I have completed sevral ak47s and now i want to take on something with a little bit more challenge. i have access to an end mill and im considering building my first ar15. I am pretty ignerent to where to begin when it comes to what receiver blank to start with. how complete can i find one that is still less than 80% and where should i look for it at. again any and all information for a first time builder would be appriciated.