If you have a Sig P938, you might want to think about the mainspring housing failures. On some first generation guns, the sear spring moves around, eventually grinding down the boss on the mainspring housing. This allows the sear spring to move out of position, rendering your pistol about as useful as a rock. There isn't any warning before the failure occurs--the gun works fine until you pull the trigger one time and find that it's become a rock.
A couple weeks ago I called Sig and asked them to send me out a second generation mainspring housing and spring. The gun has about 800-1000 rounds through it. It functioned flawlessly, but the potential for failure was a concern. They wouldn't do it, but said that if I sent them the gun, they would look it over, fix any problems, and install the new parts. Since the gun was over a year old, I would have to pay the $55 for shipping to them. Instead, I asked them to sell me the parts. Together, it was eight bucks plus six bucks shipping. Installing the parts myself would void the warranty, but who cares? It's out of warranty anyway.
Although I've never done this before, installing the new parts wasn't horribly difficult. If you're not comfortable taking apart stuff and putting it back together, you'll want to have someone else do this.
Here's a brief summary of the procedure:
Do a basic field strip of the gun. Pull out the safety, hammer, and mainspring. The pin that holds the mainspring housing is not tight. You can just push it out at this point. You can't slide the mainspring housing down just yet, as the sear spring will catch on the ejector.
You can push out the sear pin all the way, allowing the ejector and sear to fall free. Instead, I pushed the pin out just enough to allow the ejector to come out. Now you can pull out the mainspring housing.
Assembly is in reverse, but as you push the sear spring up, you'll need to use a chopstick to push the two fingers of the sear spring into position. Keep an eye on the sear, as it can spin around and you might not end up with it right side up. You can put the ejector back into place, then push the sear pin back. If you find that the sear is not in the right alignment, you can still rotate it around. Gently push the mainspring housing all the way up, and replace the pin. Replace the mainspring and hammer. Now you can test the functionality of the hammer, trigger, sear, reset, etc. Since you were careful, everything will work perfectly, but it doesn't hurt to double check.
Attached are a couple pictures. Look at the Gen 1 MSH. Note the polished area around the top where the spring moves. Also attached is a video showing the of the two pieces right after I removed them. I was very surprised by the amount of movement. I don't know if this is the way it's supposed to be, but the Gen 2 parts are rock solid.