The ‘rifling’ outside the barrel was done to counter act the spin a rifled bullet imparts on the gun. In theory it would allow the gun to stay on target better. In practice it achieved this. But unfortunately for the inventor of this machine pistol, Russ Robinson, any effects are so minor they can’t be appreciated.

Also the magazine ejects once emptied, so you can put a new on in faster.

https://smallarmsreview.com/robinsons-model-11/