If I read your message right, you're mixing up the inventory screen with the combat screen. The inventory ("I") only lets you decide which weapons you have on your person, not which weapon Shepherd's currently holding. The easiest way to cycle between weapons is to use these hotkeys:
F3: Assault Rifle
F4: Sniper Rifle
Hope this helps.
[QUOTE=MaxfireXSA;1088826]No I did not perform an orgy, and yes I need tips on how to do this.[/QUOTE]