I think that tuning a revolver with a trigger job, grips, sights, etc. is customizing or even tuning. A custom goes a whole lot farther in the number and severity of the modifications.

This revolver was merely customized at this point:



Before it became a full custom at this point:


Max Prasac

Semper Fidelis

BIG IRON: https://www.youtube.com/watch?v=L6aXjMH5C30

Gun Digest TV's Modern Shooter: https://www.youtube.com/watch?v=nGo-KMpXPpA&t=7s