First off, you have a platform there that you could build pretty much anything you can imagine on up to and including all the RUMs and the wildcats based off them. Only rounds I would stay away from with that Stainless Steel receiver would be the very large ones based on the Lapua and big Wby cases. If it were a chrome moly receiver I would feel comfortable with those as well but not on the softer Stainless receivers. More then likely you would have no problems but that is not good enough to recommend it.
600 yards is pretty easy to reach consistantly with todays modern chamberings and bullets. The simplest way to get this would be to leave the receiver set up for the '06 size case head rounds and wildcats. Three come to mind right away and most have already been mentioned, 25-06 AI, 6.5-06 AI and the 280 Rem AI.
I like the AI rounds based on the '06. They will in most cases get you so close in performance to the standard length belted magnums in the same caliber its almost embarrassing, especially in barrels in the 24" lengths. Longer barrels will get you more with the magnums but the shorter ones will not be so impressive.
Also, with the correct bullet selection, all three of these will get you easy 600 yard reach on deer size game. The 25-06 AI is probably the most challanging to get to this range effectively as the list of factory bullet options is somewhat limited but if you were to use the Wildcat 125 to 142 gr ULD RBBT you would easily have this much reach. The 110 gr Accubond and 115 gr Ballistic Tip will work but will be blown around a bit more in the wind and retained velocity and energy will be less then the more efficent Wildcat ULD RBBTs.
The 6.5-06 AI is pretty much my idea of the all around star for deer in an '06 sized case. Bullets are easy to get for long range shooting, I have a boat load if the new 130 gr Accubonds headed my way to test in this very round and the 6.5mm WSM as well as my much larger 6.5 Allen Xpress and 6.5mm Allen Magnum.
Any of the tipped bullets in 129 to 140 gr range will be impressive performers on deer out to 600 yards easily, even farther in fact. Plus you also have the choice of alot of very quality long range match bullets such as the ones from Wildcat, Berger and Sierra. Personally, I feel the 6.5mm is a little small to be using their match bullets on big game, I perfer 7mm or larger for match bullets but I can not deny they work extremely well, just not alot of frontal area for shots on the fringes when they occur. That said, you put one through the vitals, deer is dead.
The 280 Rem AI is a great round as well, in a 24" barrel will come very close to many 7mm Rem Mag loads and will top most factory 7mm Rem Mag loads. FOr 600 yards, the 140 gr bullets will work, 150 are better and in my opinion, the 160 to 170 gr class are the best. In that class you have the 160 gr Accubond, 162 gr A-Max and the 169.5 gr ULD RBBT.
I personally feel anything heavier then these are a bit more then the 280 AI will handle properly in a hunting rifles.
Thats the easiest way to go. If you want more performance however, there is alot more to be had. If you have read any of my posts you realize I really do not know what the term overkill means!!
To me 600 yard deer hunting is still long range, in certain situations 300 yard shots on deer can be considered long range and in those cases, power is really the only thing I would be concerned about. There are many wildcats and factory chamberings that will get you to that range with alot of power. As mentioned, the 7mm RUM and 300 RUM are great choices but also are the 7mm Rem Mag, 7mm STW, 7mm Dakota, 300 Win Mag, 300 Wby.
Wildcats, obviously my personal favorites would be my own children. For deer hunting, take your pick, 257 AX, 6.5 AX, 270 AX, 257 AM, 6.5mm AM or 270 AM. All will make 600 yard reach pretty much silly ballistically. Easiest to load for would be the 270 AM and also the highest performing when looking at trajectory and retained energy as well.
You do not need a wildcat however. A 7mm STW or 7mm RUM is what some would call over kill at 600 yards but loaded with the right bullets, when the wind is blowing across a big canyon, the extra Horse power can make the shot if your practiced with the rifle whereas the smaller rounds will often fall short enough to prevent a practical shot to be attemped.
So really, the sky is the limit.
Kirby Allen(50)