I grew with a Mossberg 500, still have it. I bought a Yidiz O/U 2 years ago and love it. It's light weight and fast handling. It has all the perks of a high end O/U, and costs about half the price at $400.

Foregot to say my O/U is a 20 ga.

If your looking at a new shotgun, anything in a 20 Ga 2 3/4" would be fine. As for the Mossberg, that will be perfect, as it's not the shotgun or the gauge, but the shot size and the load. I use a Winchester 1887 Lever Action 12 GA 2 3/4", and use slugs for deer, #4 shot for turkeys, #6 shot for rabbits, #7 1/2 or 8 shot for small birds (pheasant, quail) and sporting clays.

I like a 1 to 1 1/8 oz shotload in my 12, 7/8 oz in the 20. Also save the spent shells, you can reload them probably 10 to 15 times, and save money every time. Best reloadable hulls are Remington black cases, Winchesters are ok but usually only good for 3 or 4 tops. And the MEC JR Mk 5 reloader is the cheapest way to start reloading the shells without loading or feeding problems from what I have experienced.

i youse the 12 ga with 7 1/2

an H&R excel auto 5 is a nice gun handles all kinds of loads very well with a 3" chamber it can be used for allmost anything, its semi auto action is very fast i can pull off 6 rounds in less than three seconds with it, im going to buy a stoeger p 350 wich looks pretty nice

Im running a Remington Model 1100 12 gauge down in Arizona. We have good dove seasons and pretty good quail populations as well. For dove season i throw in the modified extended choke i purchased online and for quail i go ahead and bump it up to the full choke, as most of my hunting is done with no dogs, so the quail will be flying away. If you can keep the Mossberg i would see about different choke tubes if you dont want to go out and buy another gun.