Here is a little more:
During the 1980’s, the 5.56mm cartridge was officially standardized as a NATO cartridge. A remaining weakness of the M193 load was its extremely poor penetration through heavy clothing at ranges of 500 yards and beyond. The Belgium military provided a solution with their 62 grain bullet design. The Belgium load became the standard NATO ball, designated the 5.56 NATO. Muzzle velocity is 3025fps from a 20” barrel. The M16 rifle then underwent a further upgrade to a 1:7 barrel twist rate to utilize the new load. The new model rifle was designated the M16A2.
https://fenixammo.com/pages/history-of- ... -cartridge
The point being the round used to day is not the same round from the 60s. Additionally, the shooter in Ulvalde did not use NATO 5.56x45. According to reports he used an expanding round.
You still have unanswered questions on the forum.