First off what great state and city are you in. Different locales have different rules and regualtions. I can tell you all the options in Kommiefornia.
Secondly, I believe that blowback calibers like 9mm 45 acp 10mm are actually harder on your lower reciever than 5.56/.223. You may want to rethink your choice of caliber for your polymer lowers. Either way, keep us posted with your choice and results!
Assumption is the mother of all ****ups.
Know what you know, Know what you don't know. -Paul Pedzolt
"Jack of all trades, master of none, though oftentimes better than master of one" -Mynshul