STOP ! If you hold by the mag well, yer #### will fall off!
There, I got yer attention!
Use what works for you, your body geometry, your kit setup, your tac situation (buildings, hallways, ops room, galley, above decks, inside the head, going through a hatch (Sailors know what I mean), number of zombies to engage, chest rig, belt rig, battle belt setup, plates, soft level II armor, shooting position, shooting behind cover, shooting beside cover, shooting above cover, shooting with NBC/hazmat gear on during a Meth Lab raid, yada, yada , yada. Hey, did we even mention an elevated heart rate? How about shooting in condition red or condition black (you might be wounded or injured) ? How about having to shoot weak hand because shooting from your strong hand side during an advance (on the zombie position) will get you injured. I am a paintballer by night, and I have to switch up my shooting hand deliberately while moving up on an objective....
All of you are working yerselves into a frenzy here.... quite needlessly.
Get out and shoot under different conditions (that means get off the bench parked at the 100 yard range). Discover (I'm a high school teacher) what works for you under ALL the above mentioned conditions and get back to me. You are gonna have a variety of answers for a variety of conditions.
Bottom line: I got no answers for you!

You get back to me with the answers that you have discovered. In the meantime get out to your PRA or local 3 gun or IDPA or IPSC matches and learn, learn , learn. That's the excitement of this zombie preparation process.
Now if you live in the GTA, contact Bolivar for some serious match CQB intensity!
Cheers,
Barney