Well, those first two shots (blanks) may have been needed to neutralize the threat. You may not get a chance for a third shot!
Secondly, blanks usually won't cycle the action on a semi-auto without special adapters (at least on rifles). Hell, CCI Stingers wouldn't cycle my HS Victor reliably. Just what you need is a jammed gun/malfunction drill when you need it to save your life! <NOT>
Thirdly, MGLs require ALL GUNS to be locked up or have trigger locks/other disabling device when not UNDER THE DIRECT CONTROL of a Licensed person! Leave it in a nightstand unlocked, leave the room or go to the basement for something and you could be facing serious jail time (5 yr felony IIRC).
Anyone that wants to fire warning shots (LEOs are no longer ALLOWED to do this in MA) or fire blanks, should save their money, get rid of the gun, buy a shiny SS whistle to scare the bad guys off (that is what the PDs do for the ladies when there is a rapist loose in a neighborhood) and dial 911 and die while waiting for a response!