I recall loading mechs up with nothing but machine guns in MW3. Things like Summoners and Hellbringers that could be sped up to 90km and loaded with 20+ of the things could toast assault mechs in about 10 seconds. Now you could only do it about twice before running out of ammo, but it really was hilarious.
You could do it in MW4 as well, but it took a lot longer to kill a mech. Still get about 2-3 kills before you would run out of ammo. Though anything with regular weapons would toast you.