Looks like an aluminum frame revolver and the casings are indenting the reciever. Do you keep your brass? Did you notice any spent brass with a problem?
How light were the loads? Very light loads do not create enough pressure to make the primers seal and allow gasses to escape from the flash hole/primer pocket. Those gasses will erode the breech face.
Was it steel cased ammo? I read somewhere about a 642 that had craters around the firing pin bushing after firing polymer coated steel cased ammo, and smith ended up replacing it.
I would contact smith and NONE of these words would come out of my mouth; "reloads, hand loads, remanufactured, 4.2 grns unique," etc.....
Worse thing that could happen is they do nothing. My 642 has some small dimples in the finish around the firing pin bushing, but looks nothing like yours.
Looks to me like either a primer or case failure at the outside edge of the primer pocket. Either way the result is the same - hot, high pressure gas escaping at the edge of the primer pocket. The hot gas escaping acts like a cutting torch & cuts away the metal. The softer metal of the alloy frame got the worst of it. Check all your fired cases from the shooting session that resulted in the damage.