A rifleman is simply someone who's primarily equipped with a rifle
Beyond that, there's marksmanship, "sniping", and speed accurate shooting
A gunman is someone who is equipped with guns obviously; and usually have some measure of skills and proficiency in gun handling.
So really, it comes down to.. how skilled and proficient can you train to get to, and is the title that important?
What about "ammosexual" how to we define that?