Can I double tap (as per the feat) with a .44 magnum in d20 Modern?
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.
Comes down to personal preference for realism versus action heroism.
My own preference is to be able to use double Tap for even single action revolvers, as even during Western times there were people who could cap off with an old thumb buster in seconds. As to whether or not these people could get those rounds off with any kind of accuracy, that comes down to fate and the roll of the dice. I do the same with pump and bolt action weapons, though the character sacrifices their move action to do it. All they do is cycle and shoot that turn.
I would have your character, be able to do it. Though you would be firmly planted and aiming with as much care as you could with that much stopping power recoiling in your hands. You would probably not want to akimbo that .44 and you would want to be pretty close.