The error, IIRC, is one in coding how a court-martial was done. Was it assault with domestic violence, or simple assault? Going from memory, the military uses an article in the UCMJ for “assault” since there is not one for “domestic violence”. It then needs the result interpreted as domestic assault - and in a plea bargain, the guy may have actually been convicted on a charge that would not reveal domestic violence.
It may be a case of the UCMJ - Uniform Code of Military Justice - needing to be updated. If so, it will take Congress to act.
The real truth is there is no fool-proof system, and there is no way “government” can prevent these sorts of attacks. That is why I was carrying a Beretta 92 in church yesterday...
I read that the case in question should have been entered by the Air Force into NICS, but was not. It seems it’s just plain sloth.