Knight of Light gave some good advice here and especially about bullet twists so I see no need of repeating it here.
But to answer your question whether you need a custom barrel or not really depends on what type of shooting you're going to be doing at 600 yards. If you're competing, then obviously, you want the best that's out there. I can attest to Krieger's customer service. They really know their stuff.
If you're hunting and having fun, it may not be absolutely necessary. I also put my 2 cents in with a 20 (or longer) barrel for those distances. There are some tricks and add-on's to make an otherwise stock AR shoot very well. I prefer a 24 inch barrel though instead of a 20.
First and foremost, if you use over the counter ammo, there's not much you're going to be able to do to increase velocity. But there are a few things that can be done mechanically, though with any semi auto you're going to lose valuable pressure inherently due to the gas system. This is unavoidable, but this is where the "tricks" come into play.
Adding a bolt carrier weight will keep the bolt remained locked a bit longer. I was lucky enough to get one of Devon's (SNS Industries) prototype carrier weights. In all honesty, it's very well made and does exactly what it's supposed to do.
Also, exchanging the stock buffer spring to a Tubbs/Superior shooting chrome silicone flat coil springs will also add some delay, but I also like their overall function much better than a stock buffer spring. They really are sweet.
Also adding a heavier buffer will also keep the bolt closed longer, and will also aid in reliability, though I admit I haven't ever had any problems from that anyway.
And finally, Armforte' and MGI markets an adjustable gas tube. This tube can be tuned to the individual load you're shooting to where it just barely clears the magazine and ejects the shell and then cycle another round, or can almost be turned completely off in essence creating a single shot rifle letting as much pressure possible push the bullet out of the bore and downrange where it belongs. This means a lot to accuracy, and also downrange punch.
I'll attach a pic of the whole rifle below,though not all the upgrades have been mentioned here, or are even seen in the pics, but it is one sweet shooting rifle nonetheless.
