My opinion is that a true custom is a rifle that is made to fit and suit the client. Custom barrel, fitted to FAR closer tolerances than factory is able to produce, possibly a wildcat caliber or something not readily offered by a big maker.
Most importantly is that the rifle ACTUALLY fits not only the shooters style requirements by physical size and proportion. To get the best accuracy I believe that if the rifle fits like a small condom, the shooter will shoot it better, stuff like cheek pieces that fit THE shooters face so weld spot is constant, palm swell that is a copy of the shooters hand enhances trigger feel and consistant let off.
Some people have needs different from the norm, due to height , weight or other body proportions or distortions that may not fit what the mass produced rifles fit. Or they may have a specific requirement like left port right bolt or something for a specific purpose.
Then there are the modified commercial rifles, these are reworking a factory offering and adding aftermarket goodies, these tend to be less expensive and for the most part can be made to shoot extremely well, but I do not think of them as true custom rifles just very modified factory rifles. :mrgreen: