I'm going to go out on a limb a bit and assume that the 629-6 has a frame mounted firing pin. If that is the case it's possible that the previous owner has replaced the firing pin with a longer firing pin from the aftermarket. It's also possible that the gun saw enough use than the bushing has become a bit fouled and it stuck forward because of the fouling. Finally, it's also possible that the gun was dry fired a lot and the firing pin return spring has been weakened or wadded up. Since you've described a darkened primer the most likely scenerio is that you have an aftermarket firing pin installed in your 629-6.
First a note on stuck firing pins. Personally, I consider using a hammer an extreme solution. I've had a firing pin stick once in my 610-3 and found that simply rocking the cylinder back and forth allowed it to pop free of the primer. However, in my case the primer had not been pierced, the dimple was deep enough to "capture" the tip of the firing pin. BTW, I was shooting in single action when this happened. Since you did have to use a hammer I'd strongly recomend replacing it because may have been bent. If you just want your 629 fixed and don't want to take it on yourself call the factory customer service and have it done under warranty. If your curious as to why it happened, read on. Also note you'll want some properly "fitted" gunsmithing screwdrivers so you don't bugger up the screws.
I would also urge the purchase of the Kuhnhausen S&W shop manual as a basic guide to working on these revolvers however it is a bit out of date in regards to the more recent revolvers with frame mounted firing pins, so don't take it as pure gospel. Of particular note I would NEVER EVER stone a trigger to tune for a lighter SA trigger pull, today simply changing the rebound spring for a lighter one will achieve the same goal and won't lead to future issues with pushoff.
It is NOT at all difficult to change out your firing pin PROVIDED you know how to properly remove the sideplate. That is done by removing the grips, removing all of the sideplate screws, and then TAPPING the sideplate free by striking the grip frame with a plastic or wood object across the side of the grip frame. Personally I use the plastic handle on a screwdriver. Lay the gun on it's side in your lap and rap the grip frame repeatedly below the sideplate and the vibrations will cause it to rise up out of it's nest. Once it's popped free you can then set it aside.
You'll then see that the Hammer Block has also come free and that particular part can be a bit of a challenge to get fitted correctly the first time you open up the lockwork on one of these revolvers. At one end you'll find a triangular shaped end with a slot in it. This slot is engaged by the pin that is mouned on the side of the Rebound slide. When you replace the hammer block you need to place it in the UP position so that the blocking tab is between the hammer face and the frame and tip the revolver slighly so that it stays in that position while you replace the sideplate. BTW, note that the sideplate has a small tab at the top that fits under the frame cutout for the sideplate at the top. Normally when I re-mount a sideplate I start with that tab at the top and then the front extension by the Yoke retaining screw. Then I use a firm press with my thumb to push the sideplate in place. If you can't press it down with thumb pressure it means the Hammer Block is out of position and you have to start over. BTW, the first time I had to re-fit a sideplate it took me 45 minutes to figure out the positioning for the hammer block and the orientation to hold the revolver so it wouldn't shift when getting the sideplate in place, now it takes a few seconds. Basically, plan on a bit of frustration the very first time.
Now that that has been covered, removing the firing pin is dead easy. Up at the top of the frame recess in the area of the firing pin you'll see a small cross pin. Cock the lockwork and use a small piece of wood to press the firing pin forward just a small amount to take any drag off the cross pin. Then lift the cross pin out with some tweezers. Once the cross pin has been removed the firing pin will slide right out by grasping the rear with some tweezers. You could also push it out with a toothpick from the recoil shield side. When you re-insert the firing pin you have to pay attention to the orientation and position the travel slot so that it will clear the cross pin that retains it. For the firing pin return spring, I snag it out with a small bit of hook shaped wire, such as a paper clip that's been re-shaped.
Now, about factory and aftermarket firing pins. The first thing you should do when you remove your firing pin is measure the overall length using either calipers or a micrometer, eyeballing it against a scale won't be accurate enough to identify what you have.
First there is Factory. Between 2008 and 2011 I've seen an increase of about 0.015 inch in the length of the factory firing pins. On my 2008 vintage 620 the factory firing pin measured with an overall length of 0.479 inch. On my 2011 vintage 625 the factory firing pin measured at 0.493 inch for overall length. I suspect this was done because around 2008 there were many complaints about poor ignition reliablity with S&W revolvers and that has slowly gone away.
Cylinder & Slide offers a firing pin that is 0.510 inch long and this pin also features an extended travel slot. While it works very well in a revolver that has been tuned to lighten the DA trigger it's also an invitation to a pierced primer in a gun that has the trigger tuned too heavy for this particular firing pin. BTW, when tuning for this particular firing pin I would recomend tuning the DA trigger weight to weight of 7.5 to 8.3 lbs., I've found it to be very safe with this level of tension on the mainspring. However, it is critical to use snap caps for any dry firing with this particular firing pin and I also think that replacing the firing pin return spring at regular intervals with this firing pin is a good idea. I would also consider it to be a firing pin geared for range or competition use and do not recomend it's use in a Defense revolver.
Second up for aftermarket firing pins is Apex Tactical. They currently offer 2 different length of firing pins. One is specified at 0.495 inch overall length and the XP firing pin is specified at 0.500 inch for overall length. I have had no experience with Apex Tactical's firing pins, they were unfortunately out of stock the last time I needed firing pins. However, they do have a great reputation and it's worth taking a look to see if they've come into stock. Personally, I suspect that their 0.495 inch firing pin would be a good choice for a revolver tuned to a DA trigger weight of around 10 lbs., which is near full factory tension.
If you want to keep your DA trigger at full factory levels (11.5 to 12 lbs.) you should consider ordering a factory firing pin.