Originally Posted by Gryphon
...A blowback automatic weapon will not cycle if there is no inertia from moving a bullet since it is that which provides the force necessary to blow back the bolt (remember Newton's 3rd Law?)....
Woops! Yes, you are absolutely right.
As for the blanks with plastic bullets. Yes they are more dangerous than standard crimped blanks, but for military training, I think they are the optimal solution. Proper cycling, and a nice big flame... its all there with much less recoil and no bullet holes
I doubt they are used in movies much, if at all, but they are the only way that I can think of to have a live automatic firearm operate properly with blanks with no modifications.