Im not a millitary grade person but i do know what the diffrence

.408 Magazine (The real M200 Cheytac magazine)

Parent case .505 Gibbs
Case type Rimless, bottleneck
Bullet diameter 10.36 mm (0.408 in)
Neck diameter 11.12 mm (0.438 in)
Shoulder diameter 15.24 mm (0.600 in)
Base diameter 16.18 mm (0.637 in)
Rim diameter 16.25 mm (0.640 in)
Rim thickness 0.50 mm (0.020 in)
Case length 77.21 mm (3.040 in)
Overall length 115.50 mm (4.547 in)
Case capacity 10.32 cm3 (159.3 gr H2O)
Rifling twist 330.2 mm (1 in 13 in)
Primer type Large rifle
Maximum pressure (C.I.P.) 440.00 MPa (63,817 psi)

Positive
+ It does less bullet drop
+ Higher Damage
Negative
- Accuracy

.375 Magazine (Anti-personel / Hunting Magazine)

Parent case Unique
Case type Rimless, bottleneck
Bullet diameter .375 in (9.5 mm)
Neck diameter .405 in (10.3 mm)
Shoulder diameter .515 in (13.1 mm)
Base diameter .532 in (13.5 mm)
Rim diameter .532 in (13.5 mm)
Rim thickness .050 in (1.3 mm)
Case length 2.580 in (65.5 mm)
Overall length 3.340 in (84.8 mm)
Case capacity 99 gr H2O (6.4 cm3)
Rifling twist 1-12" (304.8 mm)
Primer type Large rifle
Maximum pressure 62,000 psi (430 MPa)

Positive
+ Accuracy
Negative
- Higher bullet drop rate
- Low damage

But this info from real life
For the game i still didn't crack it
but the .408 Magazine do better than the .375 (No offense .375)