Here's what I use and it works very well as long as you have a rail on top of your AR or whatever. It locks on and stays on without moving around.
https://www.amazon.com/Caldwell-Resistant-Convenient-Picatinny-Collection/dp/B00EB5OU0Q/ref=sr_1_3?s=sporting-goods&ie=UTF8&qid=1535638475&sr=1-3&keywords=Caldwell+Pic+Rail+Brass+Catcher