So I had this weird glitch today during the mission to the Adat-Yakshi monastery, wish I was able to record it or something. The first time you see Samara in the level, her gun was about 100 times the size of her, hilarious looking because the way she swung it around would've destroyed the place... then after that, she never had it again, or any gun at all. Any time she was trying to shoot something, nothing was in her hand, and yet there were -blams- and all with bullets flying out her hands. Just sat there thinking "HA! These Banshee's are no match for Samara's pretend pistol!"
Course this continued in the cutscenes, so I ended up having to restart the mission just because despite the joke, she did manage to hold her hand up to her head and kill herself... kinda actually made me believe that the pretend pistol was the deadliest weapon ever. Restarting the level to rectify her dying fixed the issue completely, but god it was hilarious to see.
Witnessing it, I now demand pretend pistol multiplayer weapons. At least have the biotic characters hold their fingers up like a gun XD
I've had an invisible rifle in multiplayer before. I think the trick for you to use is to go to swap your weapon, then immediately climb a ladder. It could be climb a ladder, hold X while climbing though.