Battlefield 3 multiplayer question?

i just got battlefield 3. i stated multiplayer but my kit custimization is really confusing. First, i always choose medic class, half the time i am using like a m16 and the other an ak-74u, even though i never switched it in my custom kits or picked it up. Second, i got a reflex scope for my ak-74u but on my kits it won't let me put it on, as it says the gun is locked. Can you give me an explanation how the kits in battlefield 3 work?Battlefield 3 multiplayer question?
Well if you are using just the medic class the two guns you have are the m16 (US side until unlocked for the RU side) and the AK-74m(RU side until unlocked) The ak-74u is for the RU side until you unlock it for the US side and it is the engineer / AT class. If you are using it for the xbox you have to hit x and the yellow triangle means you have something new unlocked and when you customize in game it only shows what you have unlock and not what is locked. The unlocks work on a kills/point bases for the class you have equipped. If you have a scope unlocked for a gun select the the class, then when you have it selected press the A or whatever you use to select something on the gun you have unlocked and the scopes are normal in the first slot under the gun, the second slot usually has undermounts (like grips, extended mags, etc.) and the 3rd slot is for silencers, flashlights, etc.