There are some mistakes one makes only once, so horrifying are the results. For those seeking replacement SIG firing pins through the DIY route (as opposed to the eminently more sensible 'send it back for warranty service' approach), repeat the following as a mantra (as in "this is my rifle"):
The firing pin, firing pin position pin, and firing pin spring are three parts of the same assembly. One is useless without the other two. If you need to order one, you need to order the others.
I could have spent an extra $3.95 to throw in a firing pin spring. But no, I forgot in my haste and wasted four days seeking another one. Nurr.
George S. Patton once said: "A good plan, violently executed now, is better than a perfect plan next week." Firearms maintenance is NOT warfare. Get it right the first time, especially the whole 'sending for parts' bit.
The good news is that, properly armed, the process of removing the firing pin and related bits is child's play*. One needs:
1. A lump of wood with a hole drilled through it (a chunk of 2x4 is perfect)
2. A pin punch, more specifically at 3mm cup tip punch
A few whacks with a proper bludgeoning instrument and the pin exits the frame with a minimum of fuss. Whack from the right side of the frame (the ejection port side). Depressing the firing pin lock on the underside of the slide will cause the pin to exit under pressure from the firing pin spring. If the firing pin is broken and the firing pin sheared in half, some prying, wiggling, shaking, and general cajoling may be required. After disassembly, discard the firing pin positioning pin you just whacked out, as this is a sacrificial part.**
As you can see, the old firing pin and firing pin spring have neatly snapped off at the tip. This is startlingly reminiscent of the well-known CZ52*** firing pin failure, which is caused by excessive dry fire. The timing (right after whacking pretty heavily on the rear sight to get it out of the dovetail) is a wee bit suspicious, and I don't know if that jarring weakened an already stressed part - but then, this is a firearm we're talking about.
According to a thread at SIGForums that I can't for the life of me dig up (by a guy who took the Armorer's Course), the recommended replacement interval for the firing pin and spring is 20,000 rounds.
As an aside, the 'firing pin lag' seems to do a good job of keeping grime out of the firing pin channel. The characteristic 'firing pin drag' on primers is due to the firing pin retracting slowly, which is meant to keep the firing pin channel sealed. There was a small amount of carbon buildup at the front, but a great deal more carbon and oil got in around the firing pin lock. After 10,000 rounds (mostly grimy reloads), I count 6 q-tips to clean as pretty good.
Part 3: Firing pin spring arrives and we get this bad boy assembled.
* This procedure applies to new-style machined slides, as opposed to old-style folded slides (i.e. West-German style, with separate breech face/firing pin housing.
** Note that one end of the pin is knurled; this holds the pin firmly in place, but gets flattened out when the pin is ejected.
*** CZ52 firing pins are notoriously brittle, being made from cast steel. As an aside - and I don't mean to blame the design for operator error - the SIG firing pin lock is basically a scaled-up and Kraut-engineered take on the CZ52 design. Too bad the Czech design didn't get a chance to mature.