I was in the military for 6 years. I loved my m 16 A2 very much, and recently decided I wanted to build an AR 15. However I am ignorant of AR 15's for the most part and after the purchase of my first component I have a couple questions. I bought a gutten lower receiver. It is a Superior Arms 5.56MM. My questions are: Can this also fire .223 or only 5.56 rounds? Where can I find a compatible trigger assembly online? I am unsure if they are all pretty much universal or if I need to buy a specific one for my lower receiver. Thank you very much for your time.