If I were you, it would depend on how much I'm willing to carry. I get along fine with my WE M1911, with its 16 BB mags and all. Optimally you won't need that much anyways, but I wouldn't recommend getting something bulkier than a pistol. Even those really long mags are too bulky for me personally. What you could do is carry the pistol with a short mag in it, and have a long mag in a mag pouch somewhere on your kit.
At any rate, I'm mostly going with the above opinions. A pistol should be enough firepower for you, as a marksman.
At any rate, I'm mostly going with the above opinions. A pistol should be enough firepower for you, as a marksman.