When I first got the game and played normal servers I HATED the p90 and couldn't wait to get one. When I picked them up I always seemed to own with them. Then I got it, and yea I owned but I've discovered I can do the same with all the sub machine guns.
The p90's biggest selling point is that is CAN hit stuff long range, something the other subs are not that great at (though the MP5 isn't bad). The larger clip size is nice too for close quarter fighting multiple foes.
I still whore with it the most, but it really depends on the map.
On hardcore games, and games with fewer people, you will see a bigger range in weapons.
From a K/D perspective I can do better with a G3 than a P90. G3 is more of a strategic weapon but I'm good enough close with it too, and its dead on long range.
My best game though was a team death match with 115 kills or so with an MP5, but I think I was playing with the short bus crew that game
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Ok posting the first post inspired me to try an MP5 again seriously.
So hardcore game (if you aren't playing hardcore you are missing out)
MP5 red dot
Bomb Squad (Just for kicks)
UAV jammer
steady aim.
First game I join is domination Back Lot. My team is getting creamed and camped at C. I start out just trying a long range shot (killed no problem) and then went 1-6 or so as my team was just getting wiped out.
Once I get the flow of things I break out, we battle for B and A.
Long story short I finish 20-10, #2 on my side (this is after starting later). MP5 seemed to do better than the P90 at short range and about the same at long.
Second game, Bloc.
I'm not a big fan of Bloc. I kept trying for B and dying. I had a G3 set up with smoke and use that and team takes B finally. I'm like 1-8 by now and get kills with the G3 a bit. Then I switch to the MP5 and now that I'm hunting instead of charging I do excellent and end up #2 again (I was cover a lot for point captures).
Over all I think I'm going to swap out the p90 in a slot or two for the MP5.