i personally would stay away from a olympic arms, out of all the ar manufactures ive heard by many it is the worst, if your ganna spend money on one get a good one, i am a big fan of Rock River Arms product, i love mine and as far as fit and finish its practically unbeatable, tightest fitting ar ive seen, but you are open to your own opinion and get what u want im just saying you can get a nice rock river for around 850 bucks which is pretty fair, i would try to stick with the main manufacturers ie. Colt, RRA, Bushmaster,DPMS. again just my personal opinion but i hope this helps ur decision. as far as accuracy to 400 if your looking for a std 400plus yrd gun i wouldnt go with a ar15 id go with a ruger 77mk2 or a rem 700 of some sort i went with the 16 inch midlength on my rra for coyote and varmints but i was looking for a 100-250 yrd gun which i can very accurately shoot with it and its shorter and handier to move around with i didnt want a benchrest rifle, as for it shooting to 400 yrds i dont know i havnt tried it, my 22 in .223 handi rifle has no prob with being accurate at 330 yrds and so far my ar and handi shoot relatively similiar. do what you feel is right man im just trying to give a little help from what ive experienced. hope this helps
thanks
adam