To get all the weapons in the game, you must press Alt+F4....no wait thats god mode...I'm kidding that actually quits the game.
First you need to add
-devmode to the shortcut it will look like this
Once you've done that, start the game, once in game(map loaded) type
i_giveallitems and repeat till your ammo is full
It gives you every item and weapon including the TAC cannon as impotent as it REALLY is
Oh and apparently theres a few that leave you empty handed without the ability to punch
You may also want these
i_noweaponlimit = 1
g_godmode 1
more codes here(they also explain how to make it so you dont have to keep re-entering them)
http://www.mycheats.com/list/cheat/754/cheat_codes/pc
Due to Devmode the F keys do different things I don't have the list handy but
Numpad 1 gives Shotgun, MOAR/MOAC and AT-4
Numpad 2 give Unlimited ammo (Excluding Grenades and Claymores and C4)
I use the numpad to play and have found those two on my own
Cheathappens has an entire list of the F keys and such that was released to the public today
http://www.cheathappens.com/show_cheat.asp?ID=23367
Another thing they have is Trainers which do some cool stuff for this game and others, but you need to pay for an Unlimited account, if you dont want to wait for them to release them.....14.95 for a year or 19.95 for lifetime. the latter is the better deal.