Like the MK14, the Marksman has a high RoF (semi auto), but lower damage than other snipers. Being a custom weapon, its bonus is perfect accuracy.
Marksman has perfect accuracy like few other snipers, and highest fire rate of them. However, impact is still low. It can be increased in workshop, but you will mostly need to hit enemies more than once to kill them.
- Possibly a reference to the MK14 being a "Designated Marksman Rifle" in the US military.