"A micro-submachine gun. Short range, low power, high rate of fire. Very quick to aim and fire."
The machine pistol is a somewhat lightweight, automatic firearm in RimWorld with very low damage per shot (or moderate per burst), very high fire rate, short range, and moderate accuracy. Similar to the pistol in terms of overall nature, the machine pistol's short range combined with its high fire rate and 'automatic' nature makes this an excellent weapon for inaccurate shooters (95% post-process accuracy or less). This weapon is capable of basic suppression due to its rapid fire, but it is a very poor weapon choice against armoured foes as they'll absorb most of a bullet's damage, rendering it significantly less useful in that scenario.
The machine pistols main advantages lie in its high fire rate and cheapness, being a strong performer in both areas.
Machine pistols can be purchased from outlander and orbital weapon traders for a somewhat low price, or obtained from outlander and pirate raiders predominantly in the early-mid game. Machine pistols can also be crafted at a machining table once the machining research is completed; from 45 steel, 3 components, and 25,000 ticks (416.67 secs) of work.
Conclusion & Comparison
Overall, the machine pistol is a decent weapon to use in early-mid game, but that's not to say it won't be outclassed: the charge rifle has the same range, but a significantly higher damage output and accuracy; the heavy SMG has even higher damage per shot at the cost of a slightly lower fire rate, shorter range, and slightly less accuracy - but is still overall a much better option; the LMG does everything the machine pistol does but better, with more sustainable fire, higher damage per shot, and slightly longer range; and the pump shotgun trades off some fire rate and range for superior raw damage output and accuracy - making that a better option for slightly higher level shooters. This weapon can be seen as an upgrade to the pistol, indeed serving this title very well.
The machine pistol is virtually identical in design and pattern to the venerated Uzi 9mm sub-machine gun in real life. The Uzi is an SMG primarily in service with the Israeli military since 1954 to present day, using the 9x19 Parabellum cartridge with a rate of fire of over 600 rounds per minute. The Uzi has an effective range of over 200 meters and weighs 3.5kg (7.72 lbs) standard issue.
The below graphs assume an unmodified shooter using a Normal quality weapon.