Unless the P90 was modded since the last time I played [I've been in mexico a week and a half], most of you guys don't know how to use the P-90 correctly. I've used every single gun in the game at least once, on one account or another, and I've come down to the conclusion that it's really a toss up between the G-36 and the P90 on which is the best gun in the game.
Before you get to flaming me, I'll expain my reasoning.
The G-36 is simply retardedly powerful. It hits hard [takes only four or five hits to majorly screw up a guy], it's ungodly accurate and it's spred pattern is predictable.
The P-90 is my baby. I was getting a 2.0 K/D with the UMP-45 and the G-36, then the P-90 came out and I jumped to 2.3. I stopped playing CA for a few weeks and it's been variating...but my point still stands.
There's two methods of using the P-90, SMG spray and pray, or tactical shooter. Yes, it has a 50 round magazine. No, you shouldn't use all of it on one person. In real life, a Drill Instructor will smack a recruit on the back of the head with a cleaning rod if it sounds like the recruit went fully automatic on a target. I've sprayed and prayed with the P-90 before, for which it works, but is hard to control.
To use the P-90 to it's full potential, look down the site and tap rounds off at about half second rates. So for every second that passes, you should click about twice. If you time it right, you'll notice you're not getting recoil. Tinker with it a bit and you'll figure out the right tempo, but I can counter snipe snipers with the P-90 on Two Towers and Snow Valley by simply tapping the trigger instead of tossing bullets at them.
For closer quarters, the same process can be used, except in short bursts.
Just my two cents.