The Revolver Speedloader is an item added by Immersive Engineering. It is used in conjunction with a Revolver. Once the Revolver is out of ammunition, it will trade it's inventory with the Revolver Speedloader; the Revolver Speedloader acts as a second inventory for the Revolver.
To load the Speedloader, the player must shift-right click with the item in hand to open it's small GUI. Only one Cartridge/item can be put into each of the 8 slots.