When you mount a weapon, your stamina will be coming back. Meaning your dispersion will be decreasing anyways. The reason I am saying tie dispersion to the STAMINA, not the MOUNTING, is that otherwise people would be completely out of stamina and tired, but the second that they mount they have pristine aim.
Those couple of seconds of inaccuracy makes a huge difference in balance. A player should not be able to have full mobility followed by full accuracy without at least a couple of seconds in between. This is why I’m saying tie it primarily to stamina, and not at all to bracing.