Some primary weapons are limited to certain class only (such as Sniper Rifles being Recon-only). They are selected by default as your spawn, and can be switched back to by pressing "1", by scrolling the Mouse Scroll or by pressing "Q". Currently, you can switch your secondary weapon to a primary weapon that is dropped on the ground, without it replacing your current primary, and vice versa. You can also pick weapons that are not of your class as well. (For example, you can pick a Sniper Rifle as an Assault class)
Primary weapons come in different kinds, ranging from small Personal Defense Weapons (placed in the SMG category), such as the P90, to big Anti-Material Rifles (placed in the Sniper Rifle category), such as the BFG-50.