When you load a magazin into your weapon, its not updated properly into the vicinity! So when you put a magazin into your gun, and lay it on the ground, you see there is no magazine in it. If you put it on hands now, at the first seconds there is no mag in it and it will start to load the mag into the gun (i guess server is updating your inventory somehow) and in this few seconds, you can empty your chambered rounds, you also you can empty it on the ground while no mag is inside it!
Edit: i could reproduce it with amphibia s, but now im no more able to do it.
Could be depending on server load.