There are tons of options out there. I personally do not reload so that is why I would say go the 223/5.56 route. Lots more of affordable ammo out there for 223 than for 204. I have never owned a 204 so I can not say one way or the other on killing ability between the two, I have shot everything from crows to deer with my 223 all dead. If you are looking for a off the shelf highly accurate rifle with a 20 inch barrel or longer, I say Rock River. If you are wanting to go the route of putting this together yourself, build a lower yourself with a good trigger and get a complete upper built by yourself or buy from someone who can build how you want. Lots of good companies out there to build you an upper with what options you want, like what barrel, bolt carrier, bolt, handguard etc. I have a rock river advanced tactical hunter which is highly accurate. I would look at Ironworks tactical for an upper or Bison armory. Good products.