one eye Joe,

I've always believed but not really sure that
the recesses in .22 revolvers accomplish
two things:

1. It insures a more positive ignition since
I guess it's possible the power of the primer
compound could be diluted if the rim was
allowed to push outward when the firing pin
hits it. (Centerfire primers are nicely
contained and help with the pressure.)

2. Though you shouldn't dry fire a rimfire
revolver, I think the recesses lessen the
chance of firing pin damage if you do a
bit of dry firing.

Probably wrong but those are my beliefs.
(I'm not much of and never have been
much of a .22 LR revolver shooter so I've
never formally looked into the matter.)