Must have a dual inventory slot magazine in your rifle (ie. PMag, Stanag Mag, dual stack Stanag) and must have a single slot magazine (ie. 10rd CMag) anywhere in your inventory. This can be reproduced in either of two ways, fill your entire bag with stuff or set up your bag so even when the CMag is used, there are not two slots in conjunction with eachother to allow the dual slot mag to exist in the bag. Once that is done, press "Use" with your rifle out to reload to the next mag (CMag) and it will reload to the CMag and cause the previous mag to disappear.