I had an S&W M&P Sport that's purpose was recreational shooting and coyote hunting. While I know there are 5.56/.223loads that can be used on deer, I wanted something that had a bit more authority. I did weeks of research with the criteria of: 1. reliability in an AR platform, 2. cost of new upper and any other items required for the caliber change over, 3. range of bullet weights available and 4. availability of commercial ammunition. After checking into all of these, I came to the conclusion that the .300 AAC Blackout was what I was looking for.
Since it is a 5.56mm necked up to .30 caliber, the Blackout can use standard AR-15 magazines and it will reliably function in the AR platform. I found uppers ranging from $549 (CMMG) to $899 (AAC)- I chose a S&W quad rail upper for $749 (just to have an S&W upper and lower). Last of all, loads are available form 85 to 220 grain bullets covering velocity ranges from supersonic to sub-sonic. While not the fastest velocity, it should get the job done for shots under 200 yards.
Just my thoughts. I did look at the grendel, but found it too limited in several areas to be realistically considered. Hope this info proves useful to you.