And HELP says:
Select Primary Weapon: Backslash
Select Secondary Weapon: Backspace
And secondary weapon toggle is changing tank ammo.
I'm sorry Lusche, I was unclear. I looked at Help webpages and then navigated through the options as Help indicated. I was presented with the only valid choices available under Global for the Weapons Selection Secondary Key_BACK_SLASH.
There was no choice presented for Weapons Selection Primary as depected in the AHII Help documentation webpage picture. I assumed that as often happens in AH something had changed. What I used the web page help to do was to located the proper place to navigate to on the clipboard to find and map keys.
Bottom line, to answer the original question, Use the \ key to rotate through available weapons, not the BackSpace key.