I am definitely in agreement with those who say to not use a gun that could function. It's simply not a safe enough thing to do, no matter how many times it has been checked. I recently handled a pistol that I know has been disabled, because it was welded that way, decades ago. Still should be treated as if it could fire.

Otherwise, the idea behind slow practice, and I am aware of it's use in tai chi, is that you do it slow to do it PERFECT. If you have it slow and perfect, then when you seek to do it fast, it are unlikely to be good enough.