Hello everyone. So I'll have about $500-550ish doallrs to spend and I want to buy an AR15. I haven't shot one since my Army days years ago ( M16A2). This rifle will be obviously mostly for the range and I guess a back up HD. I'm hoping for my money I can get something reliable. Would like it to try to be as mil spec if that's even possible with a flat railed top so I can mount a red dot and back up flip up sights. Also hoping to keep it as M4ish as possible. I think 16 inch is as short as I can legally go? All opinions are of course welcome. Thank you.