When using a primary weapon such as the Zafir or the MX 6.5mm, where the machinegun or rifle is defaulted to fully automatic or semi automatic when picked up, there is a glitch that occurs if you switch to your handgun and then attempt to switch back to your primary weapon. Your primary weapon will then be switched back to its default fire mode, regardless of whether you have it on semi or fully auto.
For example, if one had the Zafir (which defaults to fully auto) and switched it to semi automatic, then switching to his or her handgun and then switching back to the Zafir, the Zafir would be back on fully automatic. This might be a problem where you would not expect this, e.g. using a default semi-automatic rifle and having it on fully auto, switching to handgun, and then finding that you're shooting in semi auto again when you mean to spray the enemy with bullets.