Saw one gunshow seller (Former Army Armorer self described) carrying an AR fer cell. A buyer convinced him that since the BUIS were installed backwards he'd have to remove the gas block and tube in order to put them right. I couldn't believe it. He got a $200 discount if memory serves for his 'trouble', LOL!i traded into a LWRC AR that had the iron sights on backwards...and i didnt notice til i was sighting it in...lol