Damn, that's impressive Narco - my highest killstreak (according to the leaderboards) is 21, and needless to say it was in the above game
I just can't jive with the inaccuracy of SMG's, probably just different playing styles (though I'm surprised you don't use Marathon)
I don´t use Marathon on any of my classes. From the blue perks I like Sleight Of Hand (faster ADS!), but as my secondary weapon is usually a Stinger I don´t have ammo enough to get a big killstreak, so I need to keep the Scavenger.
The secret to a high score on this game is the killstreaks. I mean, if you can kill 5 in a row (not that difficult) you can get a Predator. That means that you get at least one or two more free frags from the Predator missile, bumping you to a 7 killstreak, and that means a Harrier. If your Harrier kills 3 or 4 enemies (not difficult at all) you get a Chopper Gunner, and Chopper Gunner means game over. That thing is beastly. So, you basically just turned your 5 killstreak into a 20+ frag fest.
Maybe you will need to kill a few more guys if your Predator only killed one or the Harrier was destroyed, but that´s about it. Claymore with Scavenger is awesome to get some free kills too, as you can spread 3 at the same time on the map, and when you hear them exploding you know where the enemy is coming from (same thing when they destroy the tactical insertion).
If you´re getting your Predator easily, then you can switch it for the Nuke (making the Harrier your first killstreak), but that´s kind of just to show off, as the Chopper Gunner alone is usually enough to put the match on an irreversible score.
On PC it´s more difficult to get the killstreaks because people aim really fast. Snipers are unforgiving so Cold Blooded, Silencer and Ninja helps a lot keeping you alive. Unfortunately the only weapons that goes well with silencer without Stopping Power are the UMP45 and FAL. UMP45 is better because makes you move faster, has a good hip-fire and it´s automatic. Kills anyone on the map with 3 shots no matter what (less if you score a headshot). To kill at distance with it, the trick is to shoot one bullet at a time as if it was a semi-automatic. It´s a bit weak on extremely close quarter combat due to the low rate of fire, that´s why I avoid these situations unless I have AA-12 as my secondary.