Auxiliary can't be gunner?

Wanted to make my Auxiliary more stylish with TMG instead of a Rifle as a ranger, so I tried to switch her classes around to force it to happen, but no matter what combination of classes she was, throwing on TMG didn't change her to Gunner. In fact I had to give her a Launcher, specifically, to turn her back into a ranger, as putting on a Rifle didn't work. For a while she was a Summoner/Force with a Sigma Rifle equipped. Is it supposed to be impossible for Auxiliary's to be Gunners? Or am I doing it wrong somewhere?

Sigma weapons are all class so it will never prompt class change, just give them a random non all class twin machine gun.

So I was doign something wrong. Thank you for helping em out, I had no idea SSigma weapons were all-class