Not all scoped combat rifles are sniper rifles. You can call this a debate over semantics, but the G43 and the M21 are not sniper rifles, but Designated Marksman Rifles. The G43 was originally designed as a standard infantry rifle and was later fitted with a scope (from 1943 on). It was typically used at ranges of less than 400m against any and all targets of opportunity. This makes it a DMR, not a sniper rifle.
While the M21 was a more capable system than the G43, and many shots were made at longer ranges (500-600m), the system was not designed for long range work. It was still a standard infantry rifle (M14) that had been accurised and fitted with a scope. It was used against targets of opportunity.
As has been said, it is the role that defines the sniper rifle. The sniper rifle is meant to engage special targets (namely high-value targets such as officers) at ranges in excess of 500m. This is not to say that snipers don't make shots at less than 500m and DMs don't make shots at more than 500m, but it is a crisscrossing of roles. Nowadays, snipers do not shoot enlisted men (unless in immediate self-defense), that is the job of the DM. With the addition of large (.50) caliber sniper rifles, it can be added that a sniper's role has been modified to include special anti-materiel work.
There is a reason why almost every major military uses a bolt-action as a dedicated sniper rifle. That reason is long-range accuracy. No semi can compete with a well-made and maintained bolt gun at long ranges.