Ok so here is my problem. A friend of mine recently purchased a 16" barrel AR-15. the lower receiver is stamped Rock River arms, however the Upper has no markings. the barrel is also strange with the last inch of the muzzle being flared out (almost like a very short integral suppressor). When he purchased the weapon it had a RRA detachable AR style detachable front sight attached to the front gas block, and what we later determined to be a fake ACOG on the rear. He just purchased a MAGPUL MBUS rear sight for the gun and we tried sighting it in today at 25 meters. with the front sight post almost maxed out we had it zeroed for 25 meters, however at 40 meters the weapon is shooting almost 1 foot high! I have 5 years military and shooting competition experience, however I am by no means an all knowing master of this weapon. I have never had this problem with any AR i have every picked up. Does anyone have any ideas as to what the problem might be? also any ideas on the barrel/ upper reciever?