darkenna - I've seen it a few times. It would be a stretch with a goose gun , but not with a normal barrel length shotgun. My opinion is he shot himself in the chest as he didn't want to mess up his face. He wanted his family, (usually wife or mother) to be able to view him in the casket. That is also why women rarely shoot themselves in the head....or for that matter shoot themselves at all.
I checked some news sources and all say that Seau suffered no recorded concussions. His wife has donated his brain to the scientific/medical group (name escapes me) that is studying football concussions. He left no note so the conspiracy theorists are having a field day.