I just pulled my CS 9 out to take her out to the range tomorrow and was looking over it. Noticed a ring that has developed right under the machined outer ring just back from the muzzle of the barrel. Perfectly formed area that has noticeably eroded and is dull about a half inch wide in the bore. Will try and get a photo later but can not get my phone to focus to show this right now. Almost looks like a squib but is directly under that machined ring in the barrel.
Noticed last time on the range the rounds from the CS-9 seemed to be keyholing but what caused this is the?? Have owned this one for many years so am wondering if it's something in the temper under the machined area?
No question this barrel is toast but never seen one do anything like this before.

Uh Oh! Pretty sure we are going to want to see a photo of this.

The spot is toward the end of the bore and directly under the machined ring in the outside of the barrel. There is no two piece joint here and the bore is galling directly below where the ring is machined on the outside of the barrel. Almost like the barrel was constricted when it was machined and when I started shooting it it began galling and wearing a groove in the bore.
Here is a photo of the barrel from Midway.
Smith & Wesson Barrel S&W CS9C CS9D CS9S
This is the same pistol I had an issue with finding a correct recoil spring for as the factory spring was weak when I got it not letting it return to battery at times. Finally found several factory springs and started taking it to the range then the ring appeared.
Love the little pistol and like how it shoots and feels better than my 3914 so looks like I will have to spring for a replacement barrel. What it did however was down right odd though.

I might suggest cleaning it with a wire brush to see if the ring will come out of it. Soak it in Hoppe's no.9 first and then do the wire brush cleaning. The ring may just be a carbon buildup which will come out of it.

Key-holing the target coupled with your description of the "ring" inside the barrel sounds like the barrel has "leaded up". That "ring" is probably lead. Soak it in Hopps #9 for a couple of days and scrub it out with a metal bore brush.

Put camera on 'macro' in the settings.

Description sounds like a bad barrel that shouldn't have left factory. Odd that it only started keyholing after several years (?).

Ok guys this is the best I have on photos but you can see the dark ring in the bore directly under the machined ring on the outside. It's not leading ( Copper foul maybe but would have been hard for it to lead up as I have never run unjacketed ammo in this pistol!) this is erosion of the actual bore in this area and I have never see one do this before.
Bought the pistol a few years back but it had been shot very little . It was only when I started taking her to the range after I got the new recoil spring that the ring appeared.

I'm going to say that it is most likely the barrel and not ammo as the erosion is not uniform thru the entire barrel.
I would contact S&W even though you are a second hand owner, they would be interested to see this.

It appears to me to be a smear of lead.

Thought the same at first. Thorough cleaning with a solvent and wire brush does nothing and when you look close first hand you soon realize it is actual galling on the inside of the barrel. Strange that it is in a perfect ring directly beneath that machined ring on the outside like the barrel was slightly constricted at that spot.

Pictures do help. Well scratch my previous comment....was thinking it was on the outside of the barrel where the ring bushing in the slide interacts with the barrel near the muzzle.

Bore imperfection with a modestly tighter rough spot. It will continue to readily grab build up until that is polished out. Or? The barrel is replaced.

IT APPEARS THAT YOU HAVE A BULGED BARREL. WITH A SLIGHT BULGE LIKE THAT, WHEN THE BARRELS IS CLEAN IT'S ALMOST IMPOSSIBLE TO SEE. THAN AS SOON AS YOU FIRE IT THE ENLARGED AREA COLLECTS THE DEBRIS AND THE SLIGHT BULGE BECOMES MORE APPARENT . A BULGE IN THAT AREA OF THE BARREL WOULD ALSO EXPLAIN THE KEY HOLED TARGETS. JP

I think a trip back to the Mothership may be in order on this barrel. I suspect it will only get worse if not dealt with soon. Give S&W a call and tell them whats happening. I am sure they will take are of the problem pretty quick.

That appears to be a bulged barrel from having a squib that stuck in the bore and another round was fired behind it. It's a bit difficult to see but it appears there is a second raised ring over the bore bulge, that shouldn't e there and is more evidence of a bulge. The only "raised ring" there should be on that barrel is right at the muzzle.

You are extremely fortunate that you just got a bulge and not a split barrel that could have destroyed the slide and frame too!

Buy a new barrel.

My first thought from both the picture, now, and the previous description of hesitation when going-into-battery, would be a ringed barrel.

If you take the barrel, and insert the muzzle into the front of the slide's muzzle (barrel going into the slide backwards, outside, from the front), is there any hesitation of momentary contact between the slide's "bushing" and the outside of the barrel? Sometimes this can illustrate and helkp ID a ringed barrel if the outside bulge touches the "bushing" cut and the barrel is being held in your fingers. (Not so easy to "feel" when the slide is being released, with the barrel installed normally, and your fingers aren't in contact with the barrel without recoil spring pressure being involved.)

Can you take a picture of the barrel from the side (like the one displayed in the linked advertising picture)?

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Ret LE Firearms inst & armorer

To me that looks suspiciously like a barrel that has been Squibbed. As in someone got a squib in the barrel and then shot into the squib. With some barrel steels the result of doing this won't be a blown up gun, it will be a barrel with a slight bulge where the first bullet was sitting.

BTW during the trials that led to the Beretta 92 being adopted as the US Military sidearm they actually did a test for this. The Sig Sauer P226 passed the Squib test with no measurable harm at all, meaning the barrel wasn't even bulged. All that happened was the stuck bullet was shot out of the barrel. The Beretta also passed but the barrel was slightly bulged, enough to be measured but not enough to effect the function of the pistol. As for why the Beretta was chosen, it's because the Beretta had a lower cost.

Absolutely no squib in this barrel but that is exactly what it does look like from looking down this bore. I'm the only one who has fired this pistol in years and trust me I would instantly pull the slide and check a barrel if I had a round that was off enough to cause a squib.

How certain are you that in all the time you've owned this pistol that you have NEVER EVER done the "Tap, Rack, Bang" drill. All it takes is once to end up firing into a squib. And with some handguns with good stout barrels made using a high strength steel optimally heat treated you can fire into a squib without blowing the gun apart. If you lived in SE Michigan I'd be very willing to come over with a telescoping gage set and do some measurements of the barrel in that area and an unaffected area. Because this is a rather interesting Mystery. If you know a machinist or have a nearby Machine Shop perhaps you could stop in and have them take a few measurements.

PS; the Professor in the Library with a Fireplace Poker? ? ?

100% certain on this particular pistol. The more I think about this one the more I think a call to the mothership, is in order as this barrel is going to have to be replaced.

I can believe that. It's good to hear that I'm not the only person who thinks Tap Rack Bang is a good way to destroy a perfectly good handgun.

Well if you are engaged in a fire fight and your pistol malfunctions what are your options besides "Tap-Rack-Bang" which by the way has been changed in most training programs. One should be careful when throwing rocks. Tap-Rack-Evaluate has a place and a time especially when you are taking incoming fire.

Now if you are on the range just putzing around and you experience a malfunction you should certainly clear and check the weapon. Having said that, there are many shooters who never get fully comfortable with their pistol and because of that they never realize when they have a squib load go off so they just fire one off behind it.

Rick