Here is the best MW2 M4 loadout:

Muzzle: Echoline GS-X.
Barrel: Hightower 20  Barrel.
Rear Grip: Sakin ZX Grip.
Stock: Corio Precio Factory.
Laser: Schlager PEQ Box IV.

Best M4 Loadout MW2

Muzzle: Echoless-80.
Laser: FSS OLE-V Laser.
Optic: Slimline Pro.
Underbarrel: FTAC Ripper.
Magazine: 60 Round Mag.