Argghh!! So yeah. I'm assuming the guns are supposed to be different (so there's a tab on the side and a tab on the top) but I have two of the same guns, so one has a tab on the left and on top, the other has it on the side and on the bottom. Unless I'm doing this wrong? Tears.
You could leave the bad one in the sun until it turns yellow and then you'll have a Sunstorm repaint!