Most machine pistols are not particularly good at suppressing fire. The two key requirements for the gear are: must have FA capability and needs 20 rounds. Flipping through Core, Gun Heaven, and R&G, I don't see a lot of options. The Steyr TMP is probably the best option with a 30 round capacity and FA. Most of the others that are capable have exactly 20 rounds in the magazine, so you basically have one CT of suppressing fire and then you have to reload. At least with the TMP, you'd still have a few rounds you could shoot rather than having to reload.
SMGs tend to be a lot better at the role given their ammunition capacities and range, but are a bit bigger. Unless you go with the TMP, this is probably the best route.
As Top Dog mentioned, there is the Auto-Assault 16 that has a drum with 32 slugs in it, so it would conceivably work, but it is neither easy to carry around nor legal, so that might make it a bit challenging.