I used to do some gun repair for friends who owned a Trading Post and before hunting season in the Fall, I'd sight-in maybe 40 rifles, mostly with scopes. I never had one come back because the people couldn't hit with it. One guy with a Browning Semi-Auto had me sight in his rifle that had a receiver (peep) rear sight, that couldn't move, but he was paranoid, so I'd sight-it-in, but it never went off from my original setting.
I have a sighting-in vise that the rifle is clamped-in, after shooting a group on a target. It's called a Sweeney Sight-Align. To use it, the rifle is clamped in this portable 4-legged vise and pointed at the group on the target that you just fired. While it's fixed at that point, you make adjustments on the scope until the crosshairs are in the relationship with target center that you desire. Then, remove the rifle and shoot a shot or two at the target and the shots are usually exactly where you wanted the rifle to group.
The process worked very well and I not only made money, but didn't kill my shoulder doing it. We never had any complaints about a rifle not shooting where they wanted it to.