The biggest reason scope mounts have mounting issues are the places we choose to mount them on the firearm. We can all agree that no one likes to drill and tap their firearms? Sometimes there is no other choice but to do so but we would all rather not. As a designer, to design a no drill and tap mount, I have to pick a place or places to attach the mount to existing features on the firearm. That can consist of front sight slots, rear sight slots, existing tapped holes and so on. Here is where the problem comes in. When the manufacturer machines whatever holes or slots into your firearm they generally do not do it with the intention of us mounting something on them. They keep the tolerances they need to make the part they are attaching work, not our scope mount. Let me give an example.
RUGER® GP-100 rear sight elevation screw hole. We use that for mounting the rear portion of our mount to that particular revolver. RUGER® uses that hole strictly to mount the rear sight using the elevation screw. For their application the screw hole can move a little forward or backward and the hole still works for their purposes. We would love to have the hole in the same place all of the time but it is not there for our use. We have to design around these types of things all of the time. When we manufacture a new scope mount we will run a small batch and sell them to customers. If we get feedback from customers having problems we document it and adjust accordingly. We may have to elongate a hole or move it forward or backwards to get it to a sweet spot where it fits all applications. We document most all customer input and fine tune our dimensions until we no longer get calls about the mount having fitment issues.
This is not an excuse it is just a little explanation of why things don’t fit just perfectly all the time. We can hold +-.001 dimensionally all day but you have to come up with the sweet spot first to get the machined feature in the correct position. We strive every day to design and manufacture the best parts available. We run small lots and do regular inspections on all of our parts to keep quality superior.