So you want to buy a used revolver and don't know what to look for. Here are a few things you need to look for before you decide to buy. Knowledge is your friend and will save you money on your purchase.
First, you need to look at the overall appearance and if it looks solid we will go from there. The finish is a matter of personal preference and has no bearing on the function of the revolver. If it has a lot of surface pitting or has obvious signs of neglect, pass on it.
Check the action for proper function. To do this you must dry fire it. Most dealers don't like customers to dry fire their guns. Dry firing a revolver will not harm it in any way and is a great way to practice trigger control and sight alignment. If the dealer won't let you dry fire it, pass and go somewhere else.
You want to dry fire it double action and single action on all 6 chambers. There should be no hang-ups or binding in the action at all.
Next, check the cylinder yolk alignment and lock-up. Close the cylinder, then push on the opposite side as if you were going to open it. There should be no movement of the yolk or crane and there should be no gap between the yolk/crane and the frame. If there is an obvious gap with the cylinder closed, chances are that the yolk is sprung and will require a gunsmith's services.
Next, check cylinder lock-up on all six chambers. To do this, pull the hammer to full cock, then try to turn the cylinder back and fourth, it should stay locked-up tight with minimal movement. Check the bolt cutouts in the cylinder for signs of battering. Any problems with lock-up will cost you. Also check forward and aft movement (end shake) of the cylinder. There should be very little movement. If it is excessive, it will cost you.
Next, you need to check the timing. To do this, put a very slight drag pressure on one side of the cylinder and at the same time, slowly pull the hammer back to full cock. The cylinder should lock-up just before the hammer reaches full cock. Do this with all six chambers. If it does not lock-up in this manner, it is out of time and will need work.
Next, you need to check cylinder alignment on all six chambers. To do this you will need a range rod. Insert the range rod down the barrel and into the cylinder with the hammer at full cock. It should not touch either side of the chamber in the cylinder. If it does, it will need work.
Do this with all six chambers.
Next, check the bore using a bore-lite or some other source of light. It should be bright and shiny and the lands should be sharp and in good condition. If the bore is in question, pass.
Next, check the muzzle crown. If there are any nicks or abnormal wear, it will need to be re-crowned.
Next, check the underside of the frame top strap at the barrel/cylinder gap for excessive flame cutting (erosion). Very slight flame cutting is normal, especially in magnum calibers if it has a lot of use and will not affect the strength of the top strap. If overly excessive, pass on it.
On S&W K Frame .357 magnums, check the flat on the bottom side of the barrel forcing cone for any cracks. If you find one to be cracked, pass on it, it will need to be re-barreled.
Also check for excessive forcing cone (throat) erosion, it will affect accuracy.
Next, you need to check the barrel/cylinder gap. To do this you will need a set of feeler gauges. Ideally, the barrel/cylinder gap should be between .003" and .006" with a maximum of .008". If excessive, it will need work. You can check the head space at the same time using a dummy round and the cylinder closed. The gap between the case head and the breech face should be no more than .005".
Next, check the extractor rod for smooth and free operation on it's full stroke. If it binds, it is most likely bent.
Next, visually check all 6 chambers, that should be smooth and bright with no pitting or deep scratches.
One more thing you need to check and it is very important, is hammer push-off. To check this, pull the hammer back to full cock, then try to push it off full cock with your thumb and without your finger on the trigger. If it pushes off with your thumb, you might want to pass, as it will probably require a new hammer and trigger. The action may just be very dirty or it has a weak or miss-adjusted main spring. At any rate, it should be inspected by a gunsmith.
If the revolver you are thinking of buying passes all these tests, you should have a very good used revolver that should last you a lifetime.
I hope this answers most, if not all the questions you might have in buying a used revolver.