There seem to be a lot of these posts, and I feel like the responses are not always as precise as they could be, or the advice is good but pieced out over the thread, so I wanted to try and create a post that might get stickied.
Here is a picture of my "Sig" shooting low.
When I first bought the pistol it was worse. Generally hitting at the very bottom or below the paper. As it turns out, my sight picture was wrong.
POINT 1: The vast majority of Sigs use a "combat sight" picture. This means that the dot on your front sight post should cover your intended target.
If you are using a Center Hold
or Six O'Clock Hold Sight picture
, you will hit low.
These are incorrect sight pictures for a Sig (from the factory). They may have worked fine with whatever pistol or rifle you have/had/tried once and liked, but it is not the sight picture Sig intends for their pistols and will not work on a stock Sig.
I don't intended to debate the merits of it, just know this is how Sigs are setup from the factory. Sigs from the factory are set up to use a combat sight picture at 10 yards. Closer or farther than that and the ballistics won't match up. Once I understood what the sight picture was supposed to look like, I got "on the paper", but was still well below where I should have been. See above.
This is the same pistol, shot from a cheap plastic bench rest I purchased to test what I felt I already knew.
POINT 2: Your trigger work might not be as awesome as you had hoped. (mine sure wasn't) I have a tendency (like many people) to anticipate recoil and yank the pistol down, resulting in low shots. Perfect trigger pull is actually not common, it takes practice. If you haven't shot much before, or haven't shot much recently/for awhile, get or borrow a bench. You can purchase a bench online for about one third of what a new set of sights cost (or half of what one sight costs). Shooting from a bench will help give you a good idea how much your trigger pull is affecting your results and should be recommended by anyone before you start looking at purchasing a new set of sight(s). The below is an excellent starting point. It shows some of the many mistakes a right handed shooter can make and the error in shot placement that results.
"But my friend's P22# has #X and #X sights on it and he does fine with it! And so do I! My P22# came from the factory with #X-2 and #X-1 sights on it. Some idiot at the factory just grabbed random sights out of a bin and Sig did it wrong."
POINT 3: Sig puts particular sights on particular models/batches at the factory purposefully. Just because your new pistol doesn't match what you saw on the internet, doesn't mean it's wrong. The pistol above with the just fine bench results came with a #6 front and #6 rear sight (it happens to be a P229 Legion). A quick Google will garner for you that a P229 in 9mm should come with a #6 front and #8 rear. Or maybe a #8 front and a #8 rear, depending on the post that you find.
The fact is that Sig batch tests and picks a combo that works for the test pistols from that batch. It's likely not a perfect system. The test pistols could be poorly chosen and not representative of the rest of the batch, or the pistol you bought might not be representative of the rest of the batch. You can call Sig, give them your serial number (on recent models) and they can pull up what batch it was in and what sight combo was chosen for that batch. However, until you bench shoot it (or develop/insure perfect trigger pull) you won't know if that combo works for your pistol. I should note here that I hear customer service is hit and miss. Some people have not had luck with Sig reps. The one I spoke to was nice a easily pulled up the required info. He also offered to take the pistol back in and have them "accuracy test" it to check it. I opted to buy a bench rest myself and save the mailing time. If I had found the sights were not correct, I would politely send the pistol back to Sig and expect them to get the right sights on it (for free).
Caveats to all this mess:
If you bought a used Sig, anything is possible. On a newer used pistol, you can call Sig and see what they put on at the factory. On an older model that doesn't seem to match POA with POI, lots is possible, but bench shooting will give you a better place to start from (and end your testing with). There are good and bad posts and articles out there about sight heights and how to make adjustments. This one I found particularly informative. If you are going to pay $60 (or more) to start swapping the sights on your pistol, make sure you understand the sight heights and what you expect the swap to do. There are people out there who can give you great or poor advice, but your best bet is to make sure you understand the information yourself before you change anything.
Here is a picture of my "Sig" shooting low.
When I first bought the pistol it was worse. Generally hitting at the very bottom or below the paper. As it turns out, my sight picture was wrong.
POINT 1: The vast majority of Sigs use a "combat sight" picture. This means that the dot on your front sight post should cover your intended target.
If you are using a Center Hold
These are incorrect sight pictures for a Sig (from the factory). They may have worked fine with whatever pistol or rifle you have/had/tried once and liked, but it is not the sight picture Sig intends for their pistols and will not work on a stock Sig.
I don't intended to debate the merits of it, just know this is how Sigs are setup from the factory. Sigs from the factory are set up to use a combat sight picture at 10 yards. Closer or farther than that and the ballistics won't match up. Once I understood what the sight picture was supposed to look like, I got "on the paper", but was still well below where I should have been. See above.
This is the same pistol, shot from a cheap plastic bench rest I purchased to test what I felt I already knew.
POINT 2: Your trigger work might not be as awesome as you had hoped. (mine sure wasn't) I have a tendency (like many people) to anticipate recoil and yank the pistol down, resulting in low shots. Perfect trigger pull is actually not common, it takes practice. If you haven't shot much before, or haven't shot much recently/for awhile, get or borrow a bench. You can purchase a bench online for about one third of what a new set of sights cost (or half of what one sight costs). Shooting from a bench will help give you a good idea how much your trigger pull is affecting your results and should be recommended by anyone before you start looking at purchasing a new set of sight(s). The below is an excellent starting point. It shows some of the many mistakes a right handed shooter can make and the error in shot placement that results.
"But my friend's P22# has #X and #X sights on it and he does fine with it! And so do I! My P22# came from the factory with #X-2 and #X-1 sights on it. Some idiot at the factory just grabbed random sights out of a bin and Sig did it wrong."
POINT 3: Sig puts particular sights on particular models/batches at the factory purposefully. Just because your new pistol doesn't match what you saw on the internet, doesn't mean it's wrong. The pistol above with the just fine bench results came with a #6 front and #6 rear sight (it happens to be a P229 Legion). A quick Google will garner for you that a P229 in 9mm should come with a #6 front and #8 rear. Or maybe a #8 front and a #8 rear, depending on the post that you find.
The fact is that Sig batch tests and picks a combo that works for the test pistols from that batch. It's likely not a perfect system. The test pistols could be poorly chosen and not representative of the rest of the batch, or the pistol you bought might not be representative of the rest of the batch. You can call Sig, give them your serial number (on recent models) and they can pull up what batch it was in and what sight combo was chosen for that batch. However, until you bench shoot it (or develop/insure perfect trigger pull) you won't know if that combo works for your pistol. I should note here that I hear customer service is hit and miss. Some people have not had luck with Sig reps. The one I spoke to was nice a easily pulled up the required info. He also offered to take the pistol back in and have them "accuracy test" it to check it. I opted to buy a bench rest myself and save the mailing time. If I had found the sights were not correct, I would politely send the pistol back to Sig and expect them to get the right sights on it (for free).
Caveats to all this mess:
If you bought a used Sig, anything is possible. On a newer used pistol, you can call Sig and see what they put on at the factory. On an older model that doesn't seem to match POA with POI, lots is possible, but bench shooting will give you a better place to start from (and end your testing with). There are good and bad posts and articles out there about sight heights and how to make adjustments. This one I found particularly informative. If you are going to pay $60 (or more) to start swapping the sights on your pistol, make sure you understand the sight heights and what you expect the swap to do. There are people out there who can give you great or poor advice, but your best bet is to make sure you understand the information yourself before you change anything.