I have been using a 20" 12 guage Stoeger Coachgun for nine years of Cowboy Action Shooting matches. It has been utterly reliable. The action was a bit rough when I first started using it, but it smoothed out after a hundred rounds or so They come with fixed improved cylnder and modified chokes. Handy, sturdy,and reliable.
My other CAS gun is a Rossi, 28" 12 guage, with external hammers. If you can find a used Rossi in good shape, grab it. They are good guns.
Baikal is another frequently seen coachgun brand.
Once you get the hang of the technique, two-at-a-time reloads are pretty darn fast.
BTW, avoid heavy 3" magnum loads in Coachguns. They work, and the Stoger is proofed for them, but they will definitely let your shoulder know you set them off. "Tactical" buckshot loads work quite well in mine. They are more than "enough" for practical aplications.
Slugs are iffy, as the gun will almost certainly shoot to two different points of impact unless you find exactly the right load, and that will only work at one distance. (That is the nature of doubles.) Your best bet is to shoot the slug out of the more open choke (right barrel on the Stoeger Coachgun) and learn to hit with that barrel. Again, I would suggest avoiding "magnum" slugs. A 3" Federal slug will not be pleasant unless you really like that sort of thing.