FNH's PS-90 has a 50 rd magazine and is semi-auto; the P-90 is full-auto but you probably aren't going to see one of those unless you're on the SWAT team. 
A gang of 20 is probably going to overwhelm a guy with any kind of non-automatic weapon, but fire is different.
A pair of claymores sweeping across the lawn, why waste the gas?
FNH's PS-90 has a 50 rd magazine and is semi-auto; the P-90 is full-auto but you probably aren't going to see one of those unless you're on the SWAT team.