From 'Wiki':
"Bespoke is a British English term employed in a variety of applications to mean an item custom-made to the buyer's specification. While applied to many items now, from computer software to luxury car appointments, the term historically was only applied to tailored clothing, shirts and other parts of men's apparel involving measurement and fitting."
If you order an item such as a shotgun completely to your wants and specifications and it is built from the ground up for you, today its termed bespoke. It may also be termed custom but usually as Pete indicated the term custom is misused.
If your gun is a refinished, restocked to your specfications, it could be termed custom but not bespoke. If a standard factory model to you stock dimensions it is not bespoke but could be looseley be called custom.
All else is neither bespoke nor custom.-Dick