Courtesy of an excellent post from K-frame
I'll explain what I look for, from muzzle to butt.
(Note: This is criteria for a "shooter", not a collectable.)
1.Open the action and make certain the gun is unloaded.
If you set the gun down to turn around and sneeze, when you pick it up again, check it again. It is a mistake to assume that a gun is unloaded just because it is for sale, in a gun shop, or at a gun show!
2.Look at the muzzle.
Are there any dings, nicks, etc. at the crown?
If so, it will effect accuracy and costs about $30 to fix.
3.Look at the front sight.
Bent, dings, flats where there should be corners?
That may indicate a dropped gun and should warn you to examine the rest of the gun very closely.
4.Hold the gun up to the light and look at the barrel exterior from an angle.
The reflected lines of light ought to be straight; a bulged barrel may show up as a wave in the line.
5.Check again that the gun is unloaded.
Look down the muzzle and shine a bore light into it from the cylinder end.
(If you don't have a bore light you can reflect light into the barrel with you thumbnail.)
The edges of the rifling should be sharp.
Rounded or faint rifling indicates MUCH use.
In addition, a dark ring may indicate a bulge.
6.With the action open, GENTLY spin the cylinder. If the ejector rod wobbles it may indicate a bent ejector rod, which usually costs about $25 to fix.
7.While you've got the cylinder open, look at the forcing cone of the barrel.
Pits, scratches, and cracks are bad, usually requiring barrel setback or replacement.
May cost $40-$300 depending on the model to fix.
8.Close the action.
Look at the gap between the crane and the frame.
Less is better. Gently push side to side on cylinder.
The gap should not change much at all.
Again, less is better.
9.Look at the stop notches on the cylinder.
Peening of the notch indicates a lot of DA shooting and may mean timing problems.
10.Look at the chambers.
Check for bulges, scratches, cracks (GOD FORBID), etc.
11.Push the ejector rod. It should move smoothly and snap back when released.
If it doesn't, it may indicate a bent rod, or rust inside the works.
12.Look at the underside of the topstrap, directly above the barrel/cylinder gap. With .357's on up, after a lot of use a line will be cut into the topstrap.
This is pretty much a self-limiting problem (it won't ever cut through) but it'll give you an idea how much use the gun has seen.
13.If adjustable sighted, examine the rear sight.
Make sure the screws turn, that the blade moves when screws turn, that the blade is firm when the screws are left alone, that the blade isn't bent.
Rear sight replacement on a S&W is usually $60 or so.
14.Check to make sure the gun is unloaded.
If so, cock hammer to SA. With about 10-15 pounds of thumb force, try to push the hammer down.
If it falls without pulling the trigger, there are bad things going on inside, and the gun is dangerous.
It may cost $30-$100 to fix.
15.Now, pull the trigger and hold it rearward.
Wiggle the cylinder to check for rotational play.
I check for this on all chambers.
Less play (what I call "wobble") is better.
16.If the seller will allow it, dry fire the gun both DA and SA through all chambers.
Check for sticking, failure to rotate, glitchy trigger pull, etc.
17.Slowly cock the action & while you do so, drag a finger on the cylinder. What you are checking for is that the cylinder stop engages the stop notches before the hammer reaches full-cock. Dragging the finger will prevent rotational momentum from finishing the rotation. If the stop doesn’t click into place prior to full-cock on all chambers, odds are that the hand is worn and will probably cost $25-$40 to fix. If it fails to click into place on only one or two chambers, it is likely that the ratchet is worn and needs replacement. That usually costs $50-$75 to repair.
18.Not really a mechanical issue, but look at all the edges and lettering.
Also, look at the screw heads.
Rounded out features generally indicate refinishing by someone who didn't know what they were doing, or that the gun was in very bad shape and required a lot of polishing.
Buggered up screws generally mean that some idiot without proper tools or know-how got to the insides.
In this case, I would demand to look at the internals for rust/wear before I paid a cent.
That is pretty much as complete a mechanical evaluation that can be done without disassembly.
You will of course want to look at the finish and the grips and take it all into consideration with the asking price.
I may be forgetting a couple things.
If anyone has anything that they check for that I haven't listed, I'd love to hear it.
Hope this was helpful, -Kframe
p.s.-I've gone back and added the points that Malpaso made, so it'll be easier to cut and paste if you want to print a copy to bring along shopping.
