Let me make a comment about Blackthorne before you go buying from them.
Only two companies I know of in the industry have a rep so bad that it is legendary. Model 1 and Blackthorne. In the case of Blackthorne this is their 3rd or 4th company name (Vulcan, Hesse, etc) trying to escape from all the bad complaints online. Easier for them to change their name and fool people for a while rather than admit their mistakes, clean up their act, and earn some trust.
There is a reason their prices are the best. They don't put money and skill into their equipment. They have to sell low to entice buyers who don't know their past. They hope that price grabs you before reading the forums. Go to any of the big AR forums and ask their opinion of Blackthorne. All will point you elsewhere.
Whereas the J&T I posted above worked so hard on their rep for so long that when they created their sister company called Double Star it got rave reviews all across the internet. I'm still shocked they even still bother with the discount J&T company since they get full price on their complete Double Star AR's they build now. But I'm happy they still are around.
If you buy from Blackthorne or Model 1 you put your money at risk. Good luck with problems you may have as well. Better, imo, to spend the extra $50-100 and get something from Del-Ton, J&T, M&A, or AR15-Kits so you know you will get good gear and have someone who will take care of you if there is a problem with your order.
Caveat Emptor!
Or even just build your own! We could easily tell you everything you need to build one and how to do it. There's a reason this place is called the Armory.
Last, if you buy 5.56 you can shoot .223 in it, but not always the reverse. Also the .223 round is civilian. So you miss out on using inexpensive military surplus ammo for practice. When in doubt always go with 5.56 NATO.