The first AR-15 I owned was extremely accurate with the iron sights. I bought a Colt 4X scope and a mount that attached to the carrying handle with a large thumb screw.
I was disappointed that it was less accurate with the scope than with iron sights. I eventually learned that there was a bit of play between the upper and lower receiver. The weight of the scope caused it to move easier.
I later learned there is a cure but at the time did not know.
What is the cure should I ever encounter the same situation with a carry handle mounted 4x scope? I saw one from Brownells that is for sale.