The purpose of porting, polishing, chamfering and bird mouthing air conveyance infrastructure is to reduce drag or pressure drop and inducing velocity stack. More efficient airflow to the projectile, in turn, increases projectile velocity.
Because velocity is increased, less hammer preload is required because the internals are delivering the air more efficiently which in turn, increases shot count.
The key and most important thing to remember is that the transfer port outlet inside the barrel should never exceed 80% of the bore diameter. This only because when loading a pellet into the breech anything larger than 80% tends to grab the projectile and gouge or deform it.
The projectile will try and load into the transfer port if the transfer port is too large.
The key is to taper and smooth like a funnel towards the chamber. Bird mouth the inlet side of the transfer port in the barrel so it is larger than the outlet side of the barrel again funneling the air rather than fighting a sharp 90° transfer of direction and creating jetties or vertices that essentially reduce the flow by upwards of 50% (See attached sketch).
Just like in engine building, anything you can do to help an aspirating system loose drag or pressure drop is an improvement in performance.
