1. it's
addWeapon, not addweapons
2. the default magazine for the M16A2 is
30Rnd_556x45_Stanag, not 20Rnd_556x45_ M16A2
removeAllWeapons this; this addMagazine "30Rnd_556x45_Stanag"; this addMagazine "30Rnd_556x45_Stanag"; this addMagazine "30Rnd_556x45_Stanag"; this addMagazine "30Rnd_556x45_Stanag"; this addMagazine "30Rnd_556x45_Stanag"; this addMagazine "30Rnd_556x45_Stanag"; this addWeapon "M16A2"
But of course you can use 20 round magazines as well, the classname for them is
20Rnd_556x45_Stanag