I've been studying this for years and I'm convinced once a large piece of metal that can harm people is placed in most people's hands, their movement immediately becomes almost robotic. This could be a knife, pistol, rifle, shotgun, etc.
"Robotic" as in stiff.
I've noticed this with the normal "gun guys" crowd, the guys that have "trained at dozens of places" as well as folks with a combatives background who normally move somewhat efficiently where bare hands work is being done.
A lot of times I think it's due to tight hips, lack of doing a lot of movement in general= normal "gun guys" crowd and quite a few of the training crowd. Yet even with combatives students I notice a problem with hips. I joke in class that "we will get you all's hips loosened up and then I'll get thank you notes in the mail from your wives and girlfriends."
But it's true.
And that's just standing up movement, it gets uglier as you lower the platform.