I'm posting this in case somebody, one day, wonder about the same details.

The revolver is located in a cave in mad brothers stage. It allows you to run while shooting, unlike the knife or the rifle. It also shoots faster than the rifle. I thought it might have been viable for a speedrun, but it's not.

Grabbing it is a big time loss since the cave is not located on the main path for clearing the stage. You either do the cave twice (once to grab the revolver, once to get out), or go buy the ladder to exit it immediately, and do it just once. Either way, that's too much time lost.

Finally, it is less powerful than the rifle (I wasn't sure since every regular ennemy takes only one hit to be defeated, regardless of the weapon). The only boss defeated with firearms in this speedrun, Keith the hawk/eagle master, is killed with 39 revolver bullets VS 22 for the rifle.

That settles it. Revolver is a ton of fun for a more casual run though.