The 'Sloshing' lol great word for it, is currently one of my bugbears... Ive seen a post trying to justify it by suggesting its swaying due to fear and adrenalin. I dont buy it. I'm not an experienced reenactor or a gun enthusiast but I have hunted, shot rifles and shotguns. I've also held both an authentic Bess and Sharps rifle, they are not overly heavy or unwieldy.
However if a vet with combat experience says the sway is realistic under combat conditions I'll say no more on the subject as they trump any of my preconceptions.