Originally Posted by
Silver the Cat
It's no secret to anyone in Graal Ear that there are issues with the game. The ion carbine isn't the only thing with a bug. If you put the mini-gun on your back, switch to the other item slot, then switch back, you won't be able to fire unless you once again put it on you back and take it off. I can't be certain because I don't own an ion carbine, but your issue seems similar to the mini-gun's issue. If you switch to the non-default fire mode and switch items you may need to change fire mode twice to get it to default.
PLEASE NOTE IM NOT SAYING CHANGE THE ITEM IN YOUR EQUIPT SLOT, I MEAN SWITCHING BETWEEN THE TWO EQUIPT SLOTS.