[akdavis]

I'm sure it sounds much different being there, but having a live recording like this is rare even if they are blanks.

I was not aware blanks sounded different but perhaps a trained ear picks up on that sort of thing.

A significant part of the sound of a gun firing is the bullet exiting the muzzle at supersonic speed. Firing blanks can have number of effects on the sound of firearm, including significant changes in rate of fire in the case of automatics.

Compare the sound of the guns in your video to the sound of a 0.50 BMG shooting live ammo. For example: http://www.youtube.com/watch?v=AEhd5M9uHho

The live ammo produces a slightly deeper sound.

Err...as with most film from this time, the sounds are dubbed in and probably manufactured in a studio.