mp5, HK94 havnt seen anyone use much else
Dan Peterson prover the Suomi can compete...if you can find one.
ANY gun can compete, if the shooter can. However, certain guns make the shooter's life a little easier by virtue of their features (better sights, better reliability, easier reloads, slower ROF, etc).
If $$ was not a concern, I'd get a MP5 to run on the course. If $$ is an issue, then I'd go with an UZI.
I also helped put on ISSMC for 7 years in Richmond and got the chance to RO about 170 runs per weekend.