EDIT:
So I have found 2 ways to do this bug/exploit…
1, Join a game while the ready up is activated but remain in the lobby. While in the lobby wait till you hear the game start and then switch class. You will have what ever class attachments you had set as primary while ready up was on but when you enter the game you would have the new primary class.
2, feels more random but just waiting in the lobby when connecting to a server for a long time (5-10 min) will cause this… most likely due to the same reasons as the first point. But I feel its important to note as this is where it is likely to happen by chance.
ORIGINAL POST
So I spoke to someone from SD in a game and they told me to mention it here…
http://s8.postimg.org/6w8glhgqd/wrongattachement.png
So it seems if you switch primary when connecting to a match as it loads the map there is a small chance you can get your original primary weapons attachement.