When Markinson is about to commit suicide, he puts the gun to his head. When Tom Cruise tells his partners about it, he tells them, Markinson put a gun in his mouth?
Well, I'm a police officer, and when someone shoots themselves, be it in the head, under the chin, or even in the chest, we usually say that they "put a gun in their mouth". It's a euphamism for suicide by gunshot.
Caffey could have been informed by the MP's that Ltc Markinson was found with a gunshot to the inside of the mouth. Remember Caffey as a lawyer was probably informed by them as it was his witness.