The slits in the gauge mates are only in the gauges that are "close in size". In the picture I posted the one in the middle is a 12ga. to 20ga. and they will fall out of the gun if you aren't careful.

The slit 10ga. to 12ga. works better because they can be sprung to fit the chamber a little tighter. I shoot my 10ga. about every week...truthfully I can't even tell they are in the gun.

With the exception of dove...When I hunt with my 10 I shoot 10ga. ammo.