well congrads on your purchase, but i am a little confused by your questions. the first question, the ar 15 is a smaller version chambered in many different calibers from .204 ruger to 6.8 spc. and everything in between. mechanically they are similiar and even share parts in some instances. to be specific though an ar 10 is an armalite only, every other is a .308 ar. your other questions is where i get confused, is there a builder? uh, bushmaster?(recently purchased by dpms i believe) not sure what you are looking for here. flat top receivers are optic ready, you only need to chose an optic and a mount.your third question i cannot even begin to answer, unsure what you are asking, hope i helped a little