If you can get Silver State Armory ammo the 6.8 Remington is a great cartridge for deer. It requires new magazines (PRI makes what the general public considers the best), and has a different bolt face than the standard .223. Ammo is harder to find though.
The other I would look into if you want plentiful (and cheap) ammo is the 7.62x39. There are issues with milsurp ammo and light primer strikes, as well as magazines that are reliable (MWG 10 rounders are the only ones I have had no issues with - C Products guarantees theirs, but I have had issues with them in the past) but it is excellent for deer out to 200 yards. Again the bolt face is different than a .223.
The 300 Whisper is great if you set it up for supersonic loads (carbine length gas system or adjustable gas block) and you
don't have to swap magazines or bolt heads! You have to reload in order for this to make sense though!
Unfortunately AR10 and LR308 platforms are usually several pounds heavier than standard .223 AR's!
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