Ex. A

1. Find magnum speedloader on the ground, but do not pick it up just yet.
2. Press TAB to open inventory, right click magnum speedloader in vicinity.
3. Select "Empty" from the context menu.

Result: Speedloader is empty, but there are no rounds on the ground.

Ex. B

1. Find magnum speedloader on the ground, put it in your bag/pocket and not in your gun.
2. Press TAB to open inventory, right click magnum speedloader and select "Empty" from the context menu.

Result: Speedloader is empty, but there are no rounds on the ground or in the bag/pocket.

I wasted 20 magnum rounds with two magnum speedloaders. Rounds are consumed from the pile when loading speedloader, and they are lost when unloading speedloader.
Speedloader, rounds and magnumer where in Pristine condition.