Just started playing on hardcore TDM and HQ pro the other day and loving it. I am getting way more kills (since in normal mode I just tend to wound people as they run past me). I have been switching back and forth between these 3 classes:

Default Hardcore class:

SCAR-H w/ ACOG and Heartbeat sensor
AA-12 with grip and red dog
Flashbangs
Claymore
Bling Pro
Hardline Pro
Ninja

Machine Gun class:

AUG HBAR w/ grip and red dot sight
Stinger
flashbangs
Claymore
Bling Pro
Stopping Power Pro
Steady Aim Pro

Sniper class:

Barrett .50 w/ silencer and ACOG
Stinger
flashbangs
Claymore
Bling Pro
Cold Blooded
Steady Aim Pro

I think I am going to switch my secondary on my default class from the shotgun to maybe a pistol or machine pistol. I find that I really don't use my secondary as much in hardcore though since I either have time to reload or die before I can swap weapons due to the lower health. I also just realized that steady aim pro isn't doing much for me in my machine gun class without the on-screen HUD stuff I get 99% of my kills through the sights. I am working on unlocking the heartbeat sensor for the Barrett and AUG at the moment, although I like the ACOG on the AUG and might just leave that on there with the grip.