Most of the blank guns that are based off of a .45 usually are chambered for a differant caliber.
In the past you'd never see a 1911 in movies, because they didn't have the technology to make any kind of .45 blank. The .45, being such a large diameter, is impossible to crimp, and feed.
(exception) Thompsons, dont ask me how they fired blanks from those.
Again, most blank firing "1911's" will shoot 8mm.