This poem is about the inner life of the speaker. Many poets write about their lover, some write about nature or war. Dickinson almost always has her speakers contemplating themselves in some way. Here the speaker possesses an uncanny insight into the workings of her inner self, and is able to see the interplay of experience, emotion, and intellect with incredible clarity.
To consider just one example, the use of dashes creates the feeling that the speaker is talking. These dashes fall at places where a person might take a breath or pause to contemplate her next words. There is a chance that the speaker is a male, but a small one considering the speaker’s relationship to the male "Owner" or "Master" (they share a bed, and Dickinson was writing in the 1800s). She says she’s a gun, which is a pretty intense thing to think about. Who wakes up and says, "I feel like a sawed-off shotgun today"? Well, this speaker does. She doesn’t mess around. She can blow her top with a smile.