To be honest I don't play Call of Duty so don't be surprised if I'm wrong. (Oh and be more specific next time on which Call of Duty game as there's a whole series!)But I would say it depends on your mission. I have done some research for you and here is a website which names all the weapons for the game entitled Call of Duty http://gaming.wikia.com/wiki/Weapons_(Call_of_Duty) so I figured you can choose out of them. If this is not the CoD game you are talking about I apologise but you didn't say which version/number. I hope that helped.
Yours truly, Hstar
P.S. The reason I let you choose out of some options is because really there is no best gun, it depends how you use the gun.