I can reload 9mm for quite a bit less than .223. Not only are the bullets generally cheaper but using a 3.1 grain powder charge, I can load more than 2250 rounds from a single pound of powder. Where a 223 load using 25 grains of powder will go through a pound of powder in 280 rounds. That is an 8:1 difference right there.
I agree there isn't much reason for a 9mm rifle, .223 or 300 blk either. A good old lever action 30-30 or bolt acton 30-06 took care of everything long before any of the "new" rounds existed.
Lucky for us we don't really need a reason other than "just for fun"...think I may put that on my next form 1.
Does OP reload and will he reload for this rifle?
There are good reasons for 300blk: Performance from a short barrel, reliable centerfire rifle AR with sub-sonic bullets, same magazines and almost all parts the same as 5.56, better terminal performance than 5.56.
Spears and arrows took care of everything before firearms, and muzzle loading muskets took care of everything for a time, as did rifled muskets. THere's really no need for a fancy lever action 30-30