The marksman carbine is a semi-automatic, medium-power rifle with a 20-round magazine. Mounted with an enhanced-zoom scope that shares the same mil-dot reticle as the anti-materiel rifle and sniper rifle, but its barrel and magazine appear to have a stainless steel coating. It is highly accurate and deadly at long range with a Gunsskill of 100. Additionally, it has a very low Action Point cost of 14, lower than most pistols as well as being the lowest out of any rifle in the game.
The marksman carbine can fire a total of about 1,995 standard rounds, the equivalent of 100 reloads, from full condition before breaking.
One of them can be found near Securitron de-construction plant on the pipe to the east from the main entrance. Just follow the pipe until finding a spot in which the player character can jump up onto the pipe.
Carried by marked men if the player character reaches high levels. If the player character level is 40 or above, they will use armor-piercing ammo against the Courier.
The magnification level of the scope is 2.86x on this weapon.
When experiencing a malfunction with the marksman carbine (or any other weapons which share its reload animation) the Courier will use proper procedure and utilize the forward assist to fully seat the bolt carrier group.
Despite having the same ammo type and capacity as the service rifle, the marksman carbine has a larger magazine.
Behind the scenes
According to J.E. Sawyer, the marksman carbine is loosely based around "paratrooper-oriented marksman rifles" and are present due to New Vegas' proximity to Nellis AFB.
View through the scope
Marksman carbine with the extended magazine modification that was cut from the game
↑Appears in the GECK under static\weapons\2handautomatic\1stpersonmarksmanext