The first time Vortex said it was something inside the side focus mechanism that broke loose. My scope was from their first AMG production run and they said they knew it might be a problem. The second time they tried to suggest I didn't know what I was doing and that my barrel was worn out... despite carefully tracking the problem and having the same batch of ammo shoot a roughly 4 inch group at 100 yards with the AMG, and then turn around with a brand new VX6HD shoot a 2.6 inch group at 1000 yards. They never claimed anything wrong with the scope the second time but did send me a brand new AMG, which I sold.
My thoughts are the scope couldn't handle the recoil from my braked 9.5 pound 338 ultra mag shooting 300 grain bullets. Definitely a heavy recoiling setup. The problem originated as changing points of impact between groups that slowly transitioned into it shooting all over the place.
I would hope new AMG's are made a little better, and I like to think they kept that scope I sent back as a tool to help them do that.
The VX6HD has held the same point of impact now for two years on this 338. I thought long and hard about the 4-16x50 ATACR, and will go that route if the VX6HD ever goes south.