Bad experience - Leupold Mark AR MOD 1 6-18x40mm Rifle Scopes

I purchased one of these mounted it on an ar15 and proceeded to sight it in. It seemed to be adjusting properly for the first 6-9 shots and then it was like something loosened inside and it started shooting erratically. It was all over the place and wouldn't adjust out. I returned it to leupold and they sent me a new one. Same thing! We repeated this process x3! Each time I would remove the scope and mount my 3-9x40 leupold on the gun and it would drive tacks so I'm pretty sure it was the scope. The good news is that eventually leupold made it right and sent me a different more expensive scope and it was fine. So not bashing leupold at all cause I've always had good luck with their products but not on this one. Just my experience.