The day I got the game I was playing and saw that you couldn't swap your weapon with a marine or another odst if it was empty. A few seconds later I figured out how to do it. [Pretty sure this is the first post about this] Have a weapon with ammo in it and stand by an ally until you see the press button to swap weapon. Switch your weapon to your empty one and immediately swap with ally if you did it fast enough you will have swapped your empty weapon for your allys. After that the AI drops the empty weapon and then walks around with no weapon, they still rush into combat but they can't do anything.