I gave her a Hunting shotgun but she is always complaining .
Stuff like " Might as well be throwing rocks " etc.
Is it because we were hunting DCs and she didn't have appropriate ammo ?

Will she use a rifle ? Or just shotguns ?

I have a Riot shotgun but I didn't give it her because of her trigger happy ways.

Should I just keep her on shotguns and swap out her ammo depending on enemy ? Uggh...sounds like a pain right there.

I think I'll grab Veronica ( Pushy ) to finish Dead Wind caves...got everything pretty easy except the Legendary.

Cass....weapon..?

I believe her favorite is the riot shotgun.

She does have a perk like shotgun surgeon, so shotguns are her thing. And the riot shotgun mentioned above, is probably the best you could give her. It has high DAM+DPS for a shotgun.
As for her trigger happiness, check if you have told her to be aggressive.

Personally, I have given her the Survivalist's rifle and she does a good job with it, but I think it's more effective on Raul's hands. The Riot shotgun should work excellent with her perk though, I'm using one myself and it's great in close quarters.

Riot Shotgun + Slugs makes Cass a very deadly girl. If you feel she is wasting too much ammo, give her Dinner Bell or a regular hunting shotgun instead. She'll do a good job with any other gun you give her though.

I'll try the Riot shotgun and make her some more slug ammo.

Dual Riot Shotguns with Slug rounds should clear Dead Wind pretty quick. ( I have the SS and SB perks )

Give her slug rounds.

The throwing rocks line means that she can't get past its DT. I find that she's most effective with the Medicine Stick, SWC rounds and the Hand of Vengeance perk.

she has shotgun surgeon built in like the others have said, so to use it fully you have to give her shotguns obviously, but you can give her any weapon and she will use it. (Guns) I like giving her Dinner Bell and special ammo that helps with - DT so she cam take down more armored enemies. I find riot shotgun's in her hands use up ammo too quickly most of the time. However she murders stuff wit hit.

Exactly..

"Uh, Cass ..where are those 200 rounds I just gave you go ?"

But I do want to keep it RP.

Not that I'm keeping her much longer anyway.