[*]The air disturbed by the hypersonic projectile
[*]the partial vacuum collapse in the gas track
[/LIST]
The gauss round lacks most of #1, all of #2, and has almost none of #4...
and #3 can be worked around by making the round as smooth an airflow as possible.
Apparently the trick is to bring the airstream back together again smoothly, which is easier with a needle-shaped projectile than the usual blunt-ended bullet.