In this case displacement is movement.
In any case nodes are places of minimal motion.
I don't want this to get long so I'll ask that you do your own research on what I'm about to say. You may know this and we are just not understanding each other well.
But, a barrel is not a guitar string...It's only attached at one end, making it a cantilevered beam. You can not physically move a NODE to the end of a barrel...err cantilevered beam. You can put the ANTI-NODE there.
The anti node is in fact the point where the top and bottom of a sine wave occurs. A node is in between the top and bottom anti-node and is where the most movement occurs, fastest....angularly.
Just imagine the barrel moving up and down, with some round-n-round thrown in. With a node being between the top and bottom anti-node, which do we want the bullet to exit the muzzle...the one at either end or the one in the middle? I want mine to exit just prior to the vertical apex of the barrell's oscillation.
I'm aware of tuners and that some extend beyond the muzzle. Some claim that's how a "node" gets moved to the muzzle. If that were how tuners work, a behind the muzzle tuner would NOT work...but they do.
There is no such thing as a "stopped muzzle"...that's just a physical impossibility, but there are areas where the muzzle moves less, at top and bottom.
Again, a NODE can not be moved to the end of a cantilevered beam of a given length.
FWIW, there are several different frequencies and forces action on a gun barrel when fired. A tuner at the muzzle affects the average result of those frequencies and forces....changing the frequency a tiny amount, allowing us to tune barrel to shoot a load optimally, at a given set of conditions, mostly temperature.