According to Stevens or Senich (don't have the reference handy at the moment) the majority of original XM-21 rifles were built up with SAK standard weight NM barrels. This is not necessarily because they were the best, more likely just because SAK had the contract at the time so most new barrels in the supply system were theirs. In any case, barrels were closely inspected and hand selected by the Army MTU for use on XM-21s, so a number of SAK parts seemed to have passed the tests. BTW, "Hook" Boutin was the man in charge of the XM-21 builds at that time, and he still builds rifles... who knows for how much longer.
There is another manufacturer of standard weight USGI NM barrels not mentioned yet, that is SGW (The old Schuetzen Gun Works.) I discussed this at some length with Different on this board a few years ago, and this is the best information we have to date.
SGW won a contract to make NM barrels, but they got hosed by their steel supplier. Some (not all) of the steel supplied to SGW for the barrels did not meet specs. Some number of the SGW NM barrels blew up in service. All of the SGW barrels were recalled and removed from USGI rifles.
Possibly due to the bad publicity, SGW changed their name to Olympic Arms (still in business today.) Sitting on a large pile of recalled barrels, Olympic Arms did their own hardness tests on the steel. The barrels that passed the tests had the SGW markings ground off the barrels and were restamped OLY in the same location. There are surplus OLY NM barrels floating around on the market, and in theory they have been hardness tested and are safe. There are also, rarely, NM barrels still marked SGW on the market. I would seriously beware of those, they either failed the hardness tests or were never tested.