Here's a good thread discussing the evaluation of vintage SxSs
http://www.16ga.com/forum/viewtopic.php?t=969&start=0The best part is probably this from Larry Brown:
Much of this applies to any gun, not just doubles. Check the stock dimensions--LOP, drop, and cast (if any). LOP is the easy one to fix, either by cutting the stock or adding a pad. However, if you have less than about 13 1/4-13 1/2" of wood, unless you're a midget, you'll probably need spacers as well as a pad, or you'll need to add a wooden extension. Moving drop or cast requires bending. Especially with old American stocks, do not assume they can be bent very much. Check carefully for cracks in the stock, especially right behind the receiver. Same for the forend. Look for gaps where wood and metal meet. If you're buying a used gun, is the wood still flush with the metal all around, or is the metal "proud" of the wood--that is, the wood is below the level of the metal. In the latter case, you may not be able to refinish the wood in the future. A little proud wood is good, because it permits refinishing.
Check for looseness. Two tests here: on a double, remove the forend, push the top lever to the right as if you were breaking the gun open, and see if you feel any significant "wiggle". The other test, also with the forend off, is to grasp the barrels out toward the muzzle, while holding the gun vertically, stock parallel to your body, then swing it rapidly back and forth, left and right, almost as if it were a canoe paddle. Again, you should not feel (or hear) any looseness.
Look at the breech face. If the gun is case colored and retains lots of color, no pits around the firing pin holes, that means it has not been shot a whole lot. Same thing if the breech has a shiny finish--look for pits.
Barrels--look at the lumps, the pieces that fit into the receiver. A gun can be tightened (VERY temporarily!) by a few taps with a punch or a ballpeen hammer to the lump. You should be able to spot those. Run, immediately, from such a gun. You may also see previous work done on the lump, either in the form of TIG welding, or you may spot a C-shaped piece fit into the lump that engages the hinge pin. Those are standard and acceptable ways of correcting looseness, and if the gun is tight, I would not be concerned with either of those fixes.
Hold the barrels by the lumps and strike them, all up and down, with something like a wooden pencil. They should give off a bell-like tone. If they clank, or if you hear a rattle or detect a vibration, that means the ribs are coming loose. That's an expensive fix; stay away from such guns.
Check the safety to make sure it works. Then, with snap caps in the chambers, check the trigger pulls. They should be light, crisp, and with very little or no creep. Trigger work is not a big deal on most pumps and autos; it's a much bigger deal on doubles. At the same time, check the ejectors if the gun is so equipped. They should trip together, and they should toss your snap caps the same distance. Ejector work is also fairly pricey. The top lever should be either dead center, or slightly right of center. A top lever left of center indicates wear, and a gun that will require tightening (not a cheap job) fairly soon.
Check the bores for pits, dents, or bulges. Very light pits, almost like staining, aren't a big deal. Beyond that, buyer beware. Small dents are also not a really big deal and don't necessarily even need to be removed. Look down the bores from both the breech and the muzzle to see what a dent looks like on the inside. Some that you can see (or feel) on the outside won't even show, or will barely show, in the bore. Bulges . . . stay away from any gun with a bulged barrel. Check bore diameter and choke with a gauge. "Standard" diameter can vary quite a bit, but with some doubles (especially British ones), you'll find the bore diameter stamped on the barrel flats. If your reading is significantly larger than that marked, proceed with caution. Choke work is not particularly expensive, but if you're looking at a gun with lots of choke and you're thinking about having a lot removed, take a good look at the muzzles to see that they have enough thickness to do the required reaming.
Barrel flats . . . they tell quite a story, especially on foreign guns which are required to pass proof testing in national proofhouses. With many guns, you can tell either exact or approximate age, the level of proof, the original chamber length, and whether the gun has been submitted for reproof (quite common with British guns in particular). Measure the chambers to make sure they haven't been lengthened from what's marked on the barrel flats, or elsewhere on the gun. On older American guns, lengthening 16ga chambers from the old standard 2 9/16" to 2 3/4" isn't all that critical. On British and European guns, because they tend to be light and if originally 2 1/2" chambered were designed for light loads, it can be more of an issue if they have been lengthened.
On used guns, try to get the seller to agree to let you shoot the gun, with return privileges if there's a problem. Most reputable dealers will permit this with used guns.
