5.56mm is a Nato standard round, period. But that does not
include the magazine housing the rounds. 
As far as why the export version of the SAR21 can used the M16's
mag? I would like to refer to the similarities of the U.S Bradly
ARMOURED FIGHTING VEHICLES. Despite the word armoured in the name,
the 1980s U.S version were mostly made up of light skin armour as
compared to the export version to the IDF which had modifications
to the vehicle. So whats the point here? 
The point is simple, as long as the manufactural is making weapons
technology for its host country, its making a lost in profits.
Therefore, to make up for the profits, attract buyers & to make
a stand in the arms market, manufacturals would always modify the
designs of the weapon to suit their intended buyers. 
Silencer for the weapon is not really a must since it also reduces
the max effective distance of the weapon. However, in like of
todays battle ground in urban warfare, having a silencer ensures
the firer's hearing is not affected in combat, like disorientation,
unable to hear in-coming communications, etc. 