Some kind if optic either an ACOG, red dot type, or a rifle scope. ...
Laser pointers also to aid with night aiming with night vision goggles.
Different handles for a foregrip.
Lights of different types.
A 203 grenade launcher shoots a 40mm grenade of different types high explosive, flare.
Chainsaw.
One of my personal favorites, the M4A1 is the poster child of Navy SEAL firearms. A good SOPMOD kit allows for complete configuration changes from mission to mission, giving you the opportunity to grow immensely familiar with a particular weapon while sacrificing nothing in versatility.
