Magnum, while keeping the same total loaded cartridge lenght.
In order to achieve this, the case itself is quite a good bit shorter than that of a .338 LM, BUT it's volume is not TOO significantly smaller than that of the .338 Lapua Magnum! Specially since the .338 Norma Magnum both has it's cartridge seated slightly lesser deep than that of the .388 LM, and because it's "shoulders" widen a lot faster than those of the .338LM. On top of this, it is also being able to use (5%) higher chamber pressures than the .338 LM.
And the fact that the .338NM has the bullet seated a little shallower makes the already not too significant cartrige volume diference even smaller, and the slightly higher operating pressure more than likely makes up for the nearly negligeably slightly lower volume (if not even slightly MORE than just covering just the difference)
On top of that, the .338 Norma Magunum PROJECTILE is a goood bit longer AND heavier than the usual .338 Lapua Magnum projectiles. And this fact definetely should not only make it have a higher MASS, but also have a significantly lower friction coeficient (by both increased MASS/Sectional Density, AND improved Ballistics Coeficient), but also have a better penetration than even the Navid, because while moving slower than a .338LM, it is a lot more dense.
However in relation to those properties IN GAME, the .338 NM is simply underperforming. Being simply modeled as literally a "NERFED .338 Lapua Magnum". This is because it not only has the same friction value as the .338 LM, it also has a LOWER muzzle velocity (as expected) but while basically using the exact same projectile... (which means massively lower damage AND penetration, penetrating noticeably worse than even a 7.62x51/54mm rife)
SPMG needs a buff to its damage, Friction Coeficient, and penetration. Because in "TL;DR" terms: it is basically a .338 LM loaded with a very heavy and long projectile that while has a slightly lower muzzle velocity than that of a .338LM, it has significantly lower friction coeficient, and significantly higher sectional density.
(Because when taking all the facts combined this should make up for the lower Muzzle Velocity at very long ranges when compared to the .338 LM because of the lower drag. And at close ranges should make their damage and penetration extremely similar, if not the same or even better!)