That's true.... it makes a big difference if the can is 'locked' in place or not. If it isn't they tend to get a big dent and roll around. If it is, then it gets the full power of the pellet. Also, you're right, if cans didn't have the grooves in them it would be a different thing to shoot through the metal....
I had posted this question before, using this can empty of course at 50 yds with my QB57 and CPHPsThis is the front And this is the backThere were no pellets in the can after, a month or so later I managed to borrow a chrony and averaged 575-580fps