“H&K MP9 submachinegun (10mm variant). A medium-sized SMG, capable of single shot and burst mode.”
The H&K MP9 10mm SMG is a small gun in Fallout and Fallout 2.
Ammo can be found almost anywhere. Ian and Sulik are very deadly with these. It is the only burst weapon Ian can use, and the first SMG player characters are likely to find for Sulik.
In Fallout 2, a female character named Buffy can get one from Becky (however, it is called an UZI in dialogue).