I fired one of these. My impression was that it was possible to hit the broad side of a barn ... if fired from the inside.
/sarc
It is one of those tools that met a need.
We had them in our small 40 man Air Force unit in the ‘60’s. I think you could get a faster rate of fire from a 1911! Not even close to an MP 40.
The idea was to throw a lot of lead into the air, making it highly hazardous to step into the zone of the spray. This strategy kept the combatants from the other side from advancing through areas without sufficient cover.
Totally different from a “marksman”. Those specialists were expected to aim and place the projectile rather accurately into a specific target.
The local range has a “Machine Gun Night” a couple of times a year.
Fun gun.
We had these in Germany. The Mechanics were issued them. It was the simplest weapon I have ever broken down and reassembled. I was a SAW Gunner and the Grease Gun was just so different and basic.