Well hell I shoot cast all the time in both 5.56mm and 300 Blk Ar's ! Works just fine!
OP I have a 16" unsurpressed 300 Blk and yes it lends itself very well to cast bullet use.
I run subs and supers in cast out of it very nicely. Now finding a complete rifle in the caliber will take some research. If your a reloader-when you go to the range make it a point to start picking up .223/5.56mm brass to convert to 300 Blk- very easy to do many video on youtube show you how. Probably be easier to get a 5.56mm AR now and then get an 300Blk upper later. For the ranges you expect to shoot at the 300 Bk fit perfectly. If you want more info on the 300 Blk feel free to PM me at anytime.