There are so many different specifications for a SAW that difers from army to army.
I don't know exactly how they are organized but....
The US have within the section M249, designated as a SAW, and M-16 & M-203.
The British seems to be undecided. They have the Minimi (M249) AND the L86 LSW, basically a souped up version of their L85 Assault Rifle. The L86 has a heavier (longer?) barrel and uses the STANAG mags. Though the L86 LSW is now kinda become the unofficial long range aimed fire weapon, with the Minimi taking up over the fire support role.
The British experiment with a souped-up assault rifle for the fire support/SAW role is a clear demonstration of the failure of this concept.
It is unlikely SAF will adopt the LSW version of the SAR-21. In the seventies SAF experimented with the M-16HB with bipod for the LSW role and found it unsuitable. In the end the answer was a dedicated SAW the Ultimax. So it is unlikely we will go back to a LSW concept based on the SAR-21.
The most interesting development in this area is the USMC's nre requirement for what they term as a IAR - Infantry Automatic Rifle. They were evaluating the Ultimax for this though I don't know the outcome.
This is interesting because this is taking place more than a decade after they have adopted the M249 as a SAW. Perhaps they are now finally realizing that that the M249 really no more than a GPMG firing 5.56mm. A S.A.W. IT IS NOT.