The AK74M is the default starting weapon for the assault class when playing as the RUSSIAN faction.
The M16 A3 is the default starting weapon for the assault class when playing as the US faction.
After you finish unlocking all weapons for the assault class, you will unlock each of those guns for the opposite faction so you can use either one regardless of what side you are on. I know how annoying this is, but you just have to deal with it. It helps when you unlock the M16a4 later on because it is not faction specific and even though it is burst fire, still a beast.
RDS + underslung rail + heavy barrel/laser/suppressor = kill machine
Hope that helps.