Can I double tap (as per the feat) with a .44 magnum in d20 Modern?
The .44 magnum is listed as a semi-automatic weapon so, yes you can double tap if you meet the following pre-requisites:
The mechanics of a revolver are VERY different from the mechanics of a semi-auto pistol. When fired, a semi-auto will fire the round, eject the spent cartridge, chamber a new round, and recock the hammer.
When a revolver fires, the gun returns to it's "safe" state. The hammer is forward and uncocked. When one attempts to "double tap" the revolver, you need to pull the trigger so that the cylinder rotates, the hammer comes back and then fires.
Double tapping on a semi-auto is much easier as the trigger is in it's "hair trigger" position.
I'd say you should not be able to do it with a revolver.