Fully accept the rifling is that of a left twist 6" barrel with a pitch of 1/12.
However, red mist descending over the inside of a pistol barrel is illogical and physically impossible with a round in the chamber.
It makes more sense for it to be a metaphor for the dimming of the pistoleer's life, so my reference to an eye's iris can be equally valid.

Just wait for Bond meeting Asimov's "I, Robot" then you'll see.

Acorn