I'd go the direction of considering it an SMG (unless you want to go the route of PDW - which is really the class the P90 is in now - a currently sexy subject) and give it a better chance at penetrating armor, but do less damage.
AP rounds generally don't do as much damage as softer ones, but this thing fires enough rounds in a burst fast enough and accurately enough that the point is really moot. So, you can figure it can do a 1, 4, or 10 round burst with 1, 2, 3 chances to hit in addition to the regular group target rule. I'd say 3D6 is good for damage, but the range ought to be short...I know you could slap a carbine length barrel on one - just have a range bonus if a player wants to add something like that to his personal weapon.
It handles well in CQB, the whole point of a PDW really, so it should be more accurate at Close and Short range than an SMG, but about the same at Medium. Long+ needs the carbine barrel mod.
The biggest advantage of a gun like this (Traveller wise) will be the monster ammo capacity, the close quarters accuracy, and the high ROF for increased chances to hit multiple targets. It'll be great in a ship, but not the king of the battlefield.
BTW: my agency recently looked at a couple of them and as a firearms instructor I played with them. I'm not bonking a cop but I play one in real life
These P90's are great and cool, but except for the tactical guys and what they do I'd rather have the standoff capability of my patrol carbine. Currently that is my AR, but it used to be an HK MP-5 in .40SW.