The M4A1 carbine is the primary weapon used by SEAL operators.

Best Modern Warfare 2 M4 loadout

Muzzle: RF Crown 50 (+0.49, +0.00)
Barrel: 11.5 3 T-H4 Barrel (-0.26, -0.40)
Optic: Cronen Mini Pro (-3.00, +0.00)
Stock: Tempus P80 Strike Stock (+2.74, -1.16)
Rear Grip: Sakin ZX Grip (+0.81, +0.00)