A properly made BFA has a small hole all the way through it. THis lets the gas pressure bleed of, but creates enough back pressure to allow the action to work. Simply plugging the bore completely would force all the gas pressure back out the action, and might not be such a good thing.
Recoil operated guns are notorious for being the most difficult to make function with blanks, as sometimes a bore restiction alone will not work. When this is the case, prop guns often have their locking systems removed (by ginding. cutting, etc) so they become simple blowback guns, and can function with blank. Depending on how this is done, they are often not fit for conversion back to functional firearms.
All else being equal (and it almost never is) bigger bullets tend to work better.