I just got a preban colt hbar and know basically nothing about it (other than i got a great deal) and wanted to know the proper breakin for the barrel.  It has fired one shot in its lifetime so far.  Also, ammo recommendations.... 5.56 or .223?  Best ammo for plinking (something clean pref.)? Best ammo for target shooting? best ammo for small game hunting?  what is the twist rate on the hbar?

Break in: Clean it out thoroughly - including the barrel - and shoot it.
Twist rate: It's should be marked on your barrel.
5.56 or .223: Depending on your barrel chamber. Colt is chambered for 5.56 unless you have a after market barrel.
Best Ammo for plinking: Cheapest stuff that will function reliably in your rifle.

I just picked up an Armalite- Eagle Arms M15-A2.  Found a paper inside the owners manual warning of proper barrel break- very extensive description including cleaning after each shot for the first 11 rounds.  Perhaps this is just an Armalite procedure- anyone have any experience with Armalites (my first one, my sixth AR)