A good point, but the puzzle is, though, that the author isn't recording those sort of velocities (or, rather, the acceleration that would lead to them) while the round is in the barrel.
Not all the gas reaches the bullet speed. We might guess that it averages half that, but that is assuming, I think, a uniform density and complete gasification of the charge (though with regard to the latter, is there a reason for there being a larger charge than that?)
This argument also suggests that the recoil firing blanks would be a substantial fraction of the recoil with a live round, which is not how I recall it.
Not all the gas reaches the bullet speed. We might guess that it averages half that, but that is assuming, I think, a uniform density and complete gasification of the charge (though with regard to the latter, is there a reason for there being a larger charge than that?)
This argument also suggests that the recoil firing blanks would be a substantial fraction of the recoil with a live round, which is not how I recall it.