1) Take a rifle that is semi-automatic by default in hand.
2) Take a handgun. Switch to your primary rifle.
3) Switch the rifle to fully automatic.
4) Switch to your handgun.
5) Switch back to the rifle. Notice the fire mode now.
6) Repeat steps 1-5, but with a gun that is fully-automatic by default, e.g. the Zafir, and by switching from fully auto to semi-auto.