heres a chart from sniper central.
http://www.snipercentral.com/223.htm
the .223 will not by any means have enough energy for a clean kill on humans, or game at 1000 yards, but it is used by many, many military marksman for there shooting compotitions.
and heres some really good reading for the .223, and its slighty bigger brother, the .223 AI (which can also be chanberd to fit into an AR-15.
http://www.6mmbr.com/223Rem.html
i should have said in my earlier post that to shoot 1000 yards accuratly, you will have to have a barrel that is made for the purpose of stabilizing the heavier bullets. from 69 grn to 90 grn. 90 grainers usually have to be feed singly into the rifle, as they tend to be loaded to an O.A.L that it just to big to fit in the 2.260" magazine. there are mags availible, made from stainless steel, that allow for longer O.A.L's, but i do not know them off the top of my head. 77 grn sierra SMk's are used alot to shoot 1000 yards and they can be loaded to do so at the max O.A.L that will fit into the AR magazine. i believe that they will stabilize in a 1/9 twist, but a 1/8 or even a 1/7 would probably be better. i think that the cut off limit for 1/9 twist is 55 grn- 69 grn. for varmiting out to 350 yards though. 55-60 grn bullets would be a the best bet. they are very accurate and retain a lot of energy to that point. after 350 yards, they seem to run out of stem. but then thats where the 77+ grners com in handy.
now, i have heard that hornady and berger are both setting out to make long range bullets specifically for the REMINGTON 6.8 SPC. which will also fit on the AR plateform. this will infinitally extend the use of the ever growing 6.8 round.
there are alot of better choices for shooting 1000 yards than that of the .223, but it will do it. and do it accuratly if the rifle and shooter can do there part.