@davidjoe - remaining, you absolutely miss what is happening in both the video you watched with your own eyes and what would happen in a rifle with a plugged barrel.
When the boy stops walking, he ceases his momentum, and that of the board, by pressing FORWARD on the board. Exactly the same as he generated his relative movement by pressing REARWARD on the board, there was an equal and opposite reaction. When he starts walking from a stop, he also starts the board moving. When he stops himself, he also stops the board.
Equally, when you fire that 1000 foot long barrel, the bullet will start moving one direction and the rifle will start recoiling in the opposite direction. When the bullet hits the plug and is caught, it will stop again, relative to the barrel, and the rifle will also stop. The center of mass will not have moved, but the rifle WILL have moved towards its buttstock - it WILL have recoiled, until it was stopped by the bullet stopping in the plug. Firing another bullet would do exactly the same thing. But the sum of the displacements of the rifle relative to the center of mass will never actually displace the rifle farther than the relative displacement of the bullet masses - in other words, the center of mass will always remain within the bounds of the rifle, so the rifle can’t actually move farther than the length of its own barrel (or even that far, just using this as an absurd and unattainable maximum).
So a normal rifle fired in space, would fire a bullet which would travel forever, and the rifle would recoil rearward forever. BUT… a rifle with a plugged muzzle which catches the bullet (neglecting the applications problem of how to move the gases from in front of the bullet(s) to behind) could never, even with infinite shots, actually ever move its center of mass in space. The rifle would move towards its buttstock slightly - less than the length of its own barrel - but could never actually move farther than the bounds of its own length.
You are simply picking the option, one of two laid out, more than two years ago,
that bullet exit is required for true recoil, above and beyond recentering.
You do realize, don’t you, that your analysis of limited motion is not a new unforeseen outcome, right? That I cast it as one of the “two horns of a dilemma” should be a tip off that no, I haven’t “missed” it.
The reason this hypothetical was devised more than two years ago, was to draw out that recoil, all but that smallest component of it, IS EXIT DEPENDENT. Did you ever read my thread about a magnum not being inherently less accurate, which thread’s thesis is that the disturbance of magnum recoil occurs mainly after exit, when the bullet is already gone? This hypothetical was developed in an organic discussion after I opined about walking on a trailer, and that slight effect on it, not being speed dependent, it’s all there, 2021, deja vu.
Please tell me that you can see why the hypothetical was devised in the first place. It was devised to force deep thinking about bullet exit being needed for true recoil to occur, or else you are stuck with an impossible rocket.
On the one hand, I thank you for supporting the thesis, on the other hand my brows are furrowed at you. Even if you blew past my references here to see my older thread, I’m calling it an
impossible rocket as I pose to you the horns of the dilemma. This means it cannot work.
Thank you for describing further why it cannot work, an already stipulated conclusion implied in calling it an impossible rocket, before it was posed, that was by no means stumbled upon, but distilled from much thought on the subject, preceding my bringing it up originally.
But now you have sided firmly with my older thread thesis, and described for yourself exactly why all of recoil that is not displacement limited - that vast majority of it, exists only post-bullet exit, meaning a magnum’s accuracy is not held back by pre-exit recoil relative to other cartridge choices.
You think you are telling me I’m wrong, but you are telling me the opposite. I do hope and trust that you appreciate that in the video, the boy traversing the board analogizes only to recentering balance, that period while the bullet remains in the barrel, and that only if he leapt off of the board, and it careened back, (recoil) would that system replicate the analog of a bullet leaving the barrel of the gun, and it would reset the system, which is a gut feeling problem I felt with you saying that a plugged barrel still serves as a vent, when exit does matter.
And I hope you appreciate that the video experiment was performed on water because recentering is a weak force to overcome. All statements that say only that the rifle recoils while the bullet is in the bore, and leave it at that, mislead people about how weak and easy to stop the imbalance is, which no doubt is why so many other close up videos show no movement.
problem solved. lol. 
. )
