.450 Bushmaster, .458 SOCOM and .50 Beowolf are all fine cartridges for 200 yards and less and you'll stay in the AR-15 platform.
There is a thing called the .450Corvette. It was conceived by Tim LeGendre, the developer of the .450Professional which he sold to Bushmaster and it became the .450BM. Anyway, the theory is to go to an 18" or longer barrel, faster twist and rifle length gas. It will still run the .450BM factory rounds, but with a slower powder, you can push 300-400 grain slugs at the same velocities as the 225s. I am still not done with the testing and tuning, but it is certainly capable of much more than 200 yards. BPTactical actually made my barrel for me and Tim Legendre himself did the gas port and initial testing on my upper. It is pretty impressive.