The revolver is unfortunately ambiguous in its wording, and as far as I can find there is no official ruling on whether it can be used to make Might attacks or not. However my opinion is that it should not be able to make Might attacks rolls (or at best, is useless for making Might attacks).
Firstly, even if you could use the revolver for Might attacks it would provide no additional benefit as the revolver card states:
Roll 1 additional die on your Speed attack roll
This explicitly makes the bonus die only available on a Speed attack.
Also it would be highly unrealistic to allow Might attack rolls at range with the revolver (you can't improve the way you shoot a gun with sheer force) so a Might attack would only be possible from within the same room (pistol-whipping, basically).
So if you can't use Might at range with the revolver, and gain no additional dice while making a Might attack with it, then a Might attack with the revolver is the equivalent of a barehanded attack.
When this is taken into consideration for the zombies in scenario 21 then, if you decide that the revolver can only make Speed attacks, then you cannot kill zombies with it (it would only stun them).
Even if you do allow Might attacks with the revolver from within the same room you may as well not even consider that you are making the attack with the revolver, and just say you are making a standard barehanded attack since they both provide the exact same chance of successfully making the attack.