The Mossberg 500C is the 20 gauge version of the "500" family of Mossberg pump shotguns. The Mossberg 500A, for example, the 12 gauge version (I'm pretty sure of this, but not 100% positive). I have a 500C, and it is a fantastic 20 gauge for several purposes. I'll get to that in a minute; first, the chokes.
The Mossberg 500C takes Mossberg "Accu-chokes," listed on the Mossberg website (www.mossberg.com
). These are interchangeable with Winchester, Browning Invector, Weatherby, and Savage chokes in 12 gauge & 20 gauge shotguns. You can buy these chokes for $12 - $20, online, at you local gun store, and at some sporting goods stores. I was lucky, and got mine for $3.75 each at Galyans in Indiana during their going-out-of-business sale. Don't waste money on a choke wrench; you can use a penney or a quarter to screw them in and out.
Now, about the 24" barrel. My Mossberg 500C came with an extra 24" slug barrel with iron sights. That's what most of the Mossberg combos come with. The newer ones have a 24" rifled slug barrel, which uses sabot-style slugs (pronounced like "saybo"). Mine (and probably yours) is a smooth-bore slug barrel, with no choke. You can tell by looking down the bore. If you see grooves cut into the length of the bore, it is a rifled barrel. If it does not have the grooves, it is a smooth-bore. A smooth-bore barrel shoots inexpensive "foster slugs," also known as "rifled slugs." Mine shoots fairly consistent 6" groups at 75 yards with an inexpensive 4X scope. Its great for deer, but don't expect to make long shots. If you expect to take a shot over 80 yards, I'd use something else (just my opinion).
The other barrel is probably a 28" ribbed barrel. With or without the rib, it is good for squirrel, rabbit, grouse, and most upland game. I had to take my 500C to the range several times to determine which ammo worked best in it. Check the markings on the barrel first, but if yours is like mine, it will take both 2.5" and 3" shells. It starts kicking a bit hard with more than and ounce of lead in 3" shells. I only use the 3" shells for turkey. for squirrel I use a modified choke and 2.5" shells, usually #6 shot, for close-in shots all the way out to 35 yards. My gun likes Winchester SuperX shells best for this type of hunting. A standard load is 7/8 of an ounce, but I usually get 1 ounce loads, for the benefit of a few more pellets. For kids and small-framed hunters, the 7/8 loads are mild enough to all but erase felt recoil.
As for the age, I'm sorry to say I can't help you much there. I couldn't get a straight answer about mine either. I believe they started putting the ribs on the barrels in the mid-late 1970's, but that's just a guess. I think mine is probably from the late 70's. Here's a hint, though. If that extra barrel is a rifled barrel, it is probably a newer version. If you could find out what year they started including the rifled slug barrel in the combo set, you could narrow it down a little.
By the way, I have heard that the Mossberg 500C can be a hard-hitting turkey gun too, but not as good as a 12 gauge, by most standards. I've tested mine at the range, and it seems to prefer Fiocchi shells in the 3" variety, with 1 and 1/4 ounces of lead (or more), for turkey. I can consistently put at least 16 pellets in the head and neck of a full-sized turkey target at 25 paces, and 10 pellets at 40. I'll find out for sure when the season opens. I'm using a full choke for close-in turkey hunting, and an extra-full choke for longer shots, realizing that most turkeys are taken inside of 35 yards.
I hope this helps. I love my Mossy. She's not perfect. For example, Mossberg 500's are notorious for rattling. I just take the bad with the good. Even with the rattles, I don't think I would ever trade her or sell her. She'll shoot loads almost as tame as a .410, and as hard-hitting as a 12 gauge heavy field load--all in one gun.[/quote]