It does bring up a good point though, which I'll cover in the article where I deep-dive into the physics: Knowing the acceleration of the rifle, I should be able to calculate how far it has moved in the same amount of time it took for the bullet to leave.
Thanks for taking the time to read it, and for your feedback!
Edit: forgot about the powder which as hot gas is subjected to pretty much the same acceleration as the bullet. With the powder being about 2-4x bullet mass it makes the rifle only 150x-300x heavier than the [bullet+powder] and that brings us to the 20-10ft/s of the rifle recoil speed, ie the article claim pretty closely matches the theory.
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.
The one extra thing I can think of so far is that the recoil reduces the effective length of the barrel by the distance it has moved by the time the bullet exits the muzzle, but again that seems too small an effect (~0.2 millifeet, four orders of magnitude less than the barrel length?)