The writer said the Oni Press title will have a conclusion, rather than running indefinitely.
"There is a definite ending in mind for the series," he told Comicosity.
"It took shape at some point during the scripting of the first story arc, so elements from those early issues have a part to play in the endgame. The major arcs are all planned out (with a little wiggle room if we need it) and the series is planned for roughly 50 issues."
The ongoing wild west title centres around six cursed pistols that will allow that possessor to change the fate of the world when brought together.
Bunn launched the title with artist Brian Hurtt in 2010.
The Sixth Gun entered a new arc, 'Winter Wolves', with this week's issue #24.
Syfy announced a television adaptation of the series last year.