Col Cooper described a rifleman as someone who could consistently hit a target as far as they could reasonably see. (He kept his in check by saying we can see the moon, but...)
I agree with that definition and elaborate with; a true rifleman is someone who knows their leads for running targets, drops for distance, and can successfully judge distance fast enough to figure both and make a hit. In order to accomplish this a rifleman should have a solid understanding and application of the basic marksmanship fundamentals including sight alignment, sight picture, trigger control, follow through, natural point & area of aim, bone support, muscle relaxation, and use of cover & support. When shooting in field conditions the rifleman should be able to determine which of these fundamentals are the most important, given the situation and environment, to make a hasty shot. Sometimes that is, "flash sight picture" cross hairs or FSP clear as a bell, and follow through. Other times one has the luxury of a solid rest, but not muscle relaxation. Hell, alot of times if a shooter can just put together follow through and trigger control when firing at running targets they are doing good!
I won't make this as a condition of my definition, but there is also something to be said about "that guy" with "that one gun". This is usually because "that guy" with "that one gun" has enough experience shooting the rifle that he exemplifies the characteristics mention above.
It would be cool if we could come up with a short, concise, but all inclusive definition of a "rifleman".