There is currently a bug with the magazine that is attached to the gun upon logging in. This magazine cannot be swapped for one in the inventory by pressing "r", although it should be possible.
HOWEVER, you can still drag a magazine from the inventory onto the "combine" slot and the magazines will then be swapped as should be.
After doing this once, you can now press "r" to cycle through the magazines until you have reached the "bugged mag" that was attached to the gun when logging in. As soon as this mag is reattached, "r" will not work anymore and you have to do it manually again.