The only scenario in recent history in which I've heard where a mob of people rushed an armed individual after he started firing was the CIA operative in the prison in Afghanistan. One of them tried to hold his ground with his 9mm, but was overwhelmed and killed. The other operative that had the full auto AK-47 couldn't keep them back and he lived because he was in better shape and could run faster. During the Phillipine War the Moros Tribal warriors tightly bound their appendages in order to slow blood loss and took narcotics to keep them fighting longer. As a result, the US Army discovered that in addition to shot placement, larger caliber weapons were needed to stop the charging waves. The return of the .45 caliber revolver and the use of the 12-gauge shotgun were two such weapons which effectively put them down. Just one of many examples that history has demonstrated that sometimes smaller does not always get the job done. More tissue destruction shot per shot is what helped in that conflict.
Colonel Jeff Cooper commented in one of his books about magazine capacity. In a shootout when the lead starts flying everyone on the battlefield runs for cover and it becomes a foxhole/trench fight. At this point, a shot or two is exchanged during brief exposures while the opponents try to move into better firing positions. There is often plenty of time to reload, so total ammunition carried is of more importance that magazine capacity. However, these exchanges are rare even for those who carry weapons for a living.....let alone civilians carrying for protection. The victor is usually the one with the long gun(shotgun or rifle) which offers a great leap ahead of the mediocre terminal effects you get from handguns.
You're not going to find anyone in the LE, military, or general shooting community that has relivant experience who will advocate the use of PDW calibers in place of classic service calibers. There is no merit to the calibers other than armour piercing and controllability during cyclic fire. What limited reports I have heard of in the law enforcement community with FN P90s is that it takes a generous burst of fire to effectively put down an armed perp and it is practically worthless when trying to shoot through auto glass.
