I agree with our Unkel. I have used the rubber bullets that X-Ring puts out in the basement to practice trigger pulls. They use a regular case and only a primer. They are not wimpy by any means. I chronoed some and they get around 500 fps which is better than the average pellet gun. I know I wouldn't want to get hit with one. The only problem is that the X-Rings are only in revolver calibers - .357 and .44.
And a caution - anyone using either of the practice rounds should use them with good ventilation. As I understand it, the primer is the main source of airborn lead. And as always use eye protection, hearing protection is not usually necessary.